ANSWER: The answer is complicated by your location and the applicable prevailing wage laws. For example, under federal prevailing wage requirements, the Department of Labor’s Regional Office needs to determine the “prevailing practice”. For example, both roofers and sheet metal workers claim metal roofing. The prevailing practice in Region 9 (California, Nevada, Arizona, etc.) is that roofers can perform that work. In Region 1 (New York, New Jersey, Connecticut, etc. ) that metal roofing goes to sheet metal workers. So, the answer is to ask the Regional DOL Wage and Hour person for which trade that claims the work.
Many of the States issue their own detailed scope of work and allow a contractor to pay in any classification which claims the work. For example, in California a Laborer can lay pipe and so can a plumber. The California Department of Industrial Relations allows the contractor to pay in either of those categories. But watch out, only a plumber can connect the piping. Each state has their own rules, so best to check with the appropriate state agency. And, if federal funding is involved, check with the regional representative at the Department of Labor.