I am looking at building a small 8x8 shed and wanted to use Gambrel Roof design. Anyone have or know how I go about calculating the angles of the roof truss cuts. Never been good with angles other than 45 and 90.
If you are building a regular gambrel, then the 4 sections are equal length and the angles between each section are 45, so each end is cut at 22.5 and butted to the next. Depenging on span, you might need a ridge board at the side joints as well as the top. The 22.5 is usually a set stop on most mitre saws.
The angle at the top plate is 22.5.
A regular gambrel is 1/2 as high as it is wide, so one way to start is to measure the total width and set a stud in the center 1/2 as high.
There are software packages that compute and draw the complete rafter set. I think it was called easyrafter and they had a trial version the last time I looked. Well over a year ago and I didn't save the URL.
Attached is a link to a picture on my web site showing interior details of a large gambrel roof.
You're starting with vertical walls on each side, so essentially what you have to do is make the total of all the roof angles equal 180 degrees. Think of it as having to go up one side, turn 180 degrees with whatever number of roof angles you have, and go back down the other side. A conventional gable roof with a 12/12 pitch will turn 45, 90 and 45.
The easiest angles for a gambrel will be 30 and 30 on each side with 60 at the peak. I found this picture of a shed kit which shows one way to frame it:
I wouldn't try this on anything bigger than a shed - a more substantial building would need more support at the hip.
While half as high as wide is one way, the designer at barnsplus.com has made nearly a lifetime study of gambrel roofs, and he favors a slightly squatter design. Here's a shot of his truss design:
He gets the slightly lower design by making the top chords a little longer than the side chords. I'm building the 32x40 (which is the size of the one pictured), and the top chords are ~13', while the side chords are only a little more than 10' to the break (they are ~12' to the bottom of the overhanging tail).