Cut rafter tweaking required

[Digmixfill]

I've run into a snag with my cut rafter plan. My ridge beams are level, and the new wall plates are level, but I didn't measure the span distance between the ridge and the walls all the way along. I foolishly assumed everything would be parallel - I really should have known better.

 

One side of the roof is proceeding as expected. 50x150mm rafters at just under 35 degrees with a birdsmouth seat length of 60mm. The side with the progressively diminishing span isn't going to work at the shortest end. The angle changes, and the projection of the rafter at the foot is quite a distance away from the eaves edge.

 

If I put a piece of 22mm timber under the worst offending rafter with the current birdsmouth cut, everything works, but i'd obviously need to use progressively smaller bits of timber on the other rafters.

 

If i reduce the birdsmouth seat length, I can get it to work with the existing wall plate, but i'll be heading toward 30-35mm seat length. What is the accepted minimum seat length? I can only find details for maximum birdsmouth depth.

 

An exaggerated plan view of the roof attached. Red=ridge beam Black=eaves.

 

How would you resolve this?

[Nickfromwales]

If that side of the house is "out of whack" then you can only build up the internal side and mask it, leaving the wonky stuff only visible outside; assuming the building is out of whack vs the plate having splayed outwards?

[Digmixfill]

It's definitely the building being somewhat less than square. The wall plate hasn't budged at all.  At the worst point the span is > 75mm less than the opposing side.

 

You would keep the 60mm birds-mouth seat and shim the wall plate height for each affected rafter?