🪵 Roof Truss Calculator
Estimate truss count, rafter length, and ridge height
Smart Roofing Calculator — Roof Truss Calculator — smartroofingcalculator.com/roof-truss-calculator/ — printed
6 = 6:12 pitch
24" typical
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🛠️ How This Calculator Works
Enter building dimensions, pitch, and truss spacing. The calculator determines truss count from length and spacing, calculates rafter length from half-span and pitch using the Pythagorean theorem, and computes ridge height.
📐 Formula
Trusses = ⌈(Length × 12 ÷ Spacing) + 1⌉ | Rafter = √((Width/2)² + (Width/2 × Rise/12)²) + Overhang📝 Example Calculation
For a 30 × 50 ft building at 6:12 pitch, 24" OC: • Trusses: (50 × 12 ÷ 24) + 1 + 2 gable = 28 trusses • Ridge height: (15 × 6/12) = 7.5 ft • Rafter length: √(15² + 7.5²) + 1 = ~17.7 ft
💡 Pro Tips from Roofers
- Order 2-3 extra trusses as spares — damaged trusses during delivery/placement are common.
- Verify truss engineering drawings match your actual building dimensions before ordering.
- Plan crane access before delivery day — trusses are too heavy to hand-carry on most spans.
- Brace trusses every 8 feet during installation to prevent domino collapse.
❓ Frequently Asked Questions
How many trusses do I need?
Divide the building length in inches by the spacing and add one, then add two gable-end trusses. A 40 ft building at 24 in centres needs 21 common trusses plus two gable ends, 23 in total. At 16 in centres it is 31 commons plus two.
What spacing should roof trusses be?
Twenty-four inches on centre is standard in residential construction. Sixteen-inch centres are used under heavy snow loads, tile roofing, or where the sheathing is thinner. Some engineered designs use 19.2 in.
How tall is a roof truss?
Half the span times the pitch over 12, plus the heel. A 30 ft span at 6/12 rises 7.5 ft from the wall plate to the ridge; at 4/12 it rises 5 ft. The truss drawing gives the exact overall height including heel and chord depth.
How much do roof trusses cost?
This site plans on $50 to $200 per standard truss, about $75 to $150 for a 30 ft common truss, plus $500 to $1,500 for delivery and crane placement. Attic, scissor and long-span trusses cost more. A truss plant quote for your exact span and pitch is the figure to budget on.
Are gable end trusses different?
Yes. Gable-end trusses have vertical studs to carry the wall sheathing rather than a load-carrying web pattern, and they are ordered as a separate item from the common trusses.
How long is a rafter or top chord for my roof?
Rafter length = √(run² + rise²) plus the overhang, where the run is half the span. A 30 ft span at 6/12 has a 15 ft run and 7.5 ft rise, so the top chord is about 16.8 ft before the overhang, or about 17.8 ft with a 12 in overhang.
Can I store things in a trussed attic?
Not safely on standard trusses. The bottom chord is designed for the ceiling below plus its own weight, not storage loads, and the webs leave little usable space. For storage or a future room, specify attic or storage-rated trusses at the design stage.
Can I cut or modify a roof truss?
Not without approval. A truss is an engineered assembly, and IRC R802.10.4 does not allow truss members to be cut, notched, drilled, spliced or otherwise altered without the approval of a registered design professional.
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We build free, accurate construction calculators for homeowners and contractors. This article is part of our roofing knowledge base — we also publish guides and tools for other home improvement projects:
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Truss Span & Spacing — Quick Reference
| Truss Type | Typical Span | Standard Spacing | Common Pitch |
|---|---|---|---|
| Common Gable | 20 – 60 ft | 24″ OC | 4/12 – 12/12 |
| Hip | 20 – 50 ft | 24″ OC | 5/12 – 8/12 |
| Scissor (vaulted) | 16 – 40 ft | 24″ OC | 8/12 top / 4/12 bot |
| Gambrel (barn) | 20 – 40 ft | 24″ OC | 18/12 lower / 4/12 upper |
| Mono (shed) | 12 – 30 ft | 24″ OC | 2/12 – 6/12 |
| Heavy Snow / Tile | any | 16″ OC | any |
How many roof trusses do I need?
Truss count = (Building Length in inches ÷ Spacing) + 1. For a 50 ft building at 24″ on-center, that's (600 ÷ 24) + 1 = 26 trusses, plus 2 gable-end trusses for a total of 28. Always order 2–3 spares — damaged trusses during delivery are common.
Truss count by building length
Common trusses = building length in inches ÷ spacing, rounded down, plus one. Gable-end trusses are ordered as two extra pieces. The table is worked from that formula, so it matches the calculator above.
| Building length | Commons at 24 in | Commons at 16 in | Total at 24 in with 2 gable ends |
|---|---|---|---|
| 20 ft | 11 | 16 | 13 |
| 24 ft | 13 | 19 | 15 |
| 30 ft | 16 | 23 | 18 |
| 32 ft | 17 | 25 | 19 |
| 36 ft | 19 | 28 | 21 |
| 40 ft | 21 | 31 | 23 |
| 48 ft | 25 | 37 | 27 |
| 50 ft | 26 | 38 | 28 |
| 60 ft | 31 | 46 | 33 |
Metric: 24 in is 610 mm and 16 in is 406 mm. For a building measured in metres, convert the length to millimetres, divide by the spacing in millimetres and add one in the same way.
Truss height: how tall is a truss for my span and pitch?
