A flat roof has to be completely waterproof. The reason for this is that gravity is not your friend. If you have a pitch, or some kind of gradient on your roof, gravity will push water down the pitch and off of your roof. In the case of a flat roof, gravity will allow water to pool up on your roof, which can then allow leaks to form. One common solution for waterproofing a flat roof is to build up alternating layers of tar and tarpaper. Given enough layers, this roofing option will work, but you will need to keep the roof in good repair to avoid leaks.
How Leaks Form in a Built-up Roof
One way that leaks form in a flat roof is when seams fail. Installers will alternate seams in a flat roof so that the seams between the sections of tarpaper in one layer fall in the middle of the sections of tarpaper on lower levels. If the upper level of your roof starts leaking, the water should get trapped between the layers, but when cold weather sets in, any water trapped between the layers can freeze, and as it expands, it can create damage which eventually leads to a leak.
How to Repair a Blister in Your Roof
You need to inspect your roof from time to time to look for signs of damage. If you see any areas that look like blisters, this is a sign that the the upper level where you find the blister is leaking. In order to make a repair, follow these steps:
1. Cut out the section of roof that is blistered.
2. Cut out two patches from a fresh roll of tarpaper. Make sure that the first patch is at least an inch wider on all sides than the hole you have cut out and that the second patch is at least an inch wider on all sides than your first patch.
3. Paint a layer of tar onto your roof and press the smaller patch into this tar.
4. Paint a second layer of tar over this first patch and make sure the patch completely soaks up the tar.
5. Push your second patch into the tar and make sure that it extends on all sides of the first patch by at least an inch.
6. Paint a third layer of tar onto your roof and make sure that it completely absorbs the tar.
As these steps should illustrate, repairing a flat roof is not difficult, but it is messy, and it does require climbing up onto your roof. If you can't or don't want to make your own repairs for any reason, you should consider hiring a professional company like B & B Sheet Metal And Roofing Inc to make your repairs.