Roof Truss Calculator
Reviewed by the My Roof Pitch editorial team · Updated · Free, no sign-up, works offline after first load
Truss count is the building length divided by the spacing, plus one. A 40 ft building at 24 in centres needs 21 trusses. Peak height is half the span times the pitch ratio, and each top chord equals half the span times the pitch multiplier.
Your measurements
Enter the values you know — results update live.
The dimension running parallel to the ridge.
Outside-to-outside width the truss bridges.
Standard fink trusses are typically 4:12 to 8:12.
Centre-to-centre spacing. 24 in is standard for residential trusses.
Analysis
Live results from your measurements.
Show the working
- Spacing in feet:
24 ÷ 12= 2 ft - Truss count:
floor(40 ÷ 2) + 1= 21 - Peak height:
14 × (6 ÷ 12)= 7 ft - Top chord:
(14 + 1.5) × 1.118= 17.33 ft - Total top chord stock:
17.33 × 2 × 21= 728 ft
What this calculator does
A roof truss is a pre-engineered triangular frame that replaces site-cut rafters and ceiling joists in one piece. Because the geometry is fixed at the plant, the numbers you need before ordering are simple: the span, the pitch, the spacing and the building length. Those give you the count, the peak height that sets your gable end framing, and the chord lengths that determine delivery and crane requirements. Trusses are almost always cheaper and faster than stick framing over about a 20 ft span, and they clear-span the building so no interior bearing wall is needed.
How to use it
- Measure the building length along the ridge direction.
- Divide that length by the truss spacing in feet.
- Round down and add one to get the truss count, including both gable ends.
- Multiply half the span by the pitch ratio to find the peak height above the plate.
- Multiply half the span plus the overhang by the pitch multiplier for the top chord length.
The formula
Truss count
count = floor(building length / spacing) + 1
The plus one accounts for the truss at the starting end of the run.
Peak height
peak height = (span / 2) × (rise / 12)
Measured from the top of the wall plate to the underside of the peak.
Top chord length
top chord = (span / 2 + overhang) × sqrt(rise^2 + 144) / 12
Includes the tail that forms the eave overhang.
Worked example
A 60 ft long garage, 32 ft span, 5:12 pitch, trusses at 24 in centres, 1 ft tails.
- Spacing: 24 ÷ 12 = 2 ft
- Count: floor(60 ÷ 2) + 1 = 31 trusses
- Peak height: 16 × 0.4167 = 6.67 ft
- Multiplier for 5:12: 1.0833
- Top chord: (16 + 1) × 1.0833 = 18.42 ft each
31 trusses, 6 ft 8 in peak height, and 18 ft 5 in top chords — 1,142 ft of top chord in total.
Reference chart
| Building length | 16 in centres | 19.2 in centres | 24 in centres |
|---|---|---|---|
| 20 ft | 16 | 13 | 11 |
| 24 ft | 19 | 16 | 13 |
| 30 ft | 23 | 19 | 16 |
| 36 ft | 28 | 23 | 19 |
| 40 ft | 31 | 26 | 21 |
| 48 ft | 37 | 31 | 25 |
| 60 ft | 46 | 38 | 31 |
Truss type by span and use
| Truss type | Typical span | Best for |
|---|---|---|
| Fink (W) | Up to 33 ft | Standard residential roofs with no attic use. |
| Howe | Up to 36 ft | Longer clear spans and heavier loads. |
| Attic | 24 to 40 ft | Creating a usable room inside the roof space. |
| Scissor | 20 to 40 ft | Vaulted ceilings without a flat bottom chord. |
| Mono | Up to 24 ft | Lean-to and single-slope extensions. |
Common mistakes
- Forgetting the plus one — you need a truss at both ends of the run.
- Ordering standard trusses for the gable ends. Gable-end frames are a different, vertically studded component.
- Measuring spacing edge to edge rather than centre to centre.
- Ignoring girder trusses where a hip or a load-bearing beam lands. Those are ordered separately.
- Assuming the truss span equals the interior room width. Span is measured outside to outside of the wall plates.
Assumptions and limits
- A simple symmetrical gable roof with even spacing over the full building length.
- The count includes both end trusses but not additional girder trusses.
- Chord lengths are geometric and exclude plate sizes and bearing details.
- Truss design and load capacity must come from the manufacturer's engineer.
Frequently asked questions
How many trusses do I need for a 40 foot building?
At 24 inch centres you need 21 trusses: 40 divided by 2 feet is 20 bays, plus one truss to close the last bay. At 16 inch centres the same building needs 31.
What is standard truss spacing?
24 inches on centre is standard for residential roof trusses in North America. 16 inch spacing is used for heavy snow loads, tile roofs or where the sheathing span demands it.
How tall is a truss for a 28 foot span at 6/12?
The peak sits 7 feet above the wall plate: half the 28 foot span is 14 feet, multiplied by the 6/12 ratio of 0.5. Add the heel height for the total truss depth.
Are trusses cheaper than rafters?
Usually yes above about a 20 foot span. Trusses use smaller members, install in a fraction of the time, and clear-span the building so you avoid an interior bearing wall.
Can I cut or notch a roof truss?
No. Every member is sized for the design load and the plates transfer force at each joint. Any modification voids the engineering and needs a truss engineer's repair detail.
What is the maximum span for a roof truss?
Standard residential fink trusses reach about 33 feet. Purpose-designed parallel chord and Howe trusses span well beyond 60 feet, but delivery and craning become the limiting factor.
Related guides
- Rafter Length and Cuts: The Complete Layout and Calculation GuideRafter math has three layers most guides skip: the difference between line length and true length, the half-ridge deduction, and the geometry of the birdsmouth. Get all three right and the framing square step-off method will put you within a sixteenth of an inch every time.
- Roof Truss Design Guide: Types, Spans, Spacing and BracingTrusses replace a room full of rafter geometry with a single engineered part, but only if you specify span, spacing, heel height and web layout correctly. Here is how the common truss types differ, what drives their cost, and when stick framing still wins.
- Hip Roof Pitch and Area: Hip Rafters, Jacks and the 17-Inch RuleHip roofs replace gable ends with sloped triangular planes, which means every hip rafter, jack rafter and area figure runs through a compound-angle formula rather than the simple gable triangle. Here is that math laid out with worked numbers.
- Roof Height and Ridge Calculation: Rise, Peak Height and Zoning LimitsRidge height is not just a rise calculation — it depends on where you measure from, how the ridge board and heel height shift the number, and whether your jurisdiction measures height to the ridge or to the mean of the roof. Here is the full worked math.
Last updated 2026-08-05. Results are estimates for planning and are not a substitute for a licensed engineer or local code review.