Friday, September 12, 2008

Trusses Square & Plumb

To determine where to place the first long roof truss, I measured 24" from the front outside wall and made a mark on the sill plate for the base of the truss. I did the same on the back wall and on the peak of the girder. For each subsequent truss, I repeated the process, measuring over from the previous truss. Now, after putting up four trusses on each side, I want to make sure that everything is square and plumb. Theoretically, if the outside walls were perfectly square, everything should line up correctly, right? However, as hard as I tried, I know the outside walls are not perfectly square, especially at the top of the walls. The weight and pressure of the concrete moved things around slightly.

The last trusses I put up will form the edge of the dormer window, so I'd like to have them as straight as possible from front to back. To do this, I strung a line from where the truss sits on the sill plate at the front of the house, to the same spot at the back of the house. Then I dropped a plumb-line down from the peak of the girder where the top of the trusses sit. If the trusses were perfectly in-line with each other, the strings would just touch each other where they intersect. You can't tell from this photo, but the strings are about 3/8" apart, so it's pretty good overall. Before I put the final nails in the truss brackets at the peak, I'll nudge them over a bit and they'll be straight.

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