Roof Truss Calculator
Estimate truss count, chord length, peak height, and roof area by building dimensions, spacing, and pitch.
The wall the trusses run along - usually the longer wall.
The distance each truss bridges.
Your Truss Estimate
- Enter building length - the wall the trusses run along (usually the longer wall).
- Enter the span - building width - the distance each truss bridges.
- Set spacing - 24 in OC is standard residential; 16 in OC for tile, heavy snow, or longer spans.
- Select pitch - sets the top-chord length, peak height, and roof area - match your plans.
- Confirm with a manufacturer - trusses are engineered products; use this for planning only.
To find how many roof trusses you need, divide the building length by the truss spacing and add one for the end truss. At standard 24-inch on-center spacing, a 40-foot building needs (40 ÷ 2) + 1 = 21 trusses; at 16-inch spacing it needs 31. Add one or two for gable ends.
How Many Roof Trusses Do I Need?
Truss count comes down to two numbers: how long the building is and how far apart the trusses sit. At standard 24-inch on-center spacing that is one truss every two feet, plus one for the end truss - the count table below covers common lengths at both 24-inch and 16-inch spacing.
Heavier loads - tile roofing, heavy-snow regions, or longer spans - may require 16-inch spacing. Add one or two extra for gable-end trusses, which are built differently from common trusses. Trusses are engineered to order, so confirm the final design and lead time (typically four to six weeks) with your manufacturer, and size the wall framing below to carry them.
Anatomy of a Roof Truss
Truss Count by Building Length
| Building length | 24" OC | 16" OC | Notes |
|---|---|---|---|
| 20 ft | 11 | 16 | Small garage or addition |
| 24 ft | 13 | 19 | Standard 2-car garage |
| 30 ft | 16 | 23 | Moderate home wing |
| 40 ft | 21 | 31 | Standard ranch home |
| 50 ft | 26 | 38 | Large home |
| 60 ft | 31 | 46 | Very large or commercial |
Computed with the same formula as the calculator above (exact count, no waste step). Includes one end truss. Hip roofs need additional hip and jack trusses - consult your manufacturer.
Worked Examples: Three Roofs
One calculator, very different roofs - what changes is the spacing and pitch you pick for the load. Three cases show how the same inputs play out:
1. Detached garage, asphalt shingles. 40 ft long, 24 ft span, 6/12 pitch, standard 24-inch spacing. That is (40 ÷ 2) + 1 = 21 trusses. The top chord runs 12 × √(144 + 36) ÷ 12 = 13.4 ft per side, the peak sits 12 × (6 ÷ 12) = 6 ft above the wall plate, and the sloped area is about 40 × 24 × 1.118 ≈ 1,073 sq ft - the figure you carry into the roof area and roofing calculators for shingle bundles. At roughly $120 per truss, the trusses alone run about $2,520, before delivery and crane.
2. Snow-region cabin. 30 ft long, 28 ft span, a steeper 8/12 pitch to shed snow, dropped to 16-inch spacing for the heavier ground-snow load. That brings the count to 23 trusses - seven more than the 16 you would use at 24-inch spacing. The top chord works out to about 16 ft 10 in, the peak to 9 ft 4 in above the plate, and the sloped area to roughly 1,010 sq ft. Confirm your local ground-snow load before settling on spacing - it, not the span alone, drives the design in cold regions.
3. Tile-roof house. 50 ft long, 32 ft span, 5/12 pitch, 16-inch spacing - here the reason for tighter spacing is dead load, not snow: concrete and clay tile are far heavier than asphalt. That is 38 trusses (versus 26 at 24-inch), a top chord near 17 ft 4 in, a 6 ft 8 in peak, and about 1,733 sq ft of roof. Tell the manufacturer the exact tile weight up front so the trusses are designed for it.
After You Size Your Trusses
Trusses set the shape of the roof. Here is the usual order to finish the estimate:
Common Truss Types and Spans
Maximum span depends on truss type and engineering. Typical residential spans:
| Truss type | Typical max span | Common pitch | Use case |
|---|---|---|---|
| Fink (W-shape) | 30-36 ft | 4/12-8/12 | Most common residential |
| Howe | 30-36 ft | 4/12-8/12 | Heavier loads |
| Attic / room-in-roof | 28-34 ft | 7/12-12/12 | Usable attic space |
| Scissor | 30-40 ft | 6/12+ | Vaulted ceilings |
| Gable end | matches span | any | End walls (vertical webs) |
Truss Dimensions by Span and Pitch
Approximate peak height (above the wall plate) and top-chord length per side for common spans at 4/12 and 6/12 pitch. Not sure of your pitch yet? The roof pitch calculator turns a measured rise and run into the pitch this tool uses. Use the figures below as a planning reference - your manufacturer's stamped drawings give the exact engineered dimensions.
