Flat roofs are built up of layers of roofing felt and tar. Leaks usually occur at low spots or where the roofing felt has been damaged. In most cases, the leak is directly below the damaged spot and the damage to the roofing felt is easy to see.
If there’s still water pooled in the leak area, mop it up or soak
it up with rags, and let the surface dry. Brush off any gravel. Look for cracks in the felt or for large blisters where the top layer has separated.
To mend a blister:
Use sharp utility knife to slice blister open down middle. Cut should penetrate to full depth of blistered layer but should not reach sound roofing felt beneath it.