Cascading use of timber-based construction materials and its contribution to the urban carbon sink

Abstract

Building with timber can reduce CO₂ emissions in the construction sector and balance the urban carbon cycle. The production of timber-based materials generates lower carbon emissions than that of mineral-based alternatives. Timber-based materials also act as carbon reservoirs throughout their lifespan, until they are eventually burned or decomposed. The residence time of carbon in these reservoirs can be extended further through cascading use of timber, when timber is reused or recycled for multiple purposes before being disposed of. Cascading delays the release of carbon from combustion and reduces the demand for raw wood. This study examines the long-term impact of the cascading use of timber-based building materials on the urban carbon cycle using three scenarios. In the Business-as-Usual scenario, waste wood is utilized in accordance with current German legal standards. In two Cascading scenarios, it is used in cascade chains of varying complexity. The case study considered is Schumacher Quartier, planned for Berlin, Germany. During the planning stage of that residential development, six building typologies were designed, each using a different amount of timber. Our results indicate that the Dowel Laminated Timber building typology stores the greatest amount of carbon over all cascade stages. In the Cascading scenario, carbon is stored for 199 years. Sorted by the amount of carbon stored, this is followed by Glue Laminated Timber typology (for 204 years), Timber Frame (for 186 years), Light Weight Timber (for 175 years), Brick (for 188 years), and Reinforced Concrete (for 191 years). In the Extended Cascading scenario, one to three additional cascade stages are implemented, which delay carbon emissions by 62–167 years (113 years on average) compared to the Cascading scenario. The results demonstrate that the use of waste wood in cascade chains is particularly advantageous for large solid wood parts. This study serves as a foundation for integrating the contribution of cascading waste wood use into the assessment of the impact of timber buildings on the urban carbon cycle.

1 Introduction

Cities have a particularly strong influence on the global carbon cycle, emitting around 70% of global CO₂ emissions in 2020 – a share which increased by 8% since 2015 (Lwasa et al., 2022). There are also urban anthropogenic stocks that store carbon. Churkina (2016) quantified urban carbon pools consisting of carbon in urban vegetation, soils, buildings, landfills, and humans. Even if the carbon stock in buildings was only around one percent of carbon stored aboveground, this still represented a significant amount of 6.7 Petagram of carbon (Churkina, 2016). The use of timber-based building materials is found to have multiple beneficial impacts on the urban carbon cycle, and thus on the concentration of CO₂ in the atmosphere. First, production of wood-containing building materials usually emits less CO₂ (Churkina et al., 2020; Werner and Richter, 2007). Second, timber-based materials store large amounts of carbon in their wood fibers, which is not released into the atmosphere until the materials are either burned or decomposed (Dangel, 2016; Savi and Klingler, 2022). So, wooden buildings can contribute to increasing the carbon stock of cities (Churkina et al., 2020; Lauk et al., 2012; Werner et al., 2006). Churkina et al. (2020) demonstrated that a five-storey light-frame timber construction can store up to 186 kg C/m², which is the 3.5-fold amount of carbon stored in the aboveground biomass of some natural forest ecosystems (Churkina et al., 2020). At the end of a building’s life, waste wood can be reclaimed from wood-based building materials. In Germany, 10.5 million tons of waste wood were collected in 2016, with the largest share coming from building and construction (Steger et al., 2019). Since the landfilling of waste wood was banned in Germany in 2005 (BAV, 2012), there are currently two waste management pathways for waste wood. The first is incineration, which is usually realized with energy gain. It is the dominant use case for about 80% of wooden waste in Germany (Döring and Mantau, 2021; Flamme et al., 2020). The second one is material recovery for the production of particleboards or fiberboards–currently the only material recovery application of waste wood, which is practiced on an industrial scale (BAV, 2012; Mantau et al., 2012). Waste wood that is collected for further use in new wood products must pass through various steps in processing plants: First, the wood is sorted (usually manually), and metal, plastic, or paper parts are separated from the wood using magnetic separators and airflow systems (BAV, 2012; Steger et al., 2019). The wood is then shredded and sieved, depending on the requirements for its subsequent use (BAV, 2012; Steger et al., 2019). The processed waste wood usually results in wood chips, which are used as recycled material input for producing particleboard and chipboard (BAV, 2012).

A cascading use of wood can lead to a more efficient use of timber resources. Cascading is defined as the subsequent, “multiple use of the wood resources from trees by using residues, recycling (utilization in production) resources or recovered resources (collected after consumption)” (Mantau, 2015, p. 36). In this study, at least one subsequent usage of waste wood for a different product is referred to as a cascade stage. This may also include reuse for the same purpose. Figure 1 shows potential cascade uses of wood and wood products according to Höglmeier et al. (2015a). Material losses occur between the cascade stages during the recollection, transport, and processing of waste wood (Höglmeier et al., 2015a; Risse et al., 2017). In addition, any remaining building materials adhering to the wood must be filtered out (Höglmeier et al., 2015a; Schiller et al., 2022). The extent of such material losses can vary, depending on the purity of the waste wood and its intended application in the subsequent cascade stage. Despite these material losses, cascading offers two major advantages over the one-time, conventional use of wood. First, resource efficiency is higher as more products can be produced from the same raw wood (Risse et al., 2017; Risse and Richter, 2018). As a result, the pressure on raw materials, on land use, and the competition between wood for material use and wood for incineration are reduced (Höglmeier et al., 2013; 2014; Meinlschmidt et al., 2016). Second, cascade use of waste wood extends the carbon storage in the built environment (Hart and Pomponi, 2020; Head et al., 2021), because carbon is only emitted when the wooden materials are burned or decomposed at the very end of their cascade utilization (Budzinski et al., 2020; Wolf et al., 2020).

