Flat Roofs and the Common Defects to Watch Out For

Flat roofs should not be completely flat but should lie at a subtle gradient

From porches and bay windows to extensions to provide more internal space, flat roofs are in use everywhere. Flat roofs represent good value and, particularly when it comes to adding an extension, help keep costs down. But from a surveyor’s perspective we always view a flat roof with an element of caution.

What’s the problem with flat roofs?

Typically used as a roofing solution for a structure that is an addition to a property, the main problems arise from movement, quality of build, lifespan of the material used and lack of insulation. If you’re looking to buy a property with a flat roof you should be aware that defects may well show up in the building survey. We’ve put together a summary of typical issues that you might come across.

Flat roof cracking

Older flat roofs often prompt further investigation

The age of flat roofs

Modern flat roof membranes such as EPDM, which is made from rubber, are technically very advanced and can last for in excess of 50 years without any compromise in performance. Older, more traditional membranes, such as bitumen-based materials, degrade over time and have a limited lifespan of 15-20 years. Both depend heavily on the quality of material installation.

The flat roof covering is defective

Cracks, tears, punctures and blisters are all indications of a flat roof membrane that is worn and in poor repair. There is every likelihood that water will be finding its way in. Similarly, a membrane that has been patched is a sign of previous problems and remedial work to fix leaks. Over time, these patches are also likely to fail.

Defective covering can lead to leaks and water ingress

Stretching at joints

All properties are subject to movement. With flat roofs, problems arise at the point at which they are joined to the main structure. Movement will cause the protective roof membrane to stretch and an older membrane, that has been subject to years of weather, variable temperatures and exposure to UV rays, will begin to degrade and no longer flex or perform adequately.


Puddles of water collecting on top of the flat roof are a tell-tale sign that something is wrong. Despite the name, flat roofs should not be absolutely flat. A good quality flat roof will be constructed with a subtle gradient to allow surface water to drain away and have the felt laid in the correct direction to facilitate drainage. Ponding may signify that the original construction was inferior or it may be that, over time, the membrane has started to sag. This can lead to splits and tears, allowing water to penetrate through to the room below. The timber deck may also deteriorate as a result.

Moss and other vegetation growing on the flat roof

Green roofs may be desirable but if the flat roof wasn’t specifically designed as such, the presence of vegetation growing on top of the protective membrane is a clear indication that water is collecting.


Damp walls and ceilings in a room, condensation on glass, evidence of black mould, these are all visible signs that the flat roof is poorly insulated. It may also mean that the timber deck may also be compromised, something that can’t be easily identified without closer inspection. The solution is to properly insulate the roof and provide effective ventilation where necessary, which will extract water vapour and improve ventilation to the roof. If the timbers are sound, this is relatively straightforward. However, if the timber deck can’t be salvaged, there will be more cost involved to replace this.


Another internal sign that all is not well with the roof, watermarks on the walls and/or ceiling of a room with a flat roof are a giveaway that leaks have occurred. In finding the source of the leak it’s also important to check for other damage – not only to the timber roof deck but to masonry as well.

We always pay special attention to flat roofs during a home survey. So many issues are water-related and, with the amount of rainfall we get in the UK, it’s important to check thoroughly. A small watermark may be the only visible sign of a much bigger problem hidden away – you can rely on Home-Approved to investigate, provide an honest appraisal of the condition of the flat roof and suggest suitable remedial work to cure the problem. Read more about the Home-Approved.

Photo credits: Wikipedia and Home-Approved