What is the standard spacing for roof trusses?

In residential construction, 24 inches on-center (o.c.) is the most common spacing. For heavy snow loads or specific structural requirements, 16 inches or 12 inches o.c. may be required.

How do I calculate the number of trusses needed?

Divide the total length of the building by the spacing between trusses, round up to the nearest whole number, and then add one for the starting end.

Are trusses better than hand-framed rafters?

Engineered trusses are typically faster to install, use less lumber, and can span longer distances without internal load-bearing walls. Hand-framing is better for complex roof shapes or where attic storage is the priority.

Engineered Framing Tools

Roof Truss Calculator

Simplify your roof planning with our truss estimator. Calculate the total number of trusses required for a project, determine the total roof surface area for shingles, and find the individual chord lengths for engineered truss quotes.

Roof Truss Calculator

King-Post · Queen-Post · Fink · Howe · Pratt — member lengths & lumber estimate

Fink (W-truss) — cross-section

Truss type

Roof Geometry

Building span (total width)

Roof pitch (rise)

Eave overhang (horizontal)

Truss Layout & Lumber

Truss spacing (on-centre)

Number of trusses

Lumber width (nominal section)

Lumber depth (nominal section)

Waste allowance

Tip

Choosing the right truss type

King-post suits spans up to 8 m / 26 ft. Queen-post works up to ~11 m / 36 ft. The Fink (W-truss) is the most common residential choice for spans up to 15 m / 50 ft. Howe and Pratt trusses are used in heavier commercial and industrial applications.

Results

Enter dimensions above to calculate

Truss Anatomy & Spacing

Engineered trusses use a series of triangles to transfer the roof load to the exterior walls without needing interior support.

Common Wood Truss Types

Design	Max Clear Span	Common Use
King Post	Up to 24 ft	Garages, sheds, small small additions
Fink (W-Truss)	Up to 50 ft	Standard residential homes
Howe (M-Truss)	Up to 60 ft	Large residential, commercial
Scissor	Up to 40 ft	Vaulted / Cathedral ceilings

Step-by-Step Formulas

1. Individual Truss Geometry

Rafter Length = √((Span/2)² + Rise²)

A standard King Post or Fink truss forms a large triangle. The calculator finds the length of the top chords and the bottom chord (span) to determine material per truss.

2. Total Truss Count

Count = ceil(Project Length ÷ Spacing) + 1

Divide the total building length by the on-center spacing (typically 24" or 600 mm). Adding 1 ensures a truss is placed at both the starting and ending walls.

3. Roof Surface Area

Total Area = 2 × Rafter Length × Project Length

This calculates the total square footage of the roof deck supported by the trusses. This is the primary value used for ordering sheathing and shingles.

4. Dead Load Estimation

Load = Area × Material PSF

The truss must support the weight of the roofing material (shingles, slate, or metal) plus snow and wind loads. Understanding the area helps engineers verify truss capacity.

Worked Example

Garage Project: 24 ft span, 36 ft building length, 6/12 pitch, 24" (2 ft) spacing.

Rafter length (slope) = √((24/2)² + 6²) = √(144 + 36) ≈ 13.42 ft

Truss count = (36 ÷ 2) + 1 = 18 + 1 = 19 Trusses

Total roof area = 2 × 13.42 × 36 ≈ 966.2 sq ft

Materials needed: 10 Squares of shingles

💡 Installation Pro Tip

When installing trusses, proper bracing is essential. Each truss must be plumbed and braced to the previous truss at the top chord, bottom chord, and internally through the web members. Never remove the lifting crane until enough bracing is installed to stabilize the entire system against wind and side-loading.

