Saturday, July 6, 2013
In God we truss
I've never built pitched roof rafter trusses before, only gambrel trusses, which are easier, I think.
But the basic engineering idea is easy enough to grasp -- make triangles that reduce the carrying beam portion of each truss to the allowable span. According to the American Wood Council, two by four hemlock has an allowable span of six feet without snow load, four feet with, so if we have a fifteen-foot total span, we would need to divide each truss into four sections.
I decided three sections, with a large center passage divided by a kingpost, which although not technically a triangle, will hold up the center of the bottom chord, making a flat ceiling joist. This also allows for two small crawl-able crawl spaces, where I'll need to run wires later.
Here's the detail of the rafter heel, with a solid four-foot triangle, as well as a smaller one right next door to make a vertical kneewall. These are joined by small squares and rectangles of plywood, all nail-gunned together with serrated 8-penny nail gun nails.
Using a custom-rigger 20-foot workbench and common measurements (of fifteen feet ten and a half inches wide and three feet eight inches high), it takes about forty minutes to make each one, but hanging them is hard to do by myself, because of the heat and humidity. The high today was nearly 90F and the dewpoint 70 F.
Accordingly, I was a little shaky and had to rest after lifting the first three into position. After eating a couple of sweet oranges and lying down for an hour, I went out and made another one, and was then able to put on the first little bit of roof sheathing.
We're using 7/16th OSB 24 inches on center with H-clips, which is not strong sheathing, but there will also be Grace Ice and Water Shield, then 1 x 4 purlins 16 inches on center, and a metal roof on top of all that.
There was a slight breeze on top of the roof, as well as the beginnings of shade inside the building, finally.
We are grateful for all such small mercies.
If we also get the forecast break in the humidity Monday, that will be another such blessing.