Green roofs and blue roofs are becoming more common in London and the South East, on new homes, extensions, offices and blocks of flats. They can help with drainage, wildlife and keeping buildings cooler. But they add layers on top of the waterproofing, and they still need looking after.
This guide explains how they work, which parts need regular checks and why leaks under planting are hard to trace.
What a green roof is
A green roof is a flat or low-pitched roof covered in plants. Under the plants there’s a growing medium, then drainage and filter layers, a root barrier and the waterproof membrane. Some green roofs have low, hardy plants such as sedum. Others have deeper soil and a wider range of planting.
What a blue roof is
A blue roof is designed to hold rainwater for a while and release it slowly, rather than sending it straight into the drains. It uses restricted outlets or flow controls, and sometimes a storage layer under paving or planting. When the two ideas are combined, it’s sometimes called a blue-green roof.
Because a blue roof is meant to hold water, the waterproofing, upstands and outlets have to be right. The roof structure is also designed for the weight of the stored water, so blocked flow controls matter more than on an ordinary flat roof.
Parts that need regular checks
- Outlets and inspection chambers. They must be kept clear of plants, roots and debris.
- Flow controls on blue roofs. If blocked, the roof can hold more water for longer than designed.
- Plant barrier strips. Gravel or paving around edges and outlets that stops plants spreading where they shouldn’t.
- Upstands and edges. The membrane turned up at walls and edges, with flashings and trims.
- Plants. Unwanted plants, especially tree seedlings, can put roots into layers they shouldn’t reach.
- Rooflights and penetrations through the roof.
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Why leaks under planting are hard to trace
On a green or blue roof, the waterproofing is buried under other layers. You can’t see the membrane. Water can also travel a long way between layers before it shows inside. The point where water appears on a ceiling may be well away from the actual breach.
Finding the leak usually starts with the details you can see: upstands, outlets, edges, rooflights and walls above the roof. These are common leak points on any flat roof. If those look sound, the membrane itself may need testing. Electronic leak detection can sometimes help, depending on the build-up, but on a planted roof it’s more involved. Some areas may need opening up.
Looking after a green or blue roof
Most green and blue roofs come with a maintenance plan from the designer or installer. It usually covers clearing outlets, removing unwanted plants, checking edges and topping up planting. Follow it, and keep records. Only go onto the roof if there’s safe access and edge protection, and don’t walk on fragile rooflights.
How a survey helps
A flat roof survey looks at what can be seen: outlets, upstands, edges, flashings, rooflights and the visible state of the planting and drainage. If there’s an active leak, a roof leak investigation focuses on tracing it. For suitable roofs, electronic leak detection may help find a breach in the membrane. We’ll tell you what’s possible for your roof’s build-up.
Quick answers
Do green roofs leak more than other flat roofs?
Not necessarily. They protect the membrane from sun and weather. But when they do leak, the leak is harder to find.
How often should a green roof be checked?
Follow the maintenance plan from the designer or installer. Outlets in particular need checking regularly, and after autumn leaf fall.
Can a thermal camera find a leak under a green roof?
Usually not well. Planting and soil hide surface temperature patterns. Other methods are often more useful.
Related services
More advice
- How Electronic Leak Detection Works
- Flat Roof Problems: Ponding, Blisters and Splits
- Commercial Roof Surveys: Rooflights and Sheeting
- Why Roof Leaks Are So Hard to Find
- All advice
This guide is general information about roofs and roof surveys. It isn’t legal, insurance or structural engineering advice; for those, speak to a suitably qualified professional.