SumTakeoff

Roof truss calculator

How many trusses your roof takes, and how tall and long each one is, for common, scissor and mono trusses, drawn to scale.

Start from:

You need

21 trusses

Roof rise
6 ft 0 in
Peak height
6 ft 0 in
Top chord (each side)
14 ft 6 3/8 in
Bottom chord
24 ft 0 in
Width with overhangs
26 ft 0 in
Roof angle
26.6°

For planning and quotes. The truss manufacturer's engineer designs the webs, lumber and connections for your loads and code; never cut or change a truss.

Span 24 ft 0 in Peak 6 ft 0 in
The truss outline to scale. Webs are left out; the truss designer lays them out.

How to figure roof trusses

Count the trusses along the wall they sit on, then work out each truss's shape from the span and pitch.

  1. Trusses = building length in inches ÷ spacing, rounded up, + 1 (one at each end)
  2. Run = half the span (all of it for a mono truss)
  3. Rise = run × pitch ÷ 12; add the heel height for the peak height
  4. Top chord = (run + overhang) × √(1 + (pitch ÷ 12)²)
  5. Scissor ceiling: the bottom chords slope too, rising run × ceiling pitch ÷ 12 to the middle

Worked example

A 24 × 40 ft house with a 6/12 roof and trusses 24 in on center: 480 in ÷ 24 = 20 spaces, so 21 trusses. The run is 12 ft 0 in, so the roof rises 12 ft 0 in × 6 ÷ 12 = 6 ft 0 in, and each top chord with a 12 in overhang is 14 ft 6 3/8 in long.

Roof truss height chart

How high a common truss rises above the walls, by span and pitch (before the heel).

Span 4/12 5/12 6/12 8/12 10/12 12/12
20 ft 3 ft 4 in 4 ft 2 in 5 ft 0 in 6 ft 8 in 8 ft 4 in 10 ft 0 in
22 ft 3 ft 8 in 4 ft 7 in 5 ft 6 in 7 ft 4 in 9 ft 2 in 11 ft 0 in
24 ft 4 ft 0 in 5 ft 0 in 6 ft 0 in 8 ft 0 in 10 ft 0 in 12 ft 0 in
26 ft 4 ft 4 in 5 ft 5 in 6 ft 6 in 8 ft 8 in 10 ft 10 in 13 ft 0 in
28 ft 4 ft 8 in 5 ft 10 in 7 ft 0 in 9 ft 4 in 11 ft 8 in 14 ft 0 in
30 ft 5 ft 0 in 6 ft 3 in 7 ft 6 in 10 ft 0 in 12 ft 6 in 15 ft 0 in
32 ft 5 ft 4 in 6 ft 8 in 8 ft 0 in 10 ft 8 in 13 ft 4 in 16 ft 0 in
36 ft 6 ft 0 in 7 ft 6 in 9 ft 0 in 12 ft 0 in 15 ft 0 in 18 ft 0 in
40 ft 6 ft 8 in 8 ft 4 in 10 ft 0 in 13 ft 4 in 16 ft 8 in 20 ft 0 in

How many trusses by building length

Building length 12 in o.c. 16 in o.c. 19.2 in o.c. 24 in o.c.
20 ft 21 16 14 11
24 ft 25 19 16 13
30 ft 31 24 20 16
36 ft 37 28 24 19
40 ft 41 31 26 21
48 ft 49 37 31 25
50 ft 51 39 33 26
60 ft 61 46 39 31

Roof truss questions

How many trusses do I need?

Divide the building length in inches by the spacing, round up, and add one for the far end. A 40 ft building with trusses 24 in on center needs 21; at 16 in it needs 31. The two end trusses are often gable-end trusses, built to carry the siding; ask your supplier.

What is the spacing for roof trusses?

Whatever the truss design drawing says; the designer picks it with the loads. The spacings above (12, 16, 19.2, 24 in) all put a truss under every edge of an 8 ft sheet of roof sheathing: 96 ÷ 24 = 4, 96 ÷ 16 = 6 and 96 ÷ 19.2 = 5 spaces.

What is a scissor truss?

A truss whose bottom chords slope up toward the middle like the blades of scissors, so the room below gets a vaulted ceiling. Under a 26 ft span with a 6/12 roof and a 3/12 ceiling, the ceiling rises 3 ft 3 in from the walls to the middle while the roof rises 6 ft 6 in. The ceiling pitch has to be lower than the roof pitch; choose "Scissor" above to try yours.

What is a raised heel or attic truss?

A raised (energy) heel makes the truss deeper where it sits on the wall, so full-depth insulation fits over the wall; enter its height as the heel height and the peak rises by the same amount. An attic truss leaves an open rectangle in the middle for a room or storage; its size depends on the span and pitch and is set by the truss designer.

Should I use trusses or rafters?

Trusses are built and engineered in a factory and go up fast, but they fill the attic with webs (except attic trusses). Rafters are cut on site and leave the attic open, with a ridge board or beam and ceiling joists or ties. For rafters, use the rafter calculator; for the pitch from a rise and run, the roof pitch calculator.

How much do roof trusses cost?

Trusses are priced per job by the span, pitch, type and loads, so get a quote from a truss supplier and enter the price per truss above for the total. For the roofing that goes on top, use the roofing calculator.