Roof Truss Calculator
Estimate how many trusses a gable roof needs, and the size of each truss. Enter the building length and the truss spacing for the number of trusses, including the two gable end trusses. Enter the span and the pitch for the rise, the truss height, and the top and bottom chord lengths.
- Choose US or Metric. You can still change the unit of any single field.
- For the number of trusses, enter the Building length, measured along the ridge from the outside of one end wall to the outside of the other, and choose the Truss spacing. Choose Custom spacing for a spacing that isn't listed, such as 19.2 in.
- For the size of the trusses, enter the Span, from the outside of one wall to the outside of the other, and the Pitch, as x/12 or in degrees.
- Optionally, enter the Overhang, measured level from the outside of the wall, and the Heel height from your truss drawings to see the full truss height.
- Optionally, enter the Price per truss to estimate the cost.
You only need the fields for the result you want: the building length for
the count, or the span and pitch for the size. Length fields accept whole
numbers, decimals, fractions, and mixed numbers, such as 40,
10.5, 3/4, or 4 1/2. Choose
ft + in as the unit to enter a length such as 21 ft 4 in
in two fields.
How many trusses you need
Trusses sit across the building, one at each end and the rest in between, no farther apart than the truss spacing, center to center:
Trusses = building length ÷ truss spacing, rounded up, + 1
The count includes the two gable end trusses, one over each end wall. They have the same outline as the other trusses, but are framed with vertical studs instead of angled webs, to carry the sheathing and siding. The calculator shows them separately, so you can order them as their own type. The rest are common trusses.
Pryda's installation guidelines for roof trusses lay the trusses out without going over the design spacing, and allow the last gap to be smaller than the spacing. Spacing them evenly instead takes the same number of trusses. For a 41 ft building at 24 in, that is 22 trusses, about 23 7/16 in apart.
Truss spacing
In the US and Canada, trusses are most often spaced 24 in (600 mm) on center, as Fine Homebuilding's Rolling Roof Trusses and CMHC's Canadian Wood-Frame House Construction describe. As an example from the US code, IRC Table R802.11 lists rafter and truss spacing of 12, 16, and 24 in. RMIT University's building guide gives 600, 900, or 1,200 mm in Australia, depending on the weight of the roofing.
The spacing is part of the truss design. Use the spacing on your truss design drawings or the manufacturer's layout. The calculator doesn't check whether a spacing suits your trusses.
How to calculate truss dimensions
A common truss is a triangle as wide as the building. King post, Howe, Fink, and fan trusses differ only in how the webs inside are arranged, so their outline, and the numbers below, are the same. From the span and the pitch:
Rise = span ÷ 2 × pitch ÷ 12
Top chord length = (span ÷ 2 + overhang) × √(1 + (pitch ÷ 12)²)
Bottom chord length = span
Truss height = heel height + rise
For a pitch in degrees, use the tangent of the angle in place of pitch ÷ 12. The square root is the pitch factor, the same one the Roof Pitch Calculator uses for rafters. The top chords meet at the peak, so there is no ridge board to take off.
- Span. The span runs from the outside of one wall to the outside of the other. The top and bottom chords meet at the outside of the wall, so the bottom chord is as long as the span.
- Rise. The rise is measured from the top of the top chord at the outside of the wall up to the peak. It is not the height of the truss.
- Heel height. The heel height is the height of the truss at the outside of the wall, from the bottom of the truss to the top of the top chord, as MiTek's guide to reading truss drawings defines it. It depends on the lumber and on how much insulation fits over the wall, so take it from your truss drawings. Trusses with a taller heel for insulation are called raised heel trusses: CMHC notes that the energy requirements in most building codes lead to a height of 300 mm (12 in.) or more at the outside of the wall.
- Top chord length. This is the length along the top edge of one top chord, from the peak to the end of the overhang, or to the outside of the wall with no overhang. It is not the length the lumber is cut to, which depends on the cuts at the peak and the heel and on the size of the lumber.
Truss span
There is no span table for trusses. The manufacturer designs each truss for the span, pitch, spacing, and loads of your building, and Fine Homebuilding notes that trusses can be made for spans of more than 50 ft. Enter your span to see the outline of the truss, and leave the design to the manufacturer.
Examples
A 40 × 24 ft house
At 24 in on center, 480 in ÷ 24 in = 20 spaces, so the roof needs 21 trusses: 19 common trusses and 2 gable end trusses. At 16 in on center, it needs 31.
At 6/12, half the 24 ft span is 12 ft, so the rise is 12 ft × 6 ÷ 12 = 6 ft. The top chord is 144 in × √1.25 = 160.997 in, or 13 ft 5 in, and 14 ft 6 7/16 in with a 12 in overhang. With a 12 in heel height, the truss is 7 ft tall. At $140 per truss, 21 trusses cost $2,940.
A Metric roof
A building 12 m long needs 21 trusses at 600 mm on center, 15 at 900 mm, or 11 at 1,200 mm. For a span of 8 m at 22.5°, the rise is 1,656.9 mm and the top chord is 4,329.6 mm, or 4,979.0 mm with a 600 mm overhang. With a 300 mm heel, the truss is 1,956.9 mm tall.
FAQ
21 at 24 in on center: 480 ÷ 24 = 20 spaces, plus 1 for the truss at the far end. That includes the two gable end trusses. At 16 in on center, you need 31.
Usually 24 in (600 mm) on center. Some roofs use 16 in, and in Australia 900 or 1,200 mm is also used, depending on the roofing. Use the spacing on your truss drawings.
There is no fixed limit. The manufacturer designs trusses for the span of your building, and spans of more than 50 ft are possible. Enter the span to see the rise and chord lengths.
The heel height plus the rise. A 24 ft span at 6/12 rises 6 ft, so with a 12 in heel, the truss is 7 ft tall.
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