By Mark Harris, head of sustainability at Sika
AS cities grow and climate change accelerates, flooding has become one of the most urgent issues facing our society. Hard surfaces like concrete and asphalt dominate our built environment, preventing rainwater from naturally soaking into the ground. The result? Overloaded drainage systems and more frequent urban floods. One of the smartest and most sustainable solutions is to look up, specifically, at rooftops.
Flat roofs represent a vast, underused surface area in cities. Converting them into green roofs (layers of vegetation, growing medium and drainage boards over waterproofing systems) can help mitigate the impact of heavy rainfall, slowing water runoff, retaining a significant portion of stormwater, and reduce pressure on city infrastructure.
During summer, green roofs can retain up to 80% of incident rainfall. In winter, retention ranges from 25% to 40%, depending on the system and climate conditions. That kind of capacity, scaled across a city, adds up to a major impact.
The sponge city model
The ‘sponge city’ concept has gained attention in urban sustainability circles. The idea is simple: design cities that absorb and manage water naturally, much like a sponge. Green roofs are a core part of this approach, working alongside rain gardens, permeable pavements, blue roofs and other green infrastructure.
But the benefits don’t stop with flood reduction. Green roofs also help regulate temperatures within the urban environment, and inside buildings, cutting energy use. They filter airborne pollutants, support biodiversity, and even contribute to better mental health by reintroducing greenery into dense, concrete-heavy areas.
Sika’s green roof systems
Sika has over three decades of experience developing durable green roofing solutions. Backed by more than 1.4 billion square metres of membrane production worldwide, Sika’s systems are built to perform under pressure, literally and figuratively.
• Extensive green roofs
Lightweight, low-maintenance, and perfect for large surfaces. These systems feature shallow substrates and hardy vegetation like sedum and grasses. They offer significant stormwater retention with the lowest structural impact.
• Intensive green roofs
Designed for deeper planting and heavier loads. With substrate depths typically between 300mm and one metre, these systems can support trees, shrubs, and even rooftop gardens or farms, ideal for creating usable green spaces.
• Biodiverse green roofs
Built to enhance ecology, these roofs use native plants and materials to create microhabitats for pollinators and other species. A great fit for developers looking to support urban biodiversity and ecological resilience.
All Sika green roofs are underpinned by high-performance waterproofing membranes that are resistant to root penetration and microbial damage, crucial for long-term reliability. And because these systems are suitable for both new builds and retrofits, they offer flexibility to suit a wide range of projects.
The climate crisis demands urgent action, and green roofs are one of the most practical, scalable tools we have to build urban resilience. At Sika, we believe in nature-based solutions that not only solve technical challenges like stormwater management but also improve the liveability and sustainability of our cities.
As more developers, architects and city planners embrace sponge city principles, green roofs will play a central role in shaping future-ready environments: spaces that are not just built to withstand climate extremes, but to thrive through them.
• For more information: visit: www.sika.co.uk/greenroofs
