First of all there are a few types of flat roofs but the most common would be a smooth white roof that has a mineral grit surface. This is either going to be torchdown (applied with flaming torch) or hot tar. The other common type is a gravel roof, also applied with hot tar.
The most common sign your flat roof is wearing out is pretty simple, you will start to see the granules coming off quite a bit. You’ll see the black membrane start to show through all over. If it’s just in one spot or another, it could be because of foot traffic or maybe in front of drain where the roof wears out faster but when you start to see it all over, it’s time for a new flat roof. You might be able to squeeze another few years out of it.
One tip on discovering if your flat roof was installed with at least reasonable expertise is to look at the seams where the roof overlaps. Roofing rolls are almost always 3 ft wide and you’ll see a black seam every 3 feet. If the roofers did it properly you should see a solid black line, called “bleed out” the entire length of all seams. In the case of roofing, neatness here is not what is desirable. It indicates they used the proper amount of hot tar, or, heated the torch roll sufficiently.
Sometimes they will cast extra white granules, the grit, into the bleed outs to make it look cleaner and this is good but can be misleading if you don’t know to look for it – usually you can spot this if you look closely.
Finding leaks on a flat roof can be frustrating, even for a seasoned pro but many times they are obvious: a random puncture, a seam that has come loose, and so on. The most common area for roofs to leak are the drains and fixing these usually demands expertise.
With a gravel roof it is not as easy to detect wear but if you have parapet walls or any equipment platforms, look to those places where the smooth white membranes where used for wear but this is not always instructive. If you have leaks on a gravel roof, it’s best to call a pro.