Temporary bracing for small trusses
I’m installing standard trusses with a 20′ span, 2′ overhangs, and a 4:12 pitch for a simple gable roof. The truss drawings indicate that the only permanent bracing needed is sheathing on top and a drywall ceiling directly attached on the bottom, both of which I’ll install. The generic installation instructions, BCSI-B1, a single-sheet document delivered with the trusses, however, call for five different types of temporary bracing: lateral and diagonal on the top chords and bottom chords plus diagonal bracing across the verticals in the web, which is the one I understand completely, as it it prevents racking that could otherwise easily occur right away. If I were going to climb around on the trusses before they’re sheathed, I could see why more bracing would be needed, but I’m planning to install the first row of sheathing, in full 4×8 sheets, on each side from a ladder and then each subsequent row from atop the sheathing already installed. The much longer BCSI booklet on truss installation frequently says to refer to the truss drawings. I would not take any chances with safety, but if some of temporary bracing in the installation instructions won’t be of any benefit for trusses as small as mine that I won’t be climbing upon and that don’t require any additional lumber for permanent bracing, I’d rather save whatever time, wood, and nails I safely can. In real-world practice, for small trusses like mine, which of these five types of temporary bracing are actually installed?
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Replies
Jeff,
I won't advice you to ignore the truss drawing, but here's how I stand trusses as small as those you describe.
Assuming the exterior walls are braced:
- Lay the gable truss flat and support it with several props, and sheath it.
- Nail a 2"x4" to the wall sheathing at the centre of the span extending up a few feet, stand the gable truss and fasten it to the 2"x4" and the wall below.
- Stand three more trusses, keeping them upright with bracing at the peak.
- Brace the bottom chord at the middle of the span.
- Run a diagonal brace from the peak of the gable truss to the bottom chord brace.
At this point the trusses should be rock solid.
- From then on only brace the trusses at the peak.
Many thanks, Malcolm!
I am glad to hear that you take your safety seriously during construction.
If you are in an area subject to high winds or earthquakes, I would consider additional bracing as cheap insurance. The small cost of lumber and nails for extra bracing is nothing compared to the more costly damage to finish materials after the house is complete. Also a fire is more damaging to trusses than solid lumber roof framing, so again some extra bracing would help, though unlikely needed. (Ask a fireman about the additional danger of fighting a building fire when there's a truss roof.) Finally, if your sheathing does rot decades later, there goes some of your lateral and diagonal support of the trusses. I'd guess that diagonal support from end truss peak to the bottom cord of a truss 4' interior might be the most useful to add to sheathing support, but I'm no structural engineer.
Leaving out bracing recommended by the manufacturer will probably void any warranty by the truss manufacturer which could cost far more than the cost of a few 2x4's. Avoid the temptation to design your own truss supports and bracing.
Robert and Bob,
Neither Jeff or I are suggesting leaving out the required bracing called for on the truss drawings. His question was: how much of the generic temporary bracing that is shown on all truss installation drawings is necessary for a 20 ft 4/12 gable roof? This is bracing that gets removed once the trusses are sheathed.