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Common Problems and Solutions for

Commercial Flat Roofs!

Don’t let your Commercial Flat Roof look like this!

water on flat roof

Flat roofs have been known as being problematic, with many suffering leaks. However, in our experience, issues with leaking flat roofs are mainly as a consequence of either poor installation of the roof membrane, poor maintenance by the landlord or as a result of time. A well planned and waterproofed flat roof should not create problems or leak.

Is a flat roof flat? No. For clarity, flat roofs have a slight slope to allow rainwater to drain off the roof, but they seem level when compared with more conventional steeply sloped roofs. The slope should be at least 1 in 80, but in practice, a minimum slope of 1 to 40 should be used to provide deflection of the roof and normal construction tolerances.

Why are flat roofs common? Flat roofs are inexpensive to construct. They decrease the volume of the building thus lowering energy costs and allow if needed, for the roof to be used for extra activities like a patio or simply as a green roof.

How long do waterproofing membranes last for? The membranes that are used to cover and waterproof flat roofs only last 20 to 30 years. That is less than a well built tiled roof. As a result, homeowners need a plan for replacing the membrane on their flat roofs.

What maintenance is required? Proper maintenance of flat roofs is still expected, as they are less forgiving than steeply sloped roofs. The most usual problems are that rainwater outlets become obstructed with leaves and other windblown rubble. Installing leaf guards only prolongs the inevitable as in my experience at autumn, even these can become overpowered by the sheer amount of leaves being flung around by the wind.

Few tips for designing and managing flat roofs:

When a roofer is employed to do the work, make sure they have the experience, knowledge, and insurance to carry out the job. References should be checked and a technician, or architect, engaged in specifying, detail and inspecting the work.
The detailing at the design stage is critical. A common area which needs to be created and constructed well is the joint between the flat roof and any adjacent wall. Unless it is made to allow for movement any minor deflection of the roof may cause a hole 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 prevent cold bridging and possible condensation forming on the pipe.
Allow for more than one outlet to drain an area, in the case of blockages.

Provide leaf protectors to all outlets and regularly check them to ensure they are not obstructed.
Make sure the roofs can be safely examined, with easy entrance to the roof and fall prevention systems in place to guard inspectors. This is to stop injury, and it will make it more likely that the roof will be examined if it is safe to do so. Inspections should never work during extreme poor weather conditions.

When placing roof-lights or ventilation chimneys make sure there is enough space between them and other obstacles (e.g. parapets) to allow the installation and inspection of the roof membrane.

An actual example of problems. See the photo below for what can go wrong; this was a part of a school roof which I recently inspected when there was a leak. The roof is covered to a depth of 100mm with rainwater and resembles a swimming pool because a football blocked the storm water outlet. The parapet walls stopped the water over spilling the sides. Originally there were leaf guards installed to prevent debris jamming rainwater outlets, but the school thought that these were knocked off by kids who have been known for climbing onto the rooftop. I removed the ball, and the rush of water down the drain was an amazing experience as it is rare that these problems can be this quickly solved.

There are a few lessons from this. All flat roofs must be regularly inspected to stop this type of blockage happening, and leaf guards have to be well secured. Don’t let your flat roof look like this!

Help I have a Lake on my Roof!

lake on commercial flat roof

Contact one of our Trained Commercial Roofers to inspect your roof before it’s too LATE!


White Roofing Systems

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