Your statement is contradictory. You can't set a price for labor and then search for workers. You have it set the price and the workers come to you. If 15/hr is not attracting any then you try 16/hr and so on. If yo set at 20/hr the labor will walk off the current job and come to you. I've seen it happen.
Look I just posted the average wage. I am not looking for a damn roofer now.