Roof truss calculator
Enter the building size, the roof pitch and the truss spacing. You get the number of trusses, a drawing of the truss with its rise, heel height and overall height, the length of every chord and web, the lumber in each truss and the cost.
Inputs
Result
| Member | Each | Qty |
|---|
Trusses needed by building length
Including the two gable-end trusses.
| Building length | 16 in on centre | 24 in on centre |
|---|
How it is worked out
A house 40 ft long and 28 ft wide with a 6/12 roof and trusses at 24 in on centre takes 480 / 24 = 20 spaces, so 21 trusses: 19 common trusses and 2 gable ends. Each truss rises 7 ft from the plate line to the peak; with a standard 2x4 heel of about 3-7/8 in it stands about 7 ft 4 in. The top chords are (14 + 1) x 1.118 = 16 ft 9 in each, including the 12 in tail.
Truss types
- Fink (W): the most common residential truss. Four webs in a W from the bottom chord third points, for typical house spans.
- King post: a single vertical at the centre, for short spans such as sheds, porches and garages.
- Howe: verticals with diagonals sloping to the centre; used for longer spans and heavier loads.
- Fan: webs fanning out from the bottom chord third points to the top chord, where the top chord needs support at more points.
- Scissor: sloped bottom chords for a vaulted ceiling. The ceiling pitch is commonly held to no more than half the roof pitch.
- Mono: a single slope, for lean-tos, additions and shed roofs.
Lengths here are centreline estimates for planning, ordering and checking quotes. Trusses are engineered and built by the truss maker: the IRC (R802.10) requires truss design drawings for the actual webs, plates, bracing and loads, and the IRC also requires each truss to be tied down against uplift at the walls (R802.11). The calculator counts two connectors per truss for that.
For the sheathing on top, use the roof sheathing calculator; for site-built rafters instead of trusses, the rafter span chart.
Questions
How many trusses do I need?
Divide the building length in inches by the truss spacing, round up and add one. A 40 ft building at 24 in on centre takes 21 trusses, two of them gable ends; at 16 in on centre, 31.
How far apart are roof trusses?
Usually 24 in on centre, which is what most truss designs and 7/16 in roof sheathing are made for. 16 in on centre is used under heavy roofing or high snow loads, or where the ceiling or sheathing needs it.
How tall is a roof truss?
The rise is half the span times the pitch over 12, plus the heel height. A 28 ft span at 6/12 rises 7 ft; with a standard heel of about 4 in, the truss is about 7 ft 4 in tall.
How long is the top chord of a truss?
Half the span plus the overhang, times the pitch factor. For a 28 ft span at 6/12 with a 12 in overhang: 15 ft x 1.118 = 16 ft 9 in on each side.
What is the most common roof truss?
The Fink, or W, truss: two webs on each side forming a W, used on most house spans.
What is a raised heel truss?
A truss with a taller heel at the wall, often 10 to 16 in, so ceiling insulation can run full depth to the outside of the wall instead of thinning out under the roof.
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