If you search for roofing you will not find many at all. https://play.google.com/store/search?q=roofing&c=apps
All most all the apps in the above link are simply advertising apps for roofing buisnesses.
I cannot comment on how good the paid market is as i have never made an app, although my other roofing software for desktop is doing fine.
This is a opportunity for a B4A coder to be part of an app they most probably couldnt make without having roofing experience.
The plan is to make it simple [possibly free] then build on that so it would be a long term project.