I have been asked this question more than a few times, by potential do-it-yourselfers, who are considering constructing their own buildings. Jeff, one of the Building Designers at Hansen Pole Buildings, got asked this very same question again yesterday.
I am certain most people ask this question, as they are considering how to go about lifting the trusses into place on their new buildings – by hand. My advice is – please use equipment. Lifting by hand is just an accident looking for a place to happen. Neither a broken truss, nor an injured person is worth the risk.
So, back to the question at hand….
Several factors are going to influence truss weight – how far does the truss span (the width of the building), what loads are carried by the truss, and what part of the country is the building in? To a lesser extent the slope of the truss will influence the weight as well.
The slope? Yes! In most cases a 4/12 roof slope is going to be the most efficient combination of load carrying ability and economics. Slopes flatter than 4/12, especially with wider spans or heavier roof loads, are going to need to be “beefed up” in order to carry the same loads as a steeper slope. As slopes get progressively steeper, the length of the top chord increases, as does the length of the internal members (the “webs”).
Where does span fit into the mix? In engineering calculations, the span of a beam (a truss is a beam) is squared. This means a 30 foot span is 50% wider than a 20 foot span, but must have 225% of the load carrying capacity! The load carrying capacity is handled by larger or higher graded material being used for the truss top and bottom chords, as well as the webs. Roof truss plates, the pressed in steel connectors, also get larger.
What loads are being piled on? Besides any snow loads, the trusses also must be able to carry dead, or permanent loads. Besides the weight of the truss itself, these would include roofing materials (steel roofing over purlins being relatively light, shingles over oriented strand board – osb, being eight to even 10 times as heavy).
Why would the part of the country make a difference? In most of the west, trusses are built from Western Hemlock (Hem-Fir), Spruce-Pine-Fir (SPF) or Douglas Fir (DFir or DFL). In the east and south, most often the lumber is Southern Yellow Pine (SYP). The weight of lumber is based upon specific gravity (G). For Southern Pine it is 0.55, Douglas Fir 0.50, Hem-Fir 0.43, SPF 0.42. This makes Southern Pine over 30% heavier than SPF!
With all of this said, the best I can provide is nothing short of PFA (Plucked From the Air).
My truss weight table per single truss:
24’ truss, 120-150#,
Regardless of truss span or truss weight, use common sense when lifting the trusses into place. Be careful and your roof system will perform admirably for generations.