> and they didn't look like the sort to have wasted much of it on insurance or overhead
This is in general a really bad idea. Not only can you be on the hook if they make a mistake with the install, or mess something up like venting that ends up damaging appliances or plumbing, but if one of them falls off your roof, depending on the circumstances you could be personally liable for their possibly life long medical treatment (or end up in an extended lawsuit as they try to claim that you personally directed their work). Make sure your contractors have insurance.
This is in general a really bad idea. Not only can you be on the hook if they make a mistake with the install, or mess something up like venting that ends up damaging appliances or plumbing, but if one of them falls off your roof, depending on the circumstances you could be personally liable for their possibly life long medical treatment (or end up in an extended lawsuit as they try to claim that you personally directed their work). Make sure your contractors have insurance.