🔺 Roof Truss Calculator

Understanding truss count and spacing

Spacing choice interacts with the roof sheathing rating and the truss design itself — this calculator counts and dimensions trusses geometrically but does not engineer the truss members or connections.

• Prefabricated wood or steel roof trusses are engineered products — their member sizes, web configuration and connector plates are designed by a truss manufacturer's engineer for the specific span, pitch, spacing and loads involved, and this calculator does not replace that engineering, only the geometric count and layout dimensions.
• This calculator assumes a simple symmetric gable truss with no overhang; hip roofs, dormers, or trusses with overhangs and non-standard chord configurations require a truss manufacturer's specific engineered drawings.

What does a roof truss calculator work out?

A roof truss calculator counts how many prefabricated trusses are needed along a building's length at a given on-center spacing, and calculates the geometric dimensions — top chord (sloped rafter member) length, bottom chord (span) length, and peak height — of a simple symmetric gable truss from the roof span and pitch.

Truss spacing directly affects the truss count: closer spacing (400 mm) means more trusses supporting a given roof, generally used with lighter roof sheathing or where truss design calls for it, while wider spacing (600 mm) uses fewer, more widely spaced trusses, typically paired with a sheathing product rated for the longer unsupported span between trusses.

How to use this roof truss calculator

1. Enter the building length — the horizontal distance along which trusses will be spaced out.
2. Enter the roof span — the horizontal distance the truss covers, typically the building width.
3. Enter the roof pitch as X-in-12 (for example, 6 for a 6:12 pitch).
4. Select the truss spacing: 400 mm or 600 mm on-center.
5. Read the number of trusses needed, the top chord length per side, the bottom chord length and the peak height above the bottom chord.

The formula behind truss geometry and count

Truss count equals the building length divided by the spacing, rounded down, plus one — placing a truss at both ends of the run and evenly in between. Peak height (rise) equals half the span multiplied by the pitch ratio (pitch ÷ 12). Top chord length uses the Pythagorean theorem on the right triangle formed by half the span and the rise.

Worked example (calculator defaults): a 12 m building length, 8 m span, 6:12 pitch, at 600 mm o.c. spacing. Truss count = ⌊12 ÷ 0.6⌋ + 1 = 20 + 1 = 21 trusses. Peak height = (8 ÷ 2) × (6 ÷ 12) = 4 × 0.5 = 2 m. Top chord length = √((8÷2)² + 2²) = √(16 + 4) = √20 ≈ 4.47 m per side. Bottom chord length = 8 m (the full span).

Common mistakes

• Treating the top chord length as the total rafter length including overhang — this calculator's top chord runs only from the peak to the outer edge of the span entered, with any eave overhang added separately.
• Assuming truss count alone determines roof strength — actual truss capacity depends on the manufacturer's engineered design for the member sizes, web layout and connector plates, not just the spacing chosen.
• Using truss spacing and sheathing thickness that don't match — wider truss spacing generally requires a thicker or higher-span-rated sheathing product to avoid excessive sag between supports.
• Applying this simple symmetric-gable geometry to a hip roof, dormer or other complex truss configuration without accounting for the girder trusses, jack trusses and hip trusses those roof shapes require.

Sıkça Sorulan Sorular

Kaynaklar

1. Structural Building Components Association (SBCA) — prefabricated wood roof truss design and terminology conventions.
2. American Wood Council (AWC) — Wood Frame Construction Manual, roof framing and truss spacing conventions.
3. Standard trigonometric (Pythagorean) relationships for gable-truss chord geometry from span and pitch.