Roof Truss Calculator
Estimate truss count, spacing, and total roof area by building dimensions and pitch.
Your Truss Estimate
- Enter building length — the wall the trusses run along (the longer wall for most homes).
- Enter building width — this is the truss span — the distance each truss bridges.
- Select pitch — determines truss height and roof area. Must match your plans.
- Review results — truss count, total roof area, and squares for material planning.
- Consult an engineer — trusses are engineered products. Use this for planning estimates only.
How Many Roof Trusses Do I Need?
Truss count depends on building length and spacing. Standard residential spacing is 24 inches on center (OC) — one truss every 2 feet. The formula: divide building length by spacing, then add 1 for the end truss. A 40-foot building at 24" OC needs (40 ÷ 2) + 1 = 21 trusses.
Some designs require 16-inch OC spacing for heavier loads (tile roofing, heavy snow regions) or longer spans. Always add 1–2 extra for gable-end trusses, which have a different design than common trusses. Order 4–6 weeks before your scheduled set date.
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 |
Includes 1 gable-end truss. Hip roofs require additional hip and jack trusses — consult your manufacturer.
Truss Span Limits
Maximum truss span depends on the truss type and engineering. Standard residential trusses handle these typical spans:
| Truss Type | Typical Max Span | Common Pitch | Use Case |
|---|---|---|---|
| Fink (W-shape) | 30–36 ft | 4/12–8/12 | Most common residential |
| Howe | 24–30 ft | 4/12–6/12 | Short spans, simple design |
| Scissor | 24–32 ft | 4/12–8/12 | Cathedral/vaulted ceilings |
| Attic | 24–30 ft | 8/12–12/12 | Livable attic space |
| Parallel chord | 40+ ft | Flat | Commercial, flat roofs |
Spans over 36 feet often require girder trusses or steel beams. Your truss engineer will specify the design based on span, loads, and local codes.
Truss Peak Height by Span and Pitch
| Span | 3/12 pitch | 4/12 pitch | 5/12 pitch | 6/12 pitch | 8/12 pitch |
|---|---|---|---|---|---|
| 20 ft | 2'6" | 3'4" | 4'2" | 5'0" | 6'8" |
| 24 ft | 3'0" | 4'0" | 5'0" | 6'0" | 8'0" |
| 28 ft | 3'6" | 4'8" | 5'10" | 7'0" | 9'4" |
| 30 ft | 3'9" | 5'0" | 6'3" | 7'6" | 10'0" |
| 32 ft | 4'0" | 5'4" | 6'8" | 8'0" | 10'8" |
| 36 ft | 4'6" | 6'0" | 7'6" | 9'0" | 12'0" |
| 40 ft | 5'0" | 6'8" | 8'4" | 10'0" | 13'4" |
Peak height = (Span ÷ 2) × (Rise ÷ 12). These are theoretical centerline heights — actual truss height includes heel height and varies by manufacturer. Always confirm with your truss supplier's shop drawings.
Formulas
Simple division for truss count, plus roof area:
Roof area = Length × Width × Pitch multiplier
Truss height (peak) = (Span ÷ 2) × (Rise ÷ 12)
These formulas give the common truss count. Gable-end, hip, and girder trusses are designed separately by the engineer.
Related Calculators
Truss count and spacing depends on roof pitch and span. Browse the construction calculator collection.
Frequently Asked Questions
What is standard truss spacing for a house?
24 inches on center is standard for most residential roofs with asphalt shingles and normal snow loads. Use 16" OC for heavy roofing (tile, slate), high snow areas (>40 lb/ft² ground snow), or spans over 30 feet.
How far in advance should I order trusses?
Most truss manufacturers recommend ordering 3–6 weeks in advance for residential projects. Custom or complex truss designs may take longer. Delays in truss delivery are one of the most common causes of framing schedule overruns — order as soon as your plans are finalized and permits are approved. Confirm lead time with your specific supplier.
What is the difference between a truss and a rafter?
Rafters are individual framing members cut and installed on-site, traditionally used in custom homes and complex roof shapes. Trusses are prefabricated triangulated assemblies built in a factory to exact specifications. Trusses are faster to install, more consistent, and typically cheaper for simple roof shapes. Rafters allow for finished attic space; most standard trusses do not.
How much weight can a roof truss hold?
Standard residential trusses are engineered to handle dead loads (roofing materials, ~15 psf) plus live loads (snow, occupancy — typically 20–40 psf depending on region). Total design loads of 40–55 psf are common. Never add HVAC equipment, water tanks, or storage to trusses without verifying with a structural engineer — trusses are not designed for point loads in the middle of the span.
How do I estimate the number of trusses I need?
Truss count is typically based on building length and the spacing you plan to use. Divide the run length by the spacing, then add one for the starting end. This calculator helps estimate count and spacing, but confirm with your truss supplier's layout.