My understanding (based on a conversation with Jack Sobon)is that these ridge beams were not intended to be "structural" like a posted ridge is.

They facilitated the scribing process by allowing you to lay up each roof plane as a unit with wall plate, ridge, and half the commons. This way to layout and cut the roof you only need to do 2 fairly quick and easy layups. A pentagonal ridge makes this a snap as you have one of the sides to level.

Otherwise, if each rafter joined it's opposite rafter at the peak you would have to lay up each set of commons which might require a dozen or two layups and you still haven't gotten the rafter-plate joint. So you would need to still lay up each of the roof planes. Therefore, adding the ridge eliminated quite a bit of mucking about with the rafters and kept it to two simple layups.


I believe this type of ridge fell out of use with the advent of the square rule which is pretty good circumstantial evidence to support this theory.

I always thought they would be a real pain to assemble.