Seam tapes are used to reinforce the coating at high stress areas. If seam tapes are needed, keep it flat and whole. Don't cut strips off to work around screws. Use a sharp knife to cut a small hole around the head instead.
Sealing a roof properly takes a lot more than just caulking over a seam or fastener. There are several things to consider before you just pile on more product on your roof.
Caulk fasteners after they have been inspected, tightened or replaced as necessary. Caulking a loose fastener is useless. As soon as someone steps on it, the screw moves and the coating won't be able to take the stress.
Fabric-backed, self-adhesive seam tapes such as Butyl Tapes or Eternabond® are great for emergency repairs on your roof. Just make sure you apply it on a clean surface as they are very difficult to remove and reposition.
Only use seam tapes during emergencies when you need to quickly stop the leak and don't have time to wait for the coating to dry. A dry adhesive will not stick to the surface as well as a wet adhesive. Furthermore, the adhesive backing tends to get soft and gummy during hot weather. If a dark coating was applied over it, extremely high temperatures will soften the adhesive and the tape may pull away from the seam.
Another thing to remember is that most seam tapes are pressure activated. You need to use a rubber or steel roller to get good adhesion. Be careful so that the adhesive does'nt get squeezed out of the edges of the fabric. The fabric is there to allow the coating to stick to it. Any exposed adhesive may cause problems and is definitely a dirt magnet.
Horizontal seams are tricky. When tapes fail, water backs up between the sheets and goes in the screw holes. This worsens the leak. Polyurethane Caulk works really well over fasteners & seams. Brushing the caulk into the seam will further reinforce the seam.
Vertical seams leak when the edge of the panel touches the water plane. We call it the Dirt Seal Effect.
Leaks are usually closer to the gutters. Caulking fasteners do not solve the leak as water is moving between the sheets. Leaks get worse after someone has sealed the horizontal seams.Roof pitch directly affects the severity of the leak. The higher the pitch, the worse it is.