Truss Calculator
Estimate rafter length, roof pitch, number of trusses, total lumber, and material cost for your roofing project.
| Trusses cost: | $— |
| Total cost: | $— |
Whether you are framing a new roof over a residential addition, designing a floor system for a commercial building, or engineering a long-span structure where traditional stick framing cannot reach, understanding how many trusses you need, what spans they must cover, and what loads they must carry before you order saves weeks of lead time and prevents costly field modifications. A truss calculator takes your roof or floor dimensions, your pitch or slope, your truss spacing, and your loading conditions to tell you the number of trusses required, the overall span, the required bearing width at each end, and the dead and live load each truss must support.
This guide covers the truss quantity formula, standard truss types and their applications, spacing requirements, load assumptions for residential and commercial construction, a coverage table for common roof areas, and frequently asked questions about truss estimating and ordering.
The Truss Quantity Formula
The number of trusses required for a given building width is determined by the building length and the on-center spacing of the trusses:
Number of Trusses = (Building Length / Truss Spacing) + 1
For a 40-foot long building with trusses spaced 24 inches on center:
- Number of trusses: (40 / 2) + 1 = 21 trusses
- Add 2 gable end trusses if the roof has gable ends rather than hips: 21 + 2 = 23 trusses total
- For hip roofs, hip trusses and jack trusses at each hip end must be calculated separately
The span of each truss equals the clear distance between bearing points, typically the outside-to-outside dimension of the building’s top plates. Overhang length on each side is added to determine the overall manufactured length of the truss.
Standard Truss Types and Applications
Truss type is selected based on the span, the required pitch, any need for usable attic space, and the specific structural requirements of the project. Using the wrong truss type for the application results in inadequate structural performance or wasted floor or attic area.
Truss Type | Typical Span Range | Pitch Range | Key Feature | Common Application |
Fink (W-truss) | 16 – 40 ft | 4:12 – 12:12 | Most economical, most common | Residential roofs, light commercial |
Howe truss | 20 – 50 ft | 4:12 – 10:12 | Verticals in tension, diagonals in compression | Medium spans, bridges |
Pratt truss | 20 – 60 ft | Flat or low slope | Diagonals in tension, verticals in compression | Long spans, commercial roofs |
Scissor truss | 16 – 40 ft | 3:12 – 9:12 | Sloped bottom chord creates vaulted ceiling | Cathedral ceilings, open interiors |
Raised heel truss | 16 – 50 ft | 4:12 – 12:12 | Raised top chord at eave for full insulation depth | Energy-efficient construction |
Attic truss | 16 – 36 ft | 6:12 – 12:12 | Open interior web panel for storage or room | Bonus rooms, accessible attic storage |
Mono truss | 8 – 20 ft | 1:12 – 6:12 | Single slope, one end higher than other | Additions, shed roofs, porches |
Hip truss | Varies | 4:12 – 10:12 | Progressively shorter to form hip end | Hip roof ends |
Floor truss | 16 – 40 ft | Flat | Long spans without bearing walls below | Floor systems, eliminates mid-span supports |
Truss Spacing and Load Relationship
Truss spacing determines how much tributary load each truss carries. Wider spacing means fewer trusses but each must be engineered for greater load. Typical residential truss spacing is 24 inches on center, which balances material cost against sheathing span requirements for standard 7/16-inch OSB or 1/2-inch plywood roof decking.
Spacing (on center) | Trusses per 100 ft length | Load per Truss vs 24″ OC | Sheathing Span OK? | Notes |
12 inches | 101 | 50% | Yes, always | Maximum stiffness, highest truss cost |
16 inches | 76 | 67% | Yes, always | Common for steep-pitch or heavy tile roofs |
19.2 inches | 63 | 80% | Yes for 7/16″ OSB | 5-space module for 8-ft panels |
24 inches | 51 | Baseline | Yes for 7/16″ OSB | Standard residential spacing |
32 inches | 39 | 133% | Only 5/8″ plywood or thicker | Requires heavier sheathing |
48 inches | 26 | 200% | Requires structural decking | Special engineering required |
Truss Coverage Table by Roof Size
The table below shows truss quantities for common building lengths at standard residential spacing of 24 inches on center. Quantities include standard common trusses only. Gable end trusses, hip trusses, valley sets, and girder trusses are additional.
Building Length (ft) | Trusses at 24″ OC | Trusses at 16″ OC | Trusses at 12″ OC | Typical Span (ft) |
20 | 11 | 16 | 21 | 24 – 28 |
24 | 13 | 19 | 25 | 24 – 28 |
30 | 16 | 24 | 31 | 28 – 32 |
36 | 19 | 28 | 37 | 28 – 36 |
40 | 21 | 31 | 41 | 32 – 40 |
48 | 25 | 37 | 49 | 32 – 40 |
50 | 26 | 39 | 51 | 36 – 44 |
60 | 31 | 46 | 61 | 36 – 50 |
80 | 41 | 61 | 81 | 40 – 60 |
100 | 51 | 76 | 101 | 40 – 60 |
Load Assumptions for Truss Design
Trusses are engineered for specific load combinations. Providing accurate load data to the truss manufacturer is critical. Under-specifying loads results in inadequate trusses that must be replaced. Over-specifying wastes material and money.
Load Type | Residential Typical | Light Commercial | Heavy Commercial | Notes |
Roof dead load (roofing + sheathing) | 10 – 15 psf | 15 – 20 psf | 20 – 30 psf | Tile roofs reach 25 psf; asphalt shingles 10 psf |
Roof live load (snow or construction) | 20 psf (low snow) | 20 – 30 psf | 30 – 40 psf | Snow zones may require 40 – 100+ psf ground snow |
Ceiling dead load | 5 – 10 psf | 5 – 15 psf | 10 – 20 psf | Drywall 5 psf; plaster 9 psf |
Floor dead load | 10 – 15 psf | 15 – 20 psf | 20 – 30 psf | Includes sheathing and framing weight |
Floor live load (residential) | 40 psf | 40 – 50 psf | 50 – 125 psf | IBC minimum 40 psf residential |
Bearing Width and Connection Requirements
Trusses transfer their load to the top plates of the bearing walls at each end. Minimum bearing width is required to distribute the concentrated load without crushing the wood at the truss seat cut. Standard minimum bearing is 1.5 inches for light trusses up to about 40 feet. Longer spans and heavier loads require wider bearing, often 3.5 inches (one stud width) or more. The truss manufacturer specifies minimum bearing on the engineering drawing for each truss.
Related tools and guides: rafter calculator, roof pitch calculator and beam load calculator.
Frequently Asked Questions
How long does it take to get trusses manufactured and delivered?
Lead time for custom-engineered residential trusses typically runs 3 to 6 weeks from the date engineering drawings are approved and the order is released to production. During peak construction seasons this can extend to 8 weeks or more. Floor trusses and complex roof systems with multiple hip sets and valleys can take longer. Order early — truss delays are one of the most common causes of framing schedule overruns on residential projects.
Can I modify trusses on site after delivery?
No. Trusses are precision-engineered structural components. Cutting webs, notching chords, or repositioning connector plates invalidates the engineering and creates a safety hazard. Any required modification must be designed by a licensed structural engineer and the manufacturer, and must be repaired with properly sized and positioned metal connector plates. Unauthorized field modifications to trusses are a serious structural liability.
What is the difference between a gable end truss and a common truss?
A common truss is a standard triangulated structural truss with top chords, a bottom chord, and internal web members. A gable end truss has vertical studs instead of diagonal webs because it sits flat against the gable end wall and transfers load vertically to the end wall rather than spanning between two bearing points. Gable end trusses are not structural spanning members — they provide the triangular shape and attachment surface for the rake overhang.
How do I calculate trusses for a hip roof?
A hip roof requires a hip set at each end of the building consisting of a full-length hip girder truss, a series of decreasing-length hip trusses, and jack trusses that fill in between the hip set and the common trusses. For a standard hip, the hip set length equals half the building width for a square hip. The truss manufacturer calculates the exact hip set when you provide the overall building footprint, overhang dimensions, and pitch. Always provide a scaled plan drawing to the truss company for hip roofs.
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