As London accelerates its push toward reusing existing buildings, material choice has become one of the most consequential decisions in retrofit and cut-and-carve projects. In the race to reduce embodied carbon and meet sustainability targets, timber floor systems are increasingly promoted as a fast, lightweight and environmentally positive alternative to traditional concrete. Yet when introduced into existing structures, this shift carries risks that are often underestimated or poorly understood.
Existing buildings were rarely designed with material substitution in mind. Older concrete frames, masonry structures and hybrid systems rely on known mass, stiffness and fire performance to maintain stability and compartmentation. Replacing concrete slabs with timber floors may appear straightforward on paper, but it fundamentally alters how loads are distributed, how fire spreads and how the building behaves over time. These changes are not always fully predictable, particularly when original drawings are incomplete or the structure has already been modified during its lifetime.
Fire risk is one of the most frequently cited concerns. While modern mass timber systems can be engineered to perform well in fire through charring and encapsulation, their behaviour depends heavily on detailing, workmanship and long-term maintenance. In reuse projects, fire strategies are often constrained by existing layouts, legacy compartmentation and limited tolerance for change. Introducing combustible materials into buildings that were never designed to manage them can create long-term liabilities, especially if future alterations compromise fire protection measures that were carefully designed but poorly documented.
Water ingress presents another challenge that is frequently downplayed. Existing buildings, particularly those undergoing major structural alterations, are vulnerable to temporary exposure during construction. Timber systems are far less forgiving than concrete when exposed to moisture. Swelling, degradation, mould growth and loss of structural performance can occur rapidly if protection fails, and these defects may not become apparent until years after completion. In contrast, concrete slabs offer inherent resistance to moisture and a level of durability that aligns more closely with the unpredictable realities of retrofit construction.
Beyond material performance, there is the issue of predictability. Concrete has decades of well-documented behaviour in reused buildings, with known failure modes and conservative design assumptions. Timber, particularly when used as a replacement for heavier systems, introduces changes to vibration, acoustics and robustness that can undermine the long-term usability of a building. These are not theoretical concerns; they directly affect whether reused buildings remain fit for purpose over their full lifecycle.
This debate cannot be separated from the wider question of risk management in structural reuse. As explored in A Harder Conversation on Risk for London Structural Reuse, the real danger lies not in reuse itself but in treating complex interventions as routine. Material substitution amplifies that risk by layering new uncertainties onto already imperfectly understood structures. Without intrusive surveys, conservative assumptions and a clear understanding of how materials interact with existing frames, projects risk locking in problems that will be costly, difficult or impossible to resolve later.
None of this suggests that timber has no place in London’s reuse agenda. Hybrid solutions, carefully engineered systems and robust fire and moisture strategies can deliver safe and durable outcomes when applied with discipline. However, timber should not be treated as a default sustainable choice simply because it reduces upfront embodied carbon. Sustainability must be measured over decades, not design stages.
If London is serious about reusing its existing building stock responsibly, material decisions must be driven by long-term performance, insurability and safety rather than trends. Concrete may not always be the most fashionable option, but in many reuse scenarios it remains the most predictable and resilient choice. As the industry continues to adapt historic structures for modern use, the challenge is not to choose the greenest material on paper, but the one that ensures buildings remain safe, serviceable and trusted long after the sustainability headlines fade.
Existing buildings were rarely designed with material substitution in mind. Older concrete frames, masonry structures and hybrid systems rely on known mass, stiffness and fire performance to maintain stability and compartmentation. Replacing concrete slabs with timber floors may appear straightforward on paper, but it fundamentally alters how loads are distributed, how fire spreads and how the building behaves over time. These changes are not always fully predictable, particularly when original drawings are incomplete or the structure has already been modified during its lifetime.
Fire risk is one of the most frequently cited concerns. While modern mass timber systems can be engineered to perform well in fire through charring and encapsulation, their behaviour depends heavily on detailing, workmanship and long-term maintenance. In reuse projects, fire strategies are often constrained by existing layouts, legacy compartmentation and limited tolerance for change. Introducing combustible materials into buildings that were never designed to manage them can create long-term liabilities, especially if future alterations compromise fire protection measures that were carefully designed but poorly documented.
Water ingress presents another challenge that is frequently downplayed. Existing buildings, particularly those undergoing major structural alterations, are vulnerable to temporary exposure during construction. Timber systems are far less forgiving than concrete when exposed to moisture. Swelling, degradation, mould growth and loss of structural performance can occur rapidly if protection fails, and these defects may not become apparent until years after completion. In contrast, concrete slabs offer inherent resistance to moisture and a level of durability that aligns more closely with the unpredictable realities of retrofit construction.
Beyond material performance, there is the issue of predictability. Concrete has decades of well-documented behaviour in reused buildings, with known failure modes and conservative design assumptions. Timber, particularly when used as a replacement for heavier systems, introduces changes to vibration, acoustics and robustness that can undermine the long-term usability of a building. These are not theoretical concerns; they directly affect whether reused buildings remain fit for purpose over their full lifecycle.
This debate cannot be separated from the wider question of risk management in structural reuse. As explored in A Harder Conversation on Risk for London Structural Reuse, the real danger lies not in reuse itself but in treating complex interventions as routine. Material substitution amplifies that risk by layering new uncertainties onto already imperfectly understood structures. Without intrusive surveys, conservative assumptions and a clear understanding of how materials interact with existing frames, projects risk locking in problems that will be costly, difficult or impossible to resolve later.
None of this suggests that timber has no place in London’s reuse agenda. Hybrid solutions, carefully engineered systems and robust fire and moisture strategies can deliver safe and durable outcomes when applied with discipline. However, timber should not be treated as a default sustainable choice simply because it reduces upfront embodied carbon. Sustainability must be measured over decades, not design stages.
If London is serious about reusing its existing building stock responsibly, material decisions must be driven by long-term performance, insurability and safety rather than trends. Concrete may not always be the most fashionable option, but in many reuse scenarios it remains the most predictable and resilient choice. As the industry continues to adapt historic structures for modern use, the challenge is not to choose the greenest material on paper, but the one that ensures buildings remain safe, serviceable and trusted long after the sustainability headlines fade.
