How to work out...

[allex50]

...the height where the rafters sitting on the top of the inner skin (top of the wall plate) to the top of the outer skin so that the rafters clear the wall and the soffit sits on the top of the outer skin for a given roof pitch (mine will be about 22 degrees ? Thanks.

 

[Bonni]

The tangent of 22 is .4 (or 40%). So half the width of the wall (wall plate to centre of gable) and multiply by .4. that's the height.

 

[allex50]

Ah, so from the outer face of the inner leaf to the outer face of the outer leaf I think is about 160mm incl. the cavity. So it looks like 64mm. Not much but is a low angle on the roof. Thanks, made me think it was easier with trig. So may need to trim the Botton of the rafter. Hope that aligns with your figs.

 

[^woody^]

It's not that simple.

The cavity width, the soffit level, and the depth of the rafter notch will impact where the underside of the rafters is and where it intersects the external leaf.