The rise of a common truss is half the span times the pitch over 12. A 30 ft span at 6/12 rises 15 × 6 ÷ 12 = 7.5 ft. That is the height from the top of the wall plate to the ridge line at the top chord; the heel height (the truss depth over the wall) and the chord lumber add a few inches, and the truss drawing gives the exact overall height.
| Span | 4/12 | 5/12 | 6/12 | 8/12 | 12/12 |
|---|---|---|---|---|---|
| 20 ft | 3 ft 4 in | 4 ft 2 in | 5 ft | 6 ft 8 in | 10 ft |
| 24 ft | 4 ft | 5 ft | 6 ft | 8 ft | 12 ft |
| 26 ft | 4 ft 4 in | 5 ft 5 in | 6 ft 6 in | 8 ft 8 in | 13 ft |
| 28 ft | 4 ft 8 in | 5 ft 10 in | 7 ft | 9 ft 4 in | 14 ft |
| 30 ft | 5 ft | 6 ft 3 in | 7 ft 6 in | 10 ft | 15 ft |
| 32 ft | 5 ft 4 in | 6 ft 8 in | 8 ft | 10 ft 8 in | 16 ft |
| 36 ft | 6 ft | 7 ft 6 in | 9 ft | 12 ft | 18 ft |
| 40 ft | 6 ft 8 in | 8 ft 4 in | 10 ft | 13 ft 4 in | 20 ft |
Related: Roof height calculator (ridge, eave and mean roof height) · Rafter length calculator
Roof truss cost: how to price a truss package
- Count the common trusses for your length and spacing (table above) and add the two gable ends.
- Ask a truss plant for a quote on that count, span, pitch, heel height and overhang. Trusses are engineered and priced per job, so the plant quote is the number to use.
- As a planning figure, this page uses $50 to $200 per standard truss (about $75 to $150 for a 30 ft common truss); attic, scissor and long-span trusses cost more.
- Add delivery and placement. Plan $500 to $1,500 for a crane on most house-sized packages.
- Add the sheathing, underlayment and roof covering. The plywood/OSB, shingles and metal roof calculators price those from the same footprint.
Related: Plywood / OSB sheathing calculator · Shingles calculator · Metal roof cost calculator
Truss types and when to use them
| Type | What it does | Typical use |
|---|---|---|
| Common (Fink / W web) | Triangular truss with W-shaped webs | Standard gable roofs on houses and garages |
| King post | One vertical post from apex to bottom chord | Short spans: porches, sheds, small garages |
| Queen post | Two vertical posts with a straining beam | Medium spans past a king post |
| Scissor | Sloped bottom chord | Vaulted ceilings |
| Attic | Open rectangle in the middle for a room | Room-in-roof or storage |
| Mono | Single slope, one tall end | Shed roofs, lean-tos, additions |
| Raised tie | Bottom chord set above the wall plate | Taller or partly vaulted ceilings |
| Raised heel (energy heel) | Extra depth over the wall | Full-depth attic insulation at the eaves |
Related: Attic truss · Scissor truss · Mono truss · Raised tie truss · King post truss · Queen post truss
Design, bracing and code: what the calculator does not do
- The calculator gives count, geometry and a budget. It does not size members or connector plates. Trusses are engineered by the manufacturer to ANSI/TPI 1, and the IRC (section R802.10) requires truss design drawings to be supplied to the building official and kept on site.
- Install temporary and permanent bracing exactly as the truss design drawings and the BCSI guide (SBCA/TPI, Building Component Safety Information) show. Most collapses during erection happen before the bracing is in.
- Never cut, notch, drill or splice a truss chord or web unless a registered design professional approves it (IRC R802.10.4).
- Raised-heel trusses let insulation reach full depth over the wall. Under IECC R402.2.1, R-38 meets an R-49 ceiling requirement where the full height of uncompressed R-38 extends over the wall top plate at the eaves.
Source: IRC 2021 R802.10 (wood trusses); ANSI/TPI 1; SBCA/TPI BCSI; IECC 2021 R402.2.1.
Framing the door and window openings below? The header span chart sizes each header and its jack studs from IRC Table R602.7(1).
Cutting rafters instead of ordering trusses? The birdsmouth calculator gives the seat cut, heel cut and height above plate, and checks the one-third depth rule.
Each truss needs an uplift connector at both ends — the hurricane tie calculator counts them and checks the tie against your uplift.
How tall will it stand? The roof height calculator gives ridge height, mean roof height and attic volume.
Two pitches per side? Use the gambrel roof and truss calculator — it works out each slope, the ridge height and the truss count for a barn roof.
Framing a hip roof instead? The hip rafter calculator gives the hip length, every jack rafter length and the side cuts.
Stick-framing with rafters? The collar tie calculator gives the upper-third height, tie length and spacing from IRC R802.4.6.
Not sure trusses are the right choice? Truss vs rafter compares cost, build time and attic space.
Ordering trusses by pitch? Measure the existing roof with the roof pitch calculator and its level-and-tape method before you call the truss plant.
🔗 Related Roofing Calculators
- Gambrel Truss Calculator — Two-pitch barn-style trusses
- Roof Pitch Calculator — Pitch drives rafter length & ridge height
- Plywood / OSB Sheathing Calculator — Decking sheets over trusses
- Snow Load Calculator — Determine if 16″ OC spacing is required
· Reviewed by the Smart Roofing Calculator editorial team