Hello, We are soon ready to start the roof framing on top of our ICF walls which include 2x12 rafters and SIP panels for the roof structure. Unfortunately our concrete contractor did a terrible job on getting our footing level and there is a 1.25" difference in elevation. The roof framer is not overly concerned and is planning to use a 2x12 top plate and shim level under each rafter with treated 2x shims and then they said they would spray foam the gaps. This sounds reasonable but I have several questions. 1) Will the spray foam shrink in time creating air leaks under the sill or is there a better way to do this? My second question then is related to how we design the 2x6 gable walls on top of ICF? The ICF wall is 11" wide with 2.5" foam, 6" concrete, 2.5" foam. Should we position our 2x6 wall 2.5" from outside of ICF wall, then add 1/2" zip, then 2" ridgid foam? The other thought was to effectively build a sip by using osb on exterior of 2x6 gable wall,then 2" foam, then zip board & run it down past the top plate & overlap a little of ICF foam wall to try to help air seal that gap in top plate. Doing so would create a 1/2" bump out on gable past ICF foam, but we are thinking of doing shake cedar siding on the gable and stucco below or concrete fiber board (stucco) so that might still be fine and it also creates a little drip edge. Here are some pics of what we have going on. Just wanting to make sure we get a good tight envelop here. Thanks for your ideas.

1.25" is a significant variance. A better option would be to level screed boards on each side of the ICF, insulation with rigid or low-e foam to match the insulation in your ICF, then fill the center with non-shrink grout. I think this will provide a better continuity of your air barrier and insulation while providing a level, solid surface to install your roof.

The gable end can be set in to rest directly on concrete then the difference is corrected with strapping/insulation. Insulated gables are not necessary if the attic space is not conditioned.

Once again I'm missing something. Is this a 1 or 2 story house? Why the sip roof?

Yes, this house is a 2 story house over a basement. The 2nd story is built within the attic. So the attic space is conditioned with thermal barrier at the sip roof line.

If the attic is conditioned why not use the same sip on the exterior of the gable, with the wall adjusted inwards. The framing will be on the concrete and the exterior will be flush with the icf. As far as the top.plate, if the shims are under the rafters there shouldn't be a problem. As for the foam I haven't seen shrinkage from either gun foam or can foam. I can't say the same for site mixed foam.