FIGURE 1

Höglmeier et al., 2015b, p. 336).

Previous studies compared the carbon storage potential of timber and concrete buildings (Andersen et al., 2022; Dodoo et al., 2009; Dzhurko et al., 2024), or the carbon sequestration in mature forests and timber constructions (Pomponi et al., 2020), or the overall carbon storage potential within the built environment on the regional (Pittau et al., 2022) or global scale (Churkina et al., 2020; Lauk et al., 2012). When investigating the environmental impacts of a cascading use of wooden building materials, studies compared the cascading of waste wood to using raw wood (Budzinski et al., 2020; Fraanje, 1997; Gärtner et al., 2012; Höglmeier et al., 2014; Risse and Richter, 2018) or to using mineral-based products (Gärtner et al., 2012; Höglmeier et al., 2015a). Höglmeier et al. (2014) concluded that the cascading use of waste wood is more beneficial than using raw wood, most of all because of reduced land consumption for timber harvest. Still, the authors stated that the advantages were not as pronounced as expected (Höglmeier et al., 2014). For Gärtner et al. (2013), fewer positive effects were found for multiple particleboard recycling, compared to the production of glue-laminated timber (glulam). In general, however, the authors claimed more pronounced positive substitution effects of waste wood when wooden materials replaced mineral-based building materials (Gärtner et al., 2013). Risse and Richter (2018) determined the potential for processing solid waste wood into high-quality glulam using the CaReWood process. The authors highlighted the higher resource efficiency of using waste wood in cascades and emphasized the positive economic potential of the increased use of waste wood (Risse and Richter, 2018). In addition, the authors critically examine the reduced availability of waste wood for energy generation when it is diverted into cascading use (Risse and Richter, 2018).

None of the abovementioned studies investigated the temporal delay of carbon emissions resulting from cascading. Therefore, this study was conducted using the case study of Schumacher Quartier, Berlin, Germany. Here, the approach of urban ecosystem science considers the carbon stored in the built environment as a part of the global carbon cycle (Churkina, 2016).

2 Materials and methods

The area under investigation in this study is the Schumacher Quartier on the site of the former Berlin-Tegel airport in Germany. The planned residential quarter, comprising 5,000 apartments on 46 ha, aims to implement climate change adaptation measures and contribute to climate neutrality (Tegel Projekt GmbH, 2025). Another component of the sustainability concept is to build the residential quarter with wood, to support urban carbon storage in the built environment (S. Wimmer, personal communication, 21 October 2024).

In this study, six building typologies (Table 1) are compared to support the decision-making process for planning the former Tegel airport (Tegel Projekt GmbH, n. d.). The name of each typology refers to the material that characterizes its structural system (e.g., Reinforced Concrete). Briefly outlined, there are the following building variants: Two building typologies based mainly on mineral materials, namely, on brick (assembly typology Brick) or on limestone with reinforced concrete (assembly typology Reinforced Concrete), two light-frame timber typologies with a joist system (assembly typology Light Weight Timber) or with a wooden frame system (assembly typology Timber Frame); and two mass timber typologies that either use cross-laminated timber (CLT) elements (assembly typology Glue Laminated Timber) or mass timber elements with wooden dowels for mechanically laminating panels instead of glueing them (assembly typology Dowel Laminated Timber or DLT). The design for Dowel Laminated Timber uses materials from the Austrian company Thoma Holz GmbH, which relies on local wood harvested in coordination with the moon phases (Thoma Holz GmbH, 2025). Table 1 summarizes the total weight of all building materials and the weight of the mainly wood-based materials (taken from Dzhurko et al., 2024) for all building typologies considered.

All building typologies meet identical functional requirements for thermal insulation as well as fire protection and sound insulation (Dzhurko et al., 2024). A reinforced concrete base without a basement and a green roof structure are planned for all building variants (Dzhurko et al., 2024). The materials required for these building components are not part of this analysis. The masses and specifications of the materials used were derived from the existing plans provided by Tegel Projekt GmbH. All masses calculated in this study refer to an entire building with four storeys. Each floor has a floor area of 1276.4 m², the exterior walls encompass an area of 1935.4 m² (without windows), all ceiling slabs comprise an area of 3824.3 m², and the roof an area of 1274.8 m² (Dzhurko et al., 2024). The total floor area of a single building is 5104.06 m² (Dzhurko et al., 2024). Outer walls consist of materials for the façade, the load-bearing construction, thermal insulation levels, and the interior cladding. Indoor walls were not considered for the calculation of material masses.

Since this study deals with the end-of-life options of wood-based building materials, only those materials with a biomass content of over 75%, which are to be treated as waste wood, were selected for the analysis (see Table 2).

The biomass content of the selected wood-based materials varies depending on their water content and the share of additives such as glue. Although the carbon content of wood varies depending on the age and the species of the original tree, it is assumed here 50% on average, based on DIN16449 (EN 16449:2014., 2014, p. 5). For calculations in this study, data on the carbon content of the analyzed materials were obtained from Dzhurko et al. (2024). There, the authors relied on data from the ÖKOBAUDAT database (BMWSB, 2023) or, if more specific information on the material was available, from Environmental Product Declarations (EPD) from specific accredited certifiers, such as the German Institute for Building and Environment (Institut Bauen und Umwelt e. V. – IBU). The weights of the materials used for different assemblies are also obtained from the study by Dzhurko et al. (2024) for calculations in this study.

