A roofer ideally has to know about roofing, about the city codes, OSHA safety rules, home inspector's checklist, upcoming techniques & research in roofing, and adjuster & insurance claims.
However, for real estate or rehab purposes, a roofer should know about roofing, roof repairs, and home inspector's checklist for roofs at the very least. There are a lot of roofers who know how to "shingle" but do not know what a home inspector is looking for, or know the city codes.
In fact, many roofers who shingle, have the skill to shingle but may not know about other aspects of roofing, that are particular to roof repairs. There are many different types of roof repairs, shingles missing, water flow issues, etc. The one most needed is fixing roof leaks, and it is also one of the most failed type of fix.
For example, how to identify a leak. This I have seen over and over again. Roofers go up, try to find the leak, think they found the leak, seal it, and when it rains again, it leaks again. What happened? When you see a leak in the house, it is not always the case leak is coming from right above it. The leak may be coming from a totally different side.
To shingle a roof requires a different set of skills from roof repairs. Ask your roofer, what is his method of finding leaks and ask for pictures. Most roofers do not have a method as such they go off his or her experience and is honest from his or her perspective. However, fail to fix a roof.
Regardless of the method used by the roofer to fix the leak. How can you know if the leak is fixed? Have the roofer take the hose to the roof, and run water on the roof for 15 min, and see if the leak happens. If you are an expert roofer you will how to run the hose like rain water also, but just running the hose over the suspected area, and the whole roof for 10-15 min should be a good test. Sometimes it will take ten minutes for the leak to show.
Now someone will say, they put up sealant, and run water over the sealant is not a good idea. One has to wait for the sealant to dry. That should take 10 min.
However, this is exactly the problem. There are two types of repairs, chemical repair or a mechanical repair. A chemical repair uses chemicals in a sealant. While a mechanical repair replaces the entire bad part; you do not just add sealant. Here is an example, because sealant eventually erodes. So let's say there two buildings, both have leaks. A roofer comes to building A, and adds sealant, and starts to leak again in six months. While building B the roofer replaces the bad shingle with a new one. So, a mechanical repair is always better. So, ask the roofer what type of repair they are going to do.
So, sometimes finding the leak without expensive equipment which most roofers do not have is a matter of trial and error, so it's worth using sealant to find the leak, and doing the hose test. After the leaking area is found a mechanical repair can be made.
So, sometimes you have to tell the roofer how you want things done. Most roofers will do things by what they saw others doing not scientific studies in the industry.