Loc: Bakerton, WV, USA
Since I have no idea about building, engineering standards and the approval process in the UK, I can offer no advice about requirements but Prudence and her virtuous sisters say look for local advice before building.
Ok so I've done the usual think on it overnight (always works quite well)
So if I were not able to get hold of 6x10 in fresh cut doug fir, could I then go for normal off the shelf KD 2x10 (like you said) with the eased edges like you get with normal carcassing then glue them together to make up the 6x10 would that be viable? I dont want to see metal fasteners, so could I just glue laminate them then buzz through some 5 1/2 screws from the back side?
And would this pose any issue working with fresh cut wood, fresh cut braces but KD beams? I take it as the beams are already dry I just cut the tenons a little short so they can shrink a little?
Another is to remove the oblique braces, and either use a Corbeling system and/or Truss System in concert with the Corbels. As such even a 100 mm square timber could suffice, and have seen similar with timbers as small as 75mm x 100mm when fashioned into a timber Truss Assembly...
I sent a PM as well to you with additional contact information...