In Germany, the handling of waste wood from buildings is mainly regulated by the Waste Wood Ordinance (WWO) of 2002 (AltholzV, 2002). Annex three of the WWO defines four categories into which waste wood is classified and which determine whether it might be used for material recycling or energy recovery. The classification is based on the degree of expected contamination and impurities, e.g., from coatings, paints, binders, and resins, as well as wood preservatives (Steger et al., 2019). Although waste wood classes are named differently in this study for a more intuitive understanding, the categorization is the same as in the WWO. The category Untreated Waste Wood (A1 acc. to BAV, 2012) contains natural wood, which is treated only mechanically and contains insignificant impurities (BAV, 2012), like pellets, offcuts, chips, and other construction site material from untreated solid wood. Treated Waste Wood (A2 acc. to BAV, 2012) includes glued, varnished, or otherwise treated wood like particleboards from construction sites, offcuts and chips from treated wood, floorboards, and boarding from interior finishing (BAV, 2012). Wood, which is treated with halogen-organic compounds like older kitchen furniture or mixed bulky waste, is categorized as Waste Wood with Halogen-Organic Compounds (A3 acc. to BAV, 2012). The Contaminated Waste Wood category (A4 acc. to BAV, 2012) is used for waste wood that is known to have been treated with wood preservatives, found to contain wood preservatives through testing, or assumed to contain preservatives because of its former use in load-bearing construction (BAV, 2012). This applies, for example, to impregnated wood from outdoor areas, industry, or agriculture to chips from the processing of all waste wood and timber from load-bearing constructions such as beams, windows, and exterior doors (BAV, 2012).

In order to compare different recycling and reuse options, three scenarios were developed for this study, which differ in terms of the service life of buildings and materials and the handling of waste wood. For the Business-as-Usual and Cascading scenarios, a lifetime of 60 years is assumed for the buildings, in line with assumptions from Kayo and Tonosaki (2022) (63 years), Höglmeier et al. (2013) (63 years), and Schiller et al. (2022) (60–80 years). For the Extended Cascading scenario, a longer lifetime of 100 years is projected for the buildings. This assumption represents a reasonable average, supported by estimates from Kalcher et al. (2017), who use lifetimes from 80 to 100 years for timber constructions built after 1945. Andersen et al. (2022) assume a building lifetime of 100 years for a modern five to eight-storey CLT-based construction in Scandinavia. For materials that are part of the load-bearing structures of the buildings (DLT, CLT, supporting lath, counter battens, wooden substructure, wooden beams, solid wood core beech-dowels), the same service life is assumed as for the entire building (Hafner et al., 2014; Kalcher et al., 2017).

For materials required for cladding walls and floors (e.g., all types of wood fiber insulation, parquet, larch boarding), the assumed service lives are based on information provided by EPDs or peer-reviewed studies (Andersen et al., 2022; Budzinski et al., 2020). This information was additionally verified by cross-checking it against BNB-system values (Bewertungssystem Nachhaltiges Bauen–Assessment System for Sustainable Building) and data from IEMB (Institut für die Erhaltung und Modernisierung von Bauwerken e.V. – Institute for the Maintenance and Modernization of Buildings) (Bundesinstitut für Bau-, Stadt-und Raumforschung (BBSR), 2009; Bundesinstitut für Bau-, Stadt-und Raumforschung (BBSR), 2017).

2.1 Business-as-Usual scenario

The Business-as-Usual scenario applies the management of waste wood as it is currently practiced in Germany. In the short term, this is therefore the most likely scenario for the management of waste wood in Germany. It implies that only Untreated Waste Wood, Treated Waste Wood, and Waste Wood Treated with Halogen-Organic Compounds can potentially be recovered for material use (AltholzV, 2002). In the Business-as-Usual scenario, it is assumed that 80% of waste wood from the dismantling of buildings is incinerated and 20% is recovered for material recycling. This represents an approximated allocation, which corresponds to the current utilization rates of waste wood in Germany (Höglmeier et al., 2013). For material recycling, only the use of waste wood for particleboard production is assumed, as this is the only use of relevance in terms of volume in current practice (BAV, 2012; Mantau et al., 2012). Contaminated Waste Wood is incinerated with energy gain in this scenario, according to the currently legally approved purposes (AltholzV, 2002). Since, in this scenario, the carbon stored in the wood is released fastest by burning the material residues, this scenario can be considered the worst-case scenario. Figure 2 shows the assumed end-of-life pathways of wood-based materials in the Business-as-Usual scenario.

FIGURE 2

Before particleboard production begins, material losses occur during the waste wood collection process, primarily because of limitations in waste collection methods and inefficiencies during transportation to the processing facility (Höglmeier et al., 2015a). When the components are separated into waste wood and other waste, material losses arise. These waste materials are assumed to be incinerated either together with other types of waste or, in the case of residues from the wood processing industry, directly on-site. Based on assumptions by Höglmeier et al. (2015a), these waste material losses amount to 46% for solid wood products such as sawn timber. These loss quantities appear to be high, but they are supported by the baseline study from Rüter and Diederichs (2012), who also assume a similar value for losses of 45%. Material residues and losses are usually used for energy gain by the sawmills or wood processing companies themselves (Mantau, 2015; Rüter and Diederichs, 2012).