| Span | 4/12 Peak | 4/12 Chord | 6/12 Peak | 6/12 Chord |
|---|---|---|---|---|
| 20 ft | 3 ft 4 in | 10 ft 6 in | 5 ft 0 in | 11 ft 2 in |
| 24 ft | 4 ft 0 in | 12 ft 8 in | 6 ft 0 in | 13 ft 5 in |
| 28 ft | 4 ft 8 in | 14 ft 9 in | 7 ft 0 in | 15 ft 8 in |
| 30 ft | 5 ft 0 in | 15 ft 10 in | 7 ft 6 in | 16 ft 9 in |
| 32 ft | 5 ft 4 in | 16 ft 10 in | 8 ft 0 in | 17 ft 11 in |
| 36 ft | 6 ft 0 in | 19 ft 0 in | 9 ft 0 in | 20 ft 1 in |
| 40 ft | 6 ft 8 in | 21 ft 1 in | 10 ft 0 in | 22 ft 4 in |
Code & Engineering Notes
- Wood trusses are engineered components - their web layout and metal connector plates are designed for your specific loads. The manufacturer must supply sealed, stamped drawings; this tool is for planning only.ANSI/TPI 1 (Truss Plate Institute)
- Roof live, snow, and wind loads are set by your local code adoption of the IRC climatic and geographic design criteria - ground snow load drives truss design in cold regions.IRC Table R301.2(1)
- Never cut, notch, or drill a truss member in the field. Any alteration requires approval from a registered design professional.IRC R802.10.4 - Alterations to trusses
- Trusses must be braced during and after installation per the manufacturer's bracing details to prevent buckling and collapse.BCSI - Building Component Safety Information
Truss Formulas
The geometry behind the results:
Slope multiplier = √(12² + rise²) ÷ 12, for an x/12 pitch
Top chord / side = (Span ÷ 2) × slope multiplier
Peak height = (Span ÷ 2) × (rise ÷ 12)
Sloped roof area = Length × span × slope multiplier
Top chord is the rafter length per side from the peak to the eave. Peak height is measured above the top of the wall plate, before any heel.
Related Calculators
Pitch and span drive the whole roof - size the rest in the construction calculator collection.
Frequently Asked Questions
How many trusses for a 40-foot building?
At standard 24-inch on-center spacing: (40 ÷ 2) + 1 = 21 trusses. At 16-inch spacing it is 31. Add one or two for gable-end trusses, which are built differently from common trusses. The waste step in the calculator adds those spares.
How do I figure roof area from the trusses?
Multiply building length by span, then by the slope multiplier for your pitch (√(144 + rise²) ÷ 12). A 40 × 24 ft building at 6/12 is 40 × 24 × 1.118 = about 1,073 sq ft of sloped roof. That sloped area - not the flat footprint - is what you take into the roofing and shingle calculators.
What spacing should roof trusses be?
24 inches on center is the residential standard and the most economical. Use 16-inch spacing for heavy roof loads like clay or concrete tile, in heavy-snow regions, or where the engineered design calls for it. Tighter spacing means more trusses but lighter loading per truss.
How far can a truss span without support?
Common residential Fink trusses span roughly 30 to 36 feet clear; specialty designs reach further. The real limit comes from the engineered design for your loads, so always use the manufacturer's stamped span tables rather than a rule of thumb.
How tall is a 4/12 truss?
Peak height is half the span times the pitch ratio. A 24-foot span at 4/12 peaks about 4 feet above the wall plate, a 30-foot span about 5 feet, and a 40-foot span about 6 feet 8 inches. The same 24-foot span at 6/12 peaks at 6 feet - steeper pitches raise the peak and add attic volume.
Can I build my own roof trusses?
For anything structural, no - trusses are engineered assemblies whose metal connector plates and web layout are designed and stamped for specific loads, and field-built substitutes usually will not pass inspection. Order engineered trusses from a manufacturer; use this calculator only to plan count, size, and budget.
How far ahead should I order trusses?
Engineered trusses are made to order, so plan four to six weeks of lead time, longer in busy building seasons. Confirm the set date and whether a crane is needed before ordering, since large trusses usually require one to lift into place.