Alternative built up roof repair methods
Basic repairs can be easily performed on built up roofs using the established standard three course method. Spud the roof, then apply a layer of asphalt, a layer of fabric, and another layer of asphalt. This method is applicable the vast majority of the time. There are a couple of times when years of roofing experience can give you some alternatives.
Embedded gravel. Spudding on most built up roofs will take you down to the felt level so you have a clean flat surface to repair on. Occasionally, you will find that all the gravel will not come back up due to materials previously used in repairs or the original installation process. When that happens, standard cold applied roofing asphalt may not be able to penetrate to the roof surface and give you a watertight bond. The solution is to spud, brush, apply the base course of asphalt and fabric as you typically would for a standard three course repair. The difference comes in on the third course. A horizontal grade modified bitumen adhesive can be used. The trick is to allow the adhesive to go over the edge of your first course and flow into the gravel that couldn't be removed by spudding. The thinner consistency of the adhesive will reach through the gravel and bond to the roof giving you a watertight seal.
Alligatoring. Alligatoring is when the surface of a built up roof starts to crack and resembles the scales on an alligator. The problem with performing a simple three course repair over this type of problem is the alligatoring will continue under the repair and later reappear around the edges. A good solution here is to use a cold applied modified bitumen cap sheet. This sheet can be applied by spudding the roof and applying a layer of asphalt over the damaged area. Then place the cap sheet over your asphalt and using a heavy metal roller, roll the cap sheet to fully imbed it into the asphalt. Finish your repair with a three course application around all sides and seams to reduce the possibility of fish mouths forming.
Each repair is different. If you are ever unsure about how to address a repair, it is always best to call in a professional commercial roofer. Using the wrong materials or methods will always give you, at best, unsatisfying results and at worst, further damage your roof.