For materials that already consist of small wood chips, lower loss rates of 28% are assumed when entering particleboard production, according to Höglmeier et al. (2015a). After being stored within a particleboard for 40 years, the carbon stored within the waste wood is released during subsequent incineration with energy gain. It is assumed that combustion is realized within a year.

Before the final incineration of the material, 10% of the material weight is subtracted to account for possible losses during the collection and transport of the waste wood (Höglmeier et al., 2015a). The highest proportion of waste wood – 80% of Untreated Waste Wood and Treated Waste Wood, as well as 100% of Contaminated Waste Wood–is incinerated directly after its first use in the building. Here, a 10% loss factor is applied to account for losses during the collection and transportation of waste wood.

2.2 Cascading scenario

For this scenario, an amendment of the current German Waste Wood Ordinance (WWO) is assumed, supported by its prior announcement and discussion in several publications (BAV, 2024; Flamme et al., 2020). Höglmeier et al. (2013) and Meinlschmidt et al. (2016) considered the current classification into four waste wood classes to be outdated for practitioners. Recommendations for an amendment to the Waste Wood Ordinance have been published on behalf of the German Federal Environment Agency in 2020 (Flamme et al., 2020). The Federal Association of Waste Wood Processors and Recyclers (Bundesverband der Altholzaufbereiter und -verwerter e.V.) is also in favor of revising the legislation on some points (BAV, 2012). However, the planned amendment to the law was repeatedly postponed (BAV, 2024). Based on these facts, it is likely that such a scenario is possible. However, it will take time to implement the technologies, adjust infrastructure, and improve market conditions required for this scenario. Therefore, this scenario can only be realized in the medium term.

In the Cascading scenario, however, it is assumed that the recommendations of DIN 68800-1 are implemented and that no wood preservatives are used (Willeitner, 2010). This means that even load-bearing construction elements–like CLT or wooden beams–are classified as Untreated Waste Wood or Treated Waste Wood. In addition, the Cascading scenario envisages the use of wood in cascade stages (Figure 3). Except for some material losses incurred during waste collection, transport, and further processing, it is assumed that all waste wood from building demolition can be recycled or reused beyond incineration. The waste wood is only incinerated for energy gain at the very end of the cascade chain. Compared to the Business-as-Usual scenario, the Cascading scenario results in a larger carbon amount being stored for longer time periods because the raw material is reused multiple times. The carbon stored in the wood is thus released much later, when the material residues are finally burned. The cascading scenario is therefore considered to be a favorable option.

FIGURE 3

Material losses between the cascade stages are based on calculations by Höglmeier et al. (2015b). This probably represents a conservative approach that tends to overestimate the losses. In comparison, Rüter and Diederichs (2012) state that the use of waste wood results in about 10% less material losses as it does not have to be debarked.

Apart from classification into waste wood classes, the wooden materials used within the building assemblies are differentiated according to their material strength and size. Materials that are too small to be recycled for sawn timber products, such as supporting laths and counter battens, are projected to be used as secondary material for particleboard production already in their first cascade stage. The same applies to larch boarding because this material is expected to be heavily strained after 30 years of use on the façade, and recycling options beyond crushing for particleboards seem unlikely due to a lack of stability (Bundesinstitut für Bau-, Stadt-und Raumforschung (BBSR), 2017).

2.3 Extended Cascading scenario

The Extended Cascading scenario also assumes the implementation of the amendment to the German Waste Wood Ordinance, while the categorization of waste wood according to waste wood classes remains the same as in the Cascading scenario. The classification of materials according to properties and size is also the same as in the previous scenario. In contrast to the Cascading scenario, it is assumed that a general reuse of large, wooden building parts is promoted, as well as a longer service life of buildings. Therefore, the lifetime of all assembly types is set to 100 years. It is further assumed that a further development of waste wood processing technologies allows the realization of more comprehensive cascade chains. This general assumption is supported by legislative efforts to avoid waste as far as possible and to conserve resources through reuse and recycling applications. It is expected that the new paradigm in waste management, which was formulated in the Circular Economy Act in 2012, will also be reflected in the revision of the WWO (BAV, 2012; Flamme et al., 2020; KrWG, 2012; Obert, 2022). Furthermore, it is projected for the Extended Cascading scenario that innovations in the planning and design phase of buildings will lead to the integration of dismantling concepts at early project stages, which would simplify the disassembly and separation of materials during deconstruction (Schiller et al., 2022; Wolf et al., 2020). In consequence, fewer material losses between the cascade stages are assumed in the Extended Cascading scenario. Because this scenario covers such a long time frame, it is subject to many uncertainties. While the assumptions regarding the multiple reuse of materials are technically feasible, they require extensive changes in the recycling market. Therefore, this scenario is only feasible in the long term and is based on optimistic assumptions. The most ambitious scenario represents the best-case scenario for the future, as it delays carbon emissions from the incineration of material residues for the longest period of time.

The resulting cascade chains for all mainly wood-based building materials in the Extended Cascading scenario are depicted in Figure 4.

FIGURE 4

From a real-world perspective, it is more likely that the two cascade scenarios will be implemented step by step for an increasing number of materials. However, in order to highlight the differences between the Business-as-Usual and the cascading scenarios, this paper assumes that reuse and recycling practices will be fully implemented in the cascading scenarios.

