I am building a  3/8": 12" Pitch shed roof house located in snow country, it will be supported by 11 7/8" I Joists with 5/8" ply sheath, roofing will be either standing seam or torch down, temps swing from 40C to - 25C.  Blown-in cellulose  is the common way, any other experiences?

I would never use a standing seam on a flat roof it is guaranteed to leak. why wouldn't you use SIPs for the roof system/insulation?

I have done a lot of these sorts of roofs. I think the best system is this from bottom up:

I-Joists, plywood sheathing, felt paper, poly-iso sheet foam, EPDM roll roofing glued down in pieces as wide as you can get. You can also do TPO but it is harder to seam.

You need at least two inches of the poly-iso. It has about the same r-value as any other closed cell foam (6 to 7?). Then you can fill your joist bays with fiberglass or cellulose or spray foam or whatever. Or you can just put more foam on top. You screw down the foam with clips, Piranha Clips?
Around the edge of the roof you'll need some wood to nail your flashing to. If the foam is very thick I usually make an L out of a 2 x 3 and 2 x (however thick the foam is). Then the first layer of foam is 1 1/2" thick and the next layers fill in the rest.

THe advantages of this is that sheet foam is half the price of spray foam. Your thermal break is at the roof so there is no danger of interior condensation inside the joist bays. The EPDM is bigger so less joints, no torch down dangers.

I would never use SIPs on a flat roof. I love using SIPs but if there is a leak you'll never see it until one side of your structure has been rotted away and then you're screwed. I always think it's better to know when you have a leak. I think you need a good pitch if you're using a SIP roof.

Metal roofs will leak on that low slope. My neighbor just burned down his house using torch down.

If you only put cellulose or fiberglass in the joist bays in the winter warm moist interior air will float up there and condense on the cold roof sheathing and rot your sheathing and joists. This system is not too expensive, pretty easy to do and performs well.

In short, with a flat roof you need foam on top of the sheathing.

Posted By Rsipgeo on 15 Oct 2010 07:37 AM

I would never use SIPs on a flat roof. I love using SIPs but if there is a leak you'll never see it until one side of your structure has been rotted away and then you're screwed. I always think it's better to know when you have a leak. I think you need a good pitch if you're using a SIP roof.

Our sips are rot proof

Well, I guess if they are rot proof then there wouldn't be a problem. Does this go back to magnesium SIPs?

No, this goes back to galvanized steel