Flat roofs are known as being problematic, with many suffering leaks. However in my own experience the problems with leaking flat roofs are usually as a result of either poor workmanship installing the roof membrane, poor maintenance by the building owner or as a result of age. A well designed and waterproofed flat roof should not cause problems or leak.
Is a flat roof actually flat? No. For clarity, flat roofs actually have a slight slope to allow rain water to drain off the roof, but they seem flat, when compared with more traditional steeply sloped roofs. The slope should be a minimum of 1 in 80, but in practice a minimum slope of 1 to 40 should be used to allow for deflection of the roof and for normal building tolerances.
Why are flat roofs popular? Flat roofs are economical to construct. They reduce the volume of the building thus reducing energy costs and allow, if required, for the roof to be used for other activities like a patio, positioning of services or simply as a green roof.
How long do waterproofing membranes last? The membranes used to cover and waterproof flat roofs usually only last 20 to 30 years. This is less than a well constructed tiled roof. As a result building owners need to plan for replacing the membrane on their flat roofs.
What maintenance is required? Regular maintenance of flat roofs is still required, as they are less forgiving than steeply sloped roofs. The most common problems are that rainwater outlets become blocked with leaves and other windblown debris. Installing leaf guards only slows this occurring as in my experience at this time of year, autumn, even these can become overpowered by the shear quantity of leaves being blown around by the wind.
Some tips for designing and maintaining flat roofs
- When a roofer is engaged to do the work, ensure they have the experience, skills and insurance to carry out the work. References should be checked and an engineer, or architect, engaged to specify, detail and inspect the work.
- The detailing at design stage is very important. A common area which needs to be designed and constructed properly is the joint between the flat roof and any parapet wall. Unless it is detailed to allow for movement any slight deflection of the roof could cause a break in the waterproofing membrane at the roof/ parapet wall/junction.
- Try to avoid having internal outlets and if these are unavoidable insulate them where they penetrate the warm roof to avoid cold bridging and possible condensation forming on the pipe.
- Allow for more than one outlet to drain an area, in case of blockages.
- Provide leaf guards to all outlets and regularly inspect them to ensure they are not blocked.
- Ensure the roofs can be safely inspected, with easy access to the roof and fall arrest systems in place to protect inspectors. This is to prevent injury but it will make it more likely that the roof will be inspected if it is easy and safe to do so. Inspections should never occur during extreme/ poor weather conditions.
- When positioning roof-lights or ventilation chimneys ensure there is sufficient space between them and other obstructions (e.g. parapets) to allow the installation and inspection of the roof membrane.
Actual example of problems. See the photograph below for what can go wrong; this was a section of school roof which I recently inspected where there was a leak. The roof is covered to a depth of 100mm with water and resembles a swimming pool because a rainwater outlet was blocked by a football. The parapet walls prevented the water over spilling the sides. Originally there were leaf guards installed to prevent footballs jamming rainwater outlets but it was thought by the school that these were kicked off by kids who have a habit of climbing onto the roof. I removed the football and the resultant rush of water down the drain was nice to experience as it is rare that problems with buildings can be this quickly resolved.
There are a few lessons from this example. All flat roofs should be regularly inspected to prevent this type of blockage occurring and leaf guards should be well secured.
The next photograph shows the same school roof with in this case leaves building up and beginning to block the drains in another area. These should be cleared out during a regular maintenance schedule. Obviously anyone carrying out the roof inspections should take all necessary precautions against falls, and have the assistance of another person in case of problems.