3 Results

3.1 Allocation to waste wood classes

In general, the greater the proportion of wood-based materials used in construction, the higher the absolute amount of waste wood potentially available for further use (Figure 5). On average, the typologies contain 169 t (62%) of Treated Waste Wood (range: 40-288 t, 22%–99% for all typologies) and 118.3 t (14%) of Untreated Waste Wood (range: 1-677 t, 1%–64% for all typologies).

FIGURE 5

Contaminated Waste Wood is not found within the typologies Brick and Reinforced Concrete, because there, the load-bearing construction is made from mineral-based building materials, and timber is only used for floorings and wooden substructures. For the other four typologies, the proportion of Contaminated Waste Wood is at an average of 142 t (24%) (range: 79–495 t, 0%–75%)

In the Cascading and Cascading Extended scenarios, the waste wood from all construction typologies is allocated exclusively between Treated Waste Wood class (304 t; 84% on average, range: 40-657t, 36%–100%) and Untreated Waste Wood class (125 t, 16% on average, range: 1-677t, 1%–64%, Figure 6). This considerably increases the long-term reuse and recycling possibilities of the materials, as the total amount of waste wood is suitable for material recovery at the end of the building’s service life. Especially for the Glue Laminated Timber building typology, where the heavy load-bearing components constitute the largest share of the timber materials, material recovery opportunities increase by an additional 75% of waste wood compared to the Business-as-Usual scenario.

FIGURE 6

3.1.1 Carbon storage in Business-as-Usual scenario

In the Business-as-Usual scenario, only those amounts of waste wood recovered for further material use can contribute to an extension of carbon storage beyond the end of the buildings' life cycle. Because all contaminated wood waste is burned at the end of its material life, all carbon contained in it is emitted to the atmosphere. Approximately 80% of untreated and treated wood waste is incinerated for energy recovery, resulting in the respective emissions of carbon. The remaining 20% are recovered for further material use. This means that 20% of the carbon from untreated and treated wood continues to be stored in the built environment.

Based on the classifications of waste wood, Figure 7 depicts the weight of carbon that is lost from the built environment at the end of the building life (carbon emissions), and the weight of carbon that is further stored in succeeding building materials due to material recovery (carbon storage) for the building typologies.

FIGURE 7

If the building does not contain contaminated wood waste, such as the typology Brick, only 80% of the material is used for energy recovery, and the remaining 20% is recovered for further material use, with the carbon it contains being stored for longer. On the contrary, if the building contains a high proportion of contaminated wood, such as Glue Laminated Timber, a proportionately higher amount of waste wood is used for energy, as contaminated wood waste is fully burned. As a result, more than 80% of the stored carbon is emitted, namely, 95% in the case of Glue Laminated Timber. Consequently, less waste wood is used for material recovery: only 20% of the untreated and treated waste wood share. For Glue Laminated Timber, the carbon contained in this share only accounts for 5% of the carbon contained in the total waste wood. Therefore, the lower the proportion of contaminated wood in the building, the greater the potential for extended carbon storage. In timber construction typologies, the carbon amounts that are stored longer through material recycling (14.3 tC for Glue Laminated Timber) still substantially exceed the amounts of carbon that can be stored longer through material recycling from mineral-based building typologies (3.8 tC for Brick) because significantly more wood waste is generated overall in timber construction methods.

3.1.2 Carbon storage in Cascading scenario

In the Cascading scenario, all materials are utilized through a maximum of five successive stages (Figure 8). The relative decrease in carbon storage depends on the amount of losses between the stages, which depends on the material properties. The duration of carbon storage depends on the number of cascade stages and the respective service life of the materials in each stage (Table 3).

FIGURE 8

For example, the total carbon content of all materials from Light Weight Timber decreases significantly faster than that of Glue Laminated Timber because the materials from Light Weight Timber go through fewer and shorter cascade stages. A high proportion of the materials from Light Weight Timber are particle- or fiber-based, which means that in later cascade stages they can only be used for further processing into particleboards or fiberboards. In contrast, the high proportion of mass timber in Glue Laminated Timber, for example, allows for additional durable applications, such as boards or battens, before the waste wood goes into particleboard production. On average, at the building level, the longest carbon storage period is achieved for Glue Laminated Timber typology (204 years, Figure 8), followed by Dowel Laminated Timber typology (199 years), Reinforced Concrete typology (191 years), Brick typology (188 years), Timber Frame typology (186 years), and Light Weight Timber typology (175 years). However, it should be noted that the total amounts of carbon in Reinforced Concrete and Brick building applications are initially very low, with 18.98 tons of carbon [tC] for Reinforced Concrete (Figure 8) and 16.48 tC for Brick. By contrast, materials from Dowel Laminated Timber store 546.35 tC in the initial building application.

3.1.3 Carbon storage in Extended Cascading scenario

Although overall carbon storage quantities do not differ between the cascading and extended cascading scenarios, the storage periods are longer for all assembly typologies in the Extended Cascading scenario. The maximum carbon storage duration of 408 years is achieved when eight cascade stages are implemented (Figure 9). The carbon emissions from incineration at the very end of the material’s service life are postponed, and carbon is stored longer within the urban environment.

