Mark Harris, Head of Sustainability at Sika, explores why architects can no longer treat heat resilience as an afterthought, and why the roof can play an important role in helping reduce overheating risk through design.
For years, extreme heat in the UK was framed as nothing more than a discomfort. However, estimates from the London School of Hygiene and Tropical Medicine put the death toll from May and June 2026’s record-breaking heatwaves at 2,877 people across England and Wales. Heat is no longer a seasonal inconvenience; it is a recognised public health emergency, and the built environment can influence how people experience and are exposed to extreme heat.
Much of the UK’s housing and commercial stock was designed for a temperate climate that no longer exists. Dense urban areas, dominated by dark, heat-absorbing roofing and paving, trap solar radiation and release it slowly overnight, preventing the cooling that once made summer evenings bearable. This urban heat island (UHI) effect compounds the problem: city centres can run several degrees warmer than surrounding rural areas, and it is often the most vulnerable residents, the elderly, those in poorly ventilated social housing, and people with pre-existing health conditions who are least able to escape it.
For architects, this elevates heat resilience from a sustainability consideration to an increasingly important design consideration. Overheating is already covered by Part O of the Building Regulations for new residential buildings in England, but the current wave of heat-related mortality is a signal that design thinking needs to move faster and further than compliance minimums, particularly for retrofit projects and non-domestic buildings that fall outside Part O’s scope entirely.
The roof as a thermal lever
The roof is one of several design elements that can influence a building’s thermal behaviour. It is the surface most exposed to solar radiation for the longest part of the day, and its specification has a disproportionate effect on both the internal comfort of the building beneath it and the microclimate immediately around it. Two broad strategies dominate current thinking: reflective, or “cool” roofing, and vegetated, or “green” roofing – and increasingly, hybrid approaches that combine the two.
Cool roofing works on a simple principle; the more solar radiation a surface reflects rather than absorbs, the less heat it transfers into the building and radiates back into the surrounding air. Light-coloured, high reflectivity roof surfaces may reduce heat absorption when compared with darker roof finishes, easing cooling loads and reducing the risk of overheating in the spaces below. For architects working on warehouses, retail units, schools and other buildings with large flat-roof footprints, this can provide a practical design approach for large flat roof buildings.
Green roofs address the problem from a different angle, using vegetation and growing plants to cool through evapotranspiration and shading rather than reflection alone. Beyond thermal performance, they bring additional design value that is increasingly hard to ignore. By attenuating rainwater to ease pressure on drainage infrastructure and supporting biodiversity objectives, green roofs may contribute to wider environmental and amenity benefits, depending on project design and location. For architects negotiating planning conditions in cities with declared biodiversity or climate emergencies, a green roof can help contribute toward meeting certain planning or project objectives through a single specification decision.
Matching the system to the building
Neither approach is universally superior, and the right answer depends heavily on structural capacity, budget, maintenance regime and the building’s use. A distribution centre with a lightly loaded roof structure may be a natural fit for one of Sika’s membrane options, engineered to have a high SRI value. A school or residential scheme with capacity for additional loading, and a client interested in wider sustainability credentials, may consider a green roof system where project objectives include biodiversity, amenity or surface water management considerations. Increasingly, hybrid roofs are combining both: reflective zones around plant and access routes, and vegetated zones that provide amenity and biodiversity gains, all engineered together as a single system rather than retrofitted as separate afterthoughts.
What ties these approaches together is timing. Roof specification decisions are typically made early in a project, often before the full implications of a warming climate have been modelled against the building’s operational life. Architects who build heat resilience into early-stage roof design, rather than allowing it to become a value-engineering compromise later in the process, are better placed to deliver buildings that are resilient as UK summers continue to intensify.
The data from this year’s heatwaves should reframe how the profession thinks about roofing altogether. It is not simply a weatherproofing layer sitting above the building envelope; it is an important component of building design that can influence overheating risk and occupant comfort.