We want to build up on top of the existing roof 8", spray in foam apply new deck and metal roof, should we remove the existing Architectural shingle or is it ok to go over them, the rafters are 4"x8" fir the span is 15' and the are spaced 48" o/c , the deck is t&g pine with (1/2 "?) on top I belive since no nails penatrate the ceiling, it is possible the ceiling is 2"x6" t&g not sure. My primary concern is whether this roof can withstand the wieght from heavey snow and wind if I were to leave the shingles, what do you think, Tom

Because of the unusual construction of the existing roof system, you will need to have an engineer or architect help you with this one.

Not possible for anyone else to tell you whether the existing roof system can carry the weight of the added loads.

My suspicion is that it will not and that you'd be better off demolishing the existing roof system and reframing it.

I just ran into a similar job spec as this one.

Without a structural engineer there to make a real assessment of this,from a distance there is simply no way to take in all the variables.

manhattan42 is right on the mark!

And I am sure to get an approved building permit there will have to be a study of the structure.

Who is drawing up the plans for this, do they not have an engineer to approve it