worth getting some generic drwgs done, side on stack, centre stack and ridge stack....cost between £250-£350 each and normally would cover most works required,small additions would be xtra of course but if you do a lot of stacks by putting a few £££ on the price to cover the cost it wont take long to get ur money back, cover urself with a line in your RAMS stateing its the builders responsibility to check the load ability of the roof in conjunction to ok your generics as fit for his purpose