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.

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

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

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

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

If there are any roofers out there reading this they will know just
what I mean ... I am sure :-)

cheers all,

jON