Blisters are most common on membrane roofing systems. Lots of contractors will tell you that if there are no signs of further damage, there might not be any need to repair flat roof blisters. They form when pockets of air or water get trapped between layers of the roof’s membrane or between the membrane and the frame. When the sun is out and it heats the pockets expand and stretch the membrane. If you do want to repair your flat roof blisters, there are a few ways to go about it.
If you have a membrane flat roof, cut away the membrane until you reach a place where the membrane is still stuck tightly to the roof. Reapply as many new membrane layers as you cut away over the blistered area. You might even need to do more. To make sure your patch remains watertight, you’ll need to leave at least 6 inches around the edges of the blister.
If you’ve got a felt roof, you can cut an X shape through the blister. Peel back the triangles that you’ve made and let the timber frame dry for a while (so don’t do this when rain is forecast!). Then, coat the area with lots of bitumen adhesive and leave it to dry until it feels tacky. Then press down the triangles of felt firmly into the adhesive.
After that, you’ll need to add more bitumen adhesive to the cuts you made. If you’ve got any spare roofing felt, cut a square that’s about 100mm bigger than the cuts then lay this over the adhesive. If you’ve not got any felt, it’s cheaper to use a self-adhesive flashing strip. Once you’ve laid the strip or patch over, seal the edges with more bitumen adhesive.