Roof Truss Calculator
A 24 ft wall at 24 in spacing needs 13 trusses. At 6/12, with 12 in of horizontal overhang each side, one top chord is 14.5344 ft and both sides are 29.0689 ft.
Last updated . Spacing starts at 24 in on a 24 ft wall. Pitch starts at 6, and each overhang starts at 12 in.
A roof truss calculator for how many trusses stand along a wall, and for the sloping length of the top chord. The web list is a description. It is not a design.
One common rafter, with the ridge taken off the run, is the rafter calculator. Roof area in squares is the roofing calculator. Studs in the wall below are the stud calculator.
Truss count and top chord
Count puts a truss at each end of the wall. The top chord follows the pitch out through the overhang. The bottom chord is the span alone. The manufacturer engineers the truss.
Results
How the truss count is calculated
Wall length and spacing are converted to inches. Spaces = floor(length ÷ spacing), with a tiny addition inside the floor so an exact multiple does not fall off from binary noise. Trusses = spaces + 1. The extra piece is the truss at the beginning of the wall; the far end is the last space. A 24 ft wall is 288 in. At 24 in, 288 ÷ 24 = 12 exactly, so the count is 13. At 16 in the quotient is 18 and the count is 19. At 10 in the quotient is 28.8, the floor is 28, and the count is 29.
The bottom chord is the span you type. It does not grow by the overhang. Half of a 24 ft span is 12 ft. A 12 in overhang adds 1 ft, so the horizontal run of one top chord is 13 ft. Rise over that run is the pitch times 13. At 6/12 that rise is 6.5 ft. One top chord is the square root of 13² + 6.5², which is the square root of 211.25, or 14.534441… ft, shown as 14.5344. Both sides double the unrounded root and then round, which is 29.0689 ft. Rise over the span, ignoring the tail, stays 6.0000 ft at 6/12.
Setting the overhang to zero makes the run 12 ft and the chord the square root of 180, shown as 13.4164 ft, and the two rise lines both read 6.0000 ft. A 4/12 pitch on the 13 ft run rises 4.3333 ft, and one chord shows 13.7032 ft. No ridge board is removed from the span. These lengths are the geometry of the profile you typed.
The web menu is a picture in words
King post names one upright under the peak. Queen post names a pair of uprights with a horizontal piece between them. Fink names a W. The sentence on the panel ends with “Description only” so it cannot be read as a size or a spacing of the webs. Plate size, lumber grade, camber, and the loads the truss can carry are the manufacturer’s engineering. This page does not claim a layout is adequate.
Counts and chords for the 24 ft span
| Case | Trusses | One side | Both sides | Rise over the span |
|---|---|---|---|---|
| 24 in spacing, 6/12, 12 in overhang | 13 | 14.5344 ft | 29.0689 ft | 6.0000 ft |
| 16 in spacing, same profile | 19 | 14.5344 ft | 29.0689 ft | 6.0000 ft |
| 24 in spacing, 6/12, no overhang | 13 | 13.4164 ft | 26.8328 ft | 6.0000 ft |
| 24 in spacing, 4/12, 12 in overhang | 13 | 13.7032 ft | 27.4064 ft | 4.0000 ft |
| 10 in spacing, opening profile | 29 | 14.5344 ft | 29.0689 ft | 6.0000 ft |
Truss count questions
How many trusses fit a 24 ft wall at 24 in spacing, and how long is the top chord?
Leave wall length at 24 ft, spacing at 24 in, span at 24 ft, pitch at 6, and overhang at 12 in. Leave the web pattern on King post. Trusses reads 13. Spaces reads 12. Top chord, one side reads 14.5344 ft. Top chord, both sides reads 29.0689 ft. Run with one overhang reads 13.0000 ft. Rise over the span reads 6.0000 ft. Rise over that run reads 6.5000 ft. Bottom chord reads 24.0000 ft. Web pattern reads One vertical web from the peak to the bottom chord. Description only.
What changes when the spacing is 16 in?
Set spacing to 16 in and leave the other opening values. Trusses reads 19. Spaces reads 18. Top chord, one side still reads 14.5344 ft. Top chord, both sides still reads 29.0689 ft. Run with one overhang still reads 13.0000 ft. Rise over the span still reads 6.0000 ft. Rise over that run still reads 6.5000 ft. Bottom chord still reads 24.0000 ft. Spacing changes the count only.
Why is a 24 ft wall at 10 in spacing 29 trusses and not 30?
Set spacing to 10 in. Trusses reads 29. Spaces reads 28. The wall is 288 in and 288 ÷ 10 = 28.8. Floor of 28.8 is 28 spaces, then one truss is added for the start of the wall. Rounding 28.8 up before adding the end piece would invent a 30th truss. An exact fit, such as 24 in on this 24 ft wall, is 12 spaces and 13 trusses, because floor of a whole number stays on that number.
What if the overhang is zero?
Set overhang to 0 and set spacing back to 24 in. Leave span at 24 ft and pitch at 6. Trusses reads 13. Top chord, one side reads 13.4164 ft. Top chord, both sides reads 26.8328 ft. Run with one overhang reads 12.0000 ft. Rise over the span reads 6.0000 ft. Rise over that run reads 6.0000 ft. Bottom chord reads 24.0000 ft. The two rise lines match because the tail is gone.
What does 4/12 do to the chord?
Set pitch to 4 and put the overhang back to 12 in. Spacing stays 24 in. Trusses stays 13. Top chord, one side reads 13.7032 ft. Top chord, both sides reads 27.4064 ft. Run with one overhang reads 13.0000 ft. Rise over the span reads 4.0000 ft. Rise over that run reads 4.3333 ft. Bottom chord reads 24.0000 ft.
What do the queen-post and Fink lines say?
Set the web pattern to Queen post. Web pattern reads Two vertical webs set in from the peak, plus a horizontal web between them. Description only. Set it to Fink W. Web pattern reads W-shaped webs: four members meeting the bottom chord in two places. Description only. The count and the chord lengths stay where they were. Neither sentence is a member size.
Does this page tell me the truss is strong enough?
No. Trusses are engineered by the manufacturer. The count is floor of wall length divided by spacing, plus one. The chord is geometry of the pitch and the overhang. The web menu only names a pattern. There is no lumber size, no plate, no snow load, and no pass-fail result. A single common rafter, with a ridge thickness taken off the run, is a different page.