Guide to building a timber frame home

Scribing a King Post

Overhead view of the king post truss laid out in the framing yard. The two principal rafters actually support the king post from the top. The principal rafter that Kevin is sitting on is turned upside-down, as he works on the housed mortise for the purlin. The purlins, which are supported by the principal rafters, in turn support the common rafters. The king post (in tension) holds up the middle of the summer beam, those 1 ton 12" by 12"'s that run that are supported by those big jowled posts, and in turn support all the floor joists. The king post itself is an 8" by 8", and the principal rafters are 12" by 8"'s. The arch braces are technically known as "struts" when used in a truss system. in the end, we cut these struts out of 17"x12" stock, which allowed for 5" of arc. We gave up on laminating the struts and arch braces because it was too time consuming and s a little problematic from the building code standpoint, not being factory approved The struts are scribed to the king post and principal rafters for a perfect fit.

Kevin is looking for flaws in the mating surface between the post and principal rafter. It's a long tenon and mortise, but it's even more critical to get the mating surface of the joint correct, since it's the meat on the top couple feet of the king post that actually keeps the whole thing up in the air. Since all these white oak timbers weigh over a thousand pounds, it's a bit of a hassle finessing them around to line things up. Lots of brute force and triangle rule stuff. The point at the top is the roof line, which is a 14 pitch. Then it comes time to stand the house, the truss will be assembled on the ground and crained into place.

Here's a close up of the open tenon and mortise. The photo didn't come out too good (my fault) so it's hard to see the shoulder at the bottom of the mating surface. The point of the principal rafter is actually about an inch further out from the king post then the bottom, as the king post is tapered inwards over the run of the mating surface, known as a diminished bearing shoulder joint. The whole thing should be pretty solid for a few hundred years unless a 6" by 8" section of white oak decides to let go. Even then, I wouldn't be surprised if there's not that much tension on the king post, since that 12" by 12" summer beam is pretty solid in its own right.

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