I can’t really argue the point as he’s the expert.
We have a Trussed Purlin in the loft so only 1/2 of the rear roof elevation is on the inner skin, the calcs have a diagram of this so I assume this has been taken into account.Looking at the pic of your house again, I would say that on reflection the load on the inner skin will be about right. The inner skin is usually the heavier-loaded as it normally carries the floor load and - in your case- probably one-half the load from the roof trusses as well.
My SE has specified 254 x 146 x 43 UB for inner & outer leaf. Rather than having a 254mm downstand I want the floor joists (approx 8 1/4 ") to run into the steel which will minimise it.(Fwiw, in that instance I would have used a single 203x203x47UC with 250x6 steel plate tack-welded on top to pick up the cavity wall. Padstone would bridge both skins; and NO bolts!)
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