Gable Roof Area Calculator
Calculate the total area of a gable roof using building length, width, and roof pitch. Estimate materials for shingles, underlayment, and more.
Roof Truss Spacing Calculator
Calculate the number of roof trusses needed based on building length and truss spacing. Includes total truss count and gable-end truss requirements.
Trusses Needed⧉
21.00
At 24" on-center spacing over 40 ft
Total Lumber (LF)⧉
2,201.00
Linear feet of lumber for all trusses
Estimated Cost⧉
$2,730.00
$130.00 per truss
Max Span (this config)⧉
36.8 ft
Span is within capacity
Weight per Truss⧉
110 lbs
Total weight: 2,310.00 lbs
Peak Height⧉
6 ft
Rafter length: 13.42 ft each side
Load Capacity
65% of max capacity used
| Type | Max Span (ft) | Weight/ft (lbs) | Base Cost | Members |
|---|---|---|---|---|
| King Post | 24 | 3.2 | $85.00 | 5 |
| Queen Post | 30 | 3.8 | $110.00 | 7 |
| Fink (W) | 40 | 4.1 | $130.00 | 9 |
| Howe | 36 | 4.5 | $145.00 | 9 |
| Pratt | 38 | 4.3 | $140.00 | 9 |
Planning notes, formulas, and examples
About the Roof Truss Spacing Calculator
Roof trusses are engineered structural components that support the roof deck, sheathing, and covering material. They are spaced at regular intervals (typically 24 inches on-center for residential construction) along the building length, with a truss at each end (gable-end trusses).
This calculator determines the number of trusses needed based on the building length and the truss spacing (on-center distance). The formula is simple: divide the building length by the spacing, add 1 for the starting truss, and add gable-end trusses as needed.
Accurate truss counts are essential for ordering because trusses are engineered and manufactured to order. Under-ordering delays framing; over-ordering wastes money on non-returnable custom trusses.
When This Page Helps
Trusses are manufactured to custom specifications and are typically not returnable. Getting the count right on the first order prevents framing delays. This calculator accounts for the "plus one" rule and gable-end trusses.
How to Use the Inputs
- Enter the total building length.
- Select the truss spacing (16" or 24" OC are standard).
- Specify the number of gable ends (typically 2 for a simple gable roof).
- Review the total truss count including gable-end trusses.
- Verify the count with your truss manufacturer before ordering.
Formula used
Common Trusses = (Building Length × 12 / Spacing) − 1
Gable-End Trusses = Number of Gable Ends
Total Trusses = Common Trusses + Gable-End TrussesExample Calculation
Result: 21 trusses total
Common trusses = (40 × 12) / 24 − 1 = 19 common trusses. Plus 2 gable-end trusses = 21 total trusses.
Tips & Best Practices
- 24" OC spacing is standard for most residential roofs with conventional loads.
- 16" OC spacing is used for heavy roofing (tile, slate) or high snow/wind loads.
- Gable-end trusses are flat (no bottom chord) and are different from common trusses.
- Girder trusses (doubled or tripled) are used where interior walls or beams intersect.
- Always confirm the truss design with a structural engineer or the truss manufacturer.
- Order trusses 4–6 weeks ahead of the framing schedule for manufactured designs.
Truss Types
Common trusses span the full building width and are used for the main roof structure. Gable-end trusses frame the triangular gable wall. Hip trusses step down progressively to create a hip roof. Valley trusses create intersecting roof planes. Scissor trusses have an angled bottom chord for vaulted ceilings.
Spacing Factors
The choice between 16" and 24" OC spacing depends on the roof load (material weight + snow + wind), the span (building width), and the deck material. 7/16" OSB sheathing can span 24" OC for normal loads, but 1/2" or heavier sheathing may be required for 24" spacing in heavy-load areas.
Ordering Timeline
Trusses are manufactured to order. Allow 2–4 weeks for engineering and 1–2 weeks for manufacturing. Complex designs or peak-season orders may take longer. Submit truss orders as soon as the building dimensions and roof design are finalized to avoid framing delays.
Sources & Methodology
Last updated:
Frequently Asked Questions
On-center spacing is measured from the center of one truss to the center of the next. 24" OC means 24 inches (2 feet) between truss centers. This leaves about 22.5 inches of clear space between the truss chords for insulation and utilities.
For standard asphalt shingle roofs with normal snow loads, 24" OC is acceptable. Heavy materials (concrete tile, slate), high snow load areas, and wide spans may require 16" OC or specially engineered trusses. Check local building codes.
A gable-end truss frames the triangular gable wall at each end of the building. It has vertical studs instead of the typical web pattern and is designed for the vertical gable wall load rather than the roof-spanning load of common trusses.
Trusses are delivered on a flatbed truck and offloaded by crane or by hand for small trusses. They should be stored upright on blocking, braced to prevent tipping, and protected from prolonged rain exposure. Install within 30 days of delivery.
Trusses are usually faster and cheaper for simple roof designs because they span the entire building width without interior bearing walls. Rafters (stick-framing) offer more flexibility for complex designs and attic living space. Most production homes use trusses.
Trusses must be designed by a licensed truss engineer and manufactured according to the engineered design. The building's structural engineer specifies the loading requirements, and the truss manufacturer designs trusses to meet them. This is a code requirement.
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