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Roof Structure


I really hadn't planned to build my own roof trusses.  After all, there are plenty of truss-building companies around.  But I soon learned that they only build wood trusses, and that just wouldn't do.

In my quest for a supplier, I met and became friends with Gary, the owner of a wood truss yard in nearby Oceanside.  Gary granted me the use of his facility to build my trusses.  I packed up my cutoff saw and table and spent most of the summer of 1997 there.

One advantage steel has over wood is availability in very long pieces.  This and some careful planning will minimize scrap.  The material from which I built the trusses came in 40-ft lengths.  One piece weighs only about 65 pounds, but it's very hard to turn while holding it.
I would need a precise cut list to minimize scrap, but first I had to design the trusses.  I turned to DesignCAD, which I use for almost everything, and prepared a dimensioned drawing for each of the 128 trusses.  These drawings were then given to Gary's staff who ran the engineering and produced signed drawings for the county building department to approve.
I wrote a simple program to extract the precise length of each element from the DesignCAD file and place them in an Excel worksheet.  The worksheet sorted by length and I had my cut list.  The cutting procedure was to cut the longest first and always try to find a near-size remnant to cut from.  After cutting was finished I had a large pile of very short scraps and a few remaining full-length pieces.

The most difficult part was determining the length of the web elements.  I was never able to come up with a mathematical procedure for this, so each web was precisely sized by an iteration process.

The joints between webs and chords are fastened with blue double-shear self-drilling screws like shown in the inset.  When additional metal layers are involved, such as shown here for chord joints, the longer red double-shear screws are used.  But they often break from the added torque.  So I pre-drilled the holes for all the red screws.

I build the garage trusses first and decided to try setting them before building with the rest.  I was not yet accustomed to working at height, so I built a replica of the garage top plate two feet off the ground and assembled the roof structure on it.  When finished I hired a crane to lift the assembly into place.
The steel top plate constitutes a serious thermal bridge between inside and outside - not satisfactory for this energy-efficient house.  The solution was a change in the truss profile.

Trusses for wood houses end at the outside edge of the plate.  Drooping rafter tails continue the roof structure beyond the wall to form the overhang.  The rafter tail usually passes only 3-1/2 inches above the plate and the gap between roof sheathing and plate is filled with pieces of 2X4 called Frieze blocks.

I changed the truss profile to be a structural cantilever beyond the plate.  This resulted in an 18" (instead of 3-1/2") heel-stand above the plate in which full-depth insulation could be wrapped around the plate into the soffit.  This requires special bracing to properly transfer lateral forces to the walls, as shown in this photo.


By the time the garage roof structure was fastened in place I was less apprehensive about working at height.  I decided to set the trusses individually for the rest of the house and save the crane fee.  I made a lifting fixture for my beam hoist to raise the trusses, diagonally to the walls so they'd fit between them, then I'd rotate the hoist and lower the truss into position.  What was needed next was a trip up the rolling scaffold to secure the truss against the braced line of trusses already in place.

Eventually I approached the point of insufficient clearance to raise the rotated truss.  So I stacked the rest and removed the hoist.  These had to be manually finessed into position moving each end a few inches at a time.

By Christmas, 1997, the roof structure was in place.  There were still a lot of fasteners and ridge caps yet to install and, worst of all, the sheathing.
I suppose installing the sheathing was the worst chore of the whole project.  I'm astonished that I survived it.  To make it more challenging I laminated the underside of each heavy sheet with aluminum radiant barrier material to keep the attic temperatures lower.

One little feature that has been pleasing is this triangular gable vent.  I had it custom made, and it looks much nicer than the ugly rectangular ones on other houses around here.