FIGURE 9

In the Extended Cascading scenario, the differences in carbon storage duration between different building typologies are more pronounced (Figure 9). Timber materials in Brick, Reinforced Concrete, and Light Weight Timber building typologies can undergo a maximum of six cascade stages, whereas other construction methods allow for up to eight stages. The maximum carbon storage time averaged for all materials on the building level at 408 years for Glue Laminated Timber (Figure 9), 391 years for Dowel Laminated Timber, 346 years for Timber Frame, 291 years for Reinforced Concrete, 281 years for Brick, and 230 years for Light Weight Timber. The amounts of stored carbon decrease more slowly than in the Cascading scenario, as losses between cascade stages are usually lower and the service life of the materials is generally longer (Table 4). In summary, the additional residence time of carbon in the materials in the Extended Cascading scenario exceeds the carbon residence time in the Cascading scenario by 62–167 years (on average 113 years) or 33%−76% (on average 55%).

4 Discussion

4.1 Potential for material recovery

With an average of 142 t (24%), the amount of contaminated wood in the building typologies of Schumacher Quartier is lower than the share determined by some other studies. Meinlschmidt et al. (2016) estimated that 50%–75% of all timber waste in Germany in 2010 was contaminated. Höglmeier et al. (2013) assigned 56% of the waste wood from Bavarian buildings in 2011 to the Contaminated Waste Wood and Waste Wood treated with Halogen-Organic Compounds classes. There may be two explanatory factors for this. First, modern timber construction designs are planned for the Schumacher Quartier. As such, the use of wood preservatives is expected to be avoided, as the timber construction is protected already through the design of the building (Bauhus et al., 2021; Kober, 2021). On the contrary, studies by Meinlschmidt et al. (2016), Höglmeier et al. (2013) examined a mixed pool of buildings from a variety of years of construction before 2010 and 2011, assuming that more waste wood could have been treated with hazardous substances. Second, the properties of materials planned for Schumacher Quartier are well known. Therefore, a precise assignment of materials to waste wood classes is possible.

The results regarding the distribution of waste wood classes among the typologies (Figures 5, 6) demonstrate that the building typologies that are most favorable for material recovery of waste wood are those that contain the highest quantities of timber and, at the same time, have a low proportion of Contaminated Waste Wood. In the Business-as-Usual scenario, this is mainly the case with Dowel Laminated Timber, followed by Light Weight Timber and Timber Frame. In the Cascading and Extended Cascading scenario, the amount of waste wood alone is decisive for the assessment, since Contaminated Waste Wood does not occur here. Thus, Dowel Laminated Timber has the highest potential for the material recovery of waste wood, followed by Laminated Timber, Timber Frame, and Light Weight Timber. Brick, and Reinforced Concrete also allow material recovery of waste wood, but the timber share in these typologies is very low.

An important consideration in this context is that a greater emphasis on material recovery from waste wood inevitably reduces the quantity available for energy recovery. This issue has been explored, among others, by Budzinski et al. (2020). It is questionable whether such a substitution of energy from waste wood by fossil energy carriers would still apply in the future, once the buildings of Schumacher Quartier are dismantled. In Germany, the energy generation from renewable resources is currently intended to increase and is planned to dominate the energy mix by 80% by 2030 (EEG, 2023). It can therefore be argued that the previous advantages of generating energy from the incineration of waste wood are diminishing because more renewable energy from sources other than wood will be available–an aspect also discussed by Gärtner et al. (2012). Nevertheless, the Extended Cascading scenario considers very long timeframes extending up to 408 years. These are associated with enormous uncertainties regarding the future energy supply (Head et al., 2021; Risse and Richter, 2016).

From a climate protection perspective, it can be argued that it is much more effective in the long term to not burn waste wood but to convert it into long-lasting products (Hart and Pomponi, 2020; Höglmeier et al., 2014; Kalcher et al., 2017). This approach extends carbon storage, delaying carbon emissions into the atmosphere from the incineration of waste wood until the end of the product’s service life (Budzinski et al., 2020; Head et al., 2021).

4.2 Carbon storage

Carbon loss is greatest after the service life of a building, if materials cannot be reused for the same purpose and are instead diverted to particleboard production. This is the case for small-sized solid wood materials such as counter battens, larch boarding, supporting lath, as well as for mainly particle-based components like wood joist/wall(+lvl), wood fiber insulation, and OSB. Furthermore, high material losses are expected in the first cascade stage of fiber-based materials, as these elements are often deeply integrated into composite systems, which cannot be separated without losses (Hildebrand et al., 2023). Since these materials have a high proportion within the building typology Light Weight Timber, the amount of stored carbon decreases sharply between the end of the building life and the first cascade stage.

Across all building typologies, large solid wood components integrated into the load-bearing structure, such as CLT, wooden beams, DLT, and solid wood core beech dowels, substantially increase long-term carbon storage. In the Extended Cascading scenario, such elements store carbon for an additional 280 years (+116%) compared to the Cascading scenario. In the two mass timber building assemblies, Glue Laminated Timber and Dowel Laminated Timber, large quantities of carbon (60-97 tC) remain stored within the materials, even after the maximum service life of 521 years. By comparison, Timber Frame assembly stores only 12 tC in its materials at the final cascade stage. This comparison emphasizes the clear advantages of mass timber construction over light-frame systems in terms of carbon storage potential.

Even though the amount of stored carbon decreases with each successive cascade stage, extending the service time of wooden materials remains beneficial for several reasons. By delaying carbon emissions, additional time is gained to develop and implement improved strategies and technologies, which can contribute to broader climate change mitigation efforts (Frischknecht and Pfäffli, 2023; Höglmeier et al., 2015a). This extended period might also allow for improvement of strategies for selective dismantling, higher-value use of waste wood, and reducing emissions during its processing. Plus, the supply of renewable energy from sources other than waste wood can be further developed. If that were to happen, the smaller proportion of energy gain from burning waste wood would no longer be that crucial.

However, even the maximum residence time of carbon within a building material estimated in this study (521 years) would be too short on its own to substantially reduce the CO₂ concentration in the atmosphere (Frischknecht and Pfäffli, 2023). According to calculations by Frischknecht and Pfäffli (2023), carbon would have to be stored for about 3,000–8,000 years to achieve this. Nevertheless, using wood products in cascading chains can help extend the urban carbon pool and reduce greenhouse gas emissions from the construction sector. This contribution is especially relevant in light of ongoing global urbanization and the projected increase in construction and renovation activities for the coming decades in Germany (Brunet-Navarro et al., 2016; Churkina et al., 2020; Savi and Klingler, 2022).

With an average 55% increase in carbon residence time in the Extended Cascading scenario compared to the standard Cascading scenario, key leverage points became clear: extending the lifetime of buildings and load-bearing solid wood elements, as well as the recycling of solid waste wood into sawn or glued timber products, can substantially prolong the carbon storage in the built environment. Using waste wood for particleboard production applications can also delay carbon emissions. According to a particleboard manufacturer that was consulted for this study, it is technically possible already today to reuse particleboard almost indefinitely and to increase the proportion of waste wood within it, or to even produce it entirely from waste wood (C. Möser-Benz, personal communication, 18.10 2024). So, there is potential to extend the carbon storage in light-frame timber construction typologies that contain high amounts of particleboards.

With a general increase in material recovery of waste wood, the consumer’s appreciation of waste wood as a resource might increase accordingly (Hill and Dibdiakova, 2016; Risse and Richter, 2018). In turn, increasing economic benefits from using waste wood can foster efforts to consider, even during the design phase, the need for wood materials to be sorted and separated by type after their service life (Hildebrand et al., 2023; Risse and Richter, 2016). To achieve this, knowledge about recycling and reuse opportunities should be strengthened already in the planning phase of a building, to design buildings with selective dismantling in mind (Hildebrand et al., 2023; Rosen, 2021). Standardized and mandatory building passports appear to be an effective tool for increasing the transparency regarding the building materials used in buildings and their expected fate at the end of a building’s life cycle (Hafner et al., 2014; Schiller et al., 2022). Even though such building passes already exist, they are not yet mandatory and are structured differently, depending on the provider (Hotze, 2023).

Dzhurko et al. (2024) highlighted mass timber typologies as particularly suitable for sustainable construction because their materials emit less carbon during production and store more carbon. This study supports the results of Dzhurko et al. (2024), while refining the assessment of carbon storage in the built environment by including temporal aspects. The properties of the individual materials and the amount of carbon stored in them are equally important. Large solid wood elements like wooden beams or DLT can have a particularly beneficial effect on the duration of carbon storage. The possibilities for cascade utilization are broader here, as the load-bearing components can initially be reused for the same purposes, then processed into smaller mass timber products such as planks, and only then shredded for particleboards. The construction typologies in which the proportion of these materials is higher are thus better suited for further use in cascade chains. This is the case with the mass timber typologies Dowel Laminated Timber and Glue Laminated Timber, in which the load-bearing components are designed with solid wood such as DLT, CLT, or solid wood core beech dowels. In contrast, Light Weight Timber performs less well because the high proportion of particle-based materials leads to larger quantities of waste wood being incinerated more quickly, thus releasing the carbon they contain, earlier. In addition, particle-based materials contain less carbon than solid wood materials or laminated products made from solid wood panels. As a result, less carbon can be stored in Light Weight Timber building assemblies over the longer term than in Glue Laminated Timber and Dowel Laminated Timber assemblies.

4.2.1 Uncertainties and limitations

The fact that only wood-based materials were analyzed leads to two limitations in the evaluation of the results at the building level. First, the number of materials analyzed for each building type varies, and these materials are also present in different quantities. As a result, the materials analyzed per building typology are not necessarily functionally equivalent. This can lead to certain distortions when calculating the average carbon storage. For example, in the Reinforced Concrete typology, only two wood-based materials are used. Their average carbon storage duration is high, with 191 years in the Cascading scenario and 291 years in the Extended Cascading scenario because an extensive cascade use is projected for one of them, the wooden substructure. Although the carbon weight of the wooden substructure is relatively low (0.22 tC), the long service life of this material has a pronounced influence on the calculation of the average storage period, since only two wood materials are considered for the assembly typology. By contrast, in the case of Timber Frame, for example, the sum of the material-specific carbon storage duration is divided by the total number of materials, which is 13. Some of these materials are used only for shorter periods, such as wood fiber insulation board. In consequence, these lower the overall average for the Timber Frame typology to 175 years in the Cascading scenario and 230 years in the Extended Cascading scenario, respectively. So, the overall impression might be that Reinforced Concrete is more advantageous than Timber Frame in terms of carbon storage duration. This impression is misleading, since Reinforced Concrete stores only a small amount of carbon, and it would be better from an environmental point of view to use more wood-based materials. In this study, the focus is rather on the properties of specific materials or material groups than on an assessment at the building level because material-level results are more easily transferable to other case studies than findings based on complex building constructions. Second, for a conclusive assessment at the building level, it would have been necessary to consider the end-of-life potential of non-wood-based materials as well. Construction waste containing steel or concrete, are collected at high rates of almost 80%–90% in Germany (Umweltbundesamt, 2022). Although recycling concrete is still widely used for backfilling excavations or in road construction, which is more of a downcycling effort, the use of recycled concrete aggregate in the construction sector is steadily increasing (Baunetzwissen, n.d.; Dodoo et al., 2009; Müller and Martins, 2022). The effects of carbonation, that is, the absorption of atmospheric CO₂ by concrete, on the carbon balance of concrete have been the subject of repeated investigations (Andersen et al., 2022; Dodoo et al., 2009; Pittau et al., 2022). Dodoo et al. (2009) found a carbon uptake of 23% of the amount that was emitted during the production of concrete during the lifetime of a concrete building. Dodoo et al. (2009) caution that the carbonation effect in buildings should not be overestimated, as it is offset by the high energy input during the processing of concrete waste. Although a holistic analysis of the end-of-life potential of all building materials for the Schumacher Quartier would be interesting, it would detract from the focus on the carbon pools in the built environment. Therefore, this question was not within the scope of this study.

Possible renovations that may arise during the service life of a building were also not considered, since their scope was too unclear. The interior walls, doors, and windows were also excluded, as no detailed data was available. However, since wooden doors and windows are usually well-suited for reuse, their inclusion could further enhance the overall reuse potential of building materials.

The cascade stages in the Cascading and Extended Cascading scenarios were developed based on assumptions described in the Methods section. Testing the impact of various parameters such as material loss rates or service lives on the results would unlikely offer additional insights, since the calculations are based on linear equations. Therefore a sensitivity analysis was not conducted. These reuse or recycling options assumed for the cascading scenarios are not yet realized in practice, apart from the use of waste wood for particleboard and, in some experiments, for oriented strand board (OSB) and medium-density fiberboard (MDF) (Gärtner et al., 2013; Institut für Holztechnologie Dresden gemeinnützige GmbH, 2022). These scenarios are to be understood as potential pathways. Whether the described cascade chains could be realized with the materials from the Schumacher Quarter buildings, in an economically and technically feasible way, remains uncertain. Similar scenarios have been developed by Gärtner et al. (2013), Fraanje (1997), Höglmeier et al. (2013), Höglmeier et al. (2014), Höglmeier et al. (2015), Risse and Richter (2018). Furthermore, the scenario design is supported by research on waste wood recycling, which demonstrates that developing suitable technologies and infrastructures for processing waste wood is possible and worthwhile (Institut für Holztechnologie Dresden gemeinnützige GmbH, 2022; Risse and Richter, 2018). The canton of Zurich in Switzerland serves as an example of a successful transformation of the construction industry toward a more circular economy. For over 20 years, the use of recycled concrete has been promoted there through targeted flagship projects, regulations for public tenders, and the implementation of national standards (ICLEI Europe, 2019). Between 2005 and 2018, 90% of public construction projects already incorporated recycled concrete aggregates (ICLEI Europe, 2019). The example of Zurich demonstrates that the public sector can play a pioneering role and generate demand for more sustainable construction solutions, provided these are of equivalent quality to the primary material. A similar approach is also conceivable in the long-term for the introduction of cascading use of waste wood, which is why the cascading scenarios are feasible in the medium to long term if the implementation of regulations and the adoption of new technologies work together toward this goal.

5 Conclusion and outlook

The potential for waste wood material recovery at the end of the building life depends on the properties of the materials contained in the buildings and the scenario under consideration. In the Business-as-Usual scenario, only a certain percentage of the waste wood classified as Untreated Waste Wood and Treated Waste Wood can be recovered for subsequent material use. Whereas in the Cascading and Extended Cascading scenarios, all waste wood is suitable for material recovery.

Concerning the carbon storage potential, it can be concluded that building typologies with load-bearing structures made of large solid timber elements are particularly beneficial for maximizing the carbon storage period in cascade chains. This advantage arises for two main reasons. First, they store more carbon than particle-based wood products due to their higher wood content (Dzhurko et al., 2024). Second, before being shredded for particleboard production, these solid wood components can be reused for the same purpose and subsequently recycled into sawn timber products, extending their use across three cascade stages. This delays carbon emissions from incineration at the end of the material’s service life more than for any other materials. While the total amount of stored carbon at the end of the building life does not differ between the Cascading and Extended Cascading scenarios (because the same amount of material is available from the buildings), the residence time of carbon varies according to the number and the duration of cascade stages that can be realized.

By implementing additional cascade stages in the Extended Cascading scenario, carbon storage periods are prolonged by 62–167 years (113 years on average) compared to the standard Cascading scenario, representing a 55% increase in carbon residence time. The results therefore suggest that the focus in timber construction should currently be on solid wood materials and on the development of recycling processes for them. In this way, the full cascade potential of mass timber materials can be realized over the long-term, particularly after their initial service life has ended.

This study provides a starting point for integrating the carbon storage capacity and carbon residence time of timber-based building materials into environmental impact reports. It also enables a more robust quantification of wooden buildings for urban carbon storage in future case studies.

Recognizing the importance of carbon storage as a material property could support the implementation of large-scale cascading use of waste wood in the future. In addition, the delay in CO₂ emissions from the incineration of waste wood opens up opportunities to further develop strategies for the selective dismantling of buildings, for standardized material passports, for economic incentives for the use of waste wood materials, and for scaling up construction parts exchange platforms. Effectively implementing these strategies can turn waste wood into a powerful tool for reducing the construction sector’s impacts on resource use and climate.

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