sadsjon wrote:

I have a triangular cross section roof at 45 degrees.
I have 90 degree turn in this roof over the front aspect of the house.

Something like my picture here? :

http://www.internode.co.uk/temp/angles.jpg

Internally, I therefore have timbers that are cut 45 degrees on one
plane an then 45 degrees on another plane.

You mean where they join the gully beam running down from the apex of
the roof (at G) to the wall plate?

Does anyone have any idea how to work out the angles so that I can cut
timer or insulation board to fit the voids.

In this case the ends of the beam would need to be cut with a compound
mitre matching the slope angles a and b. Since in this case they are
both 45 deg, your mitre angle will be 45/45

WIthout trial an error I am never going to get the insulation
fitted ...

You just need to allow for the thickness of the insluation when
measuring, otherwise you bits will come out too short.

I am sure there must be some rules of thumb that govern how to cut
insulation and timbers to fit this setup.

You just need to keep the orientation of the panel or beam in mind and
apply the same angles as used on the roof to it, in the same plane.


--
Cheers,

John.

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