Box gutter, bevel/mitre: What am i missing here?

[kaseryn]

I'm just starting out in 1st & 2nd fix having spent a long time in shuttering and found myself doing a bit of headscratching today. We cut in a low pitch (26deg) roof which is of equal sides i.e. square and are installing a box gutter around it. Battens and soffit are in and we were to run a certain amount of ply up the rafters to take the flashing that would go in. Being square i mistakenly assumed the the ply bevel cuts going around the hips would all be 45 but when offered up clearly they werent. What then is the relationship of that bevel to the pitch? When we manually measured it seemd pretty close to the pitch of the hips but if anything shouldnt it match the common rafter pitch? I'm obviously missing something basic here.. whats the best way of determining that cut?

 

[garyo]

The hip will be a lower pitch because its rising at a shallower angle due to running across the main roof pitch rather than directly up it. If your common pitch is 26 then the hip pitch will be 18.79. The bevel angle should be 51.92 degrees. I'm not sure if you can get a bevel angle like that from a speed square, but if not you can use trig to work it out, or easier still measure it on site with a carpenters angle finder!

 

[noseall]

The hip will be a lower pitch

I always trial and error my hip seat and plumb cuts for this very reason. Unlike the rafters, I have never been able to apply an accurate enough mathematical way of working it out on paper.

 

[kaseryn]

Looked into it a bit more after the fact. Thing is it just was a more complicated calculation/joint than i expected. With a 45 deg pitch everything else is 45 and lines up at that hence coping joints works. It's not the same as the hip pitch although in this case it happened to be very close it separates as the pitch increases. At the time just by faffing about with my speed square i got a measurement but next time i'd prob just run the calculation. Blocklayer.com has calcs for everything and the pyramid calculator gives you the bevel setting needed for a given pitch at the corners which is the cut i'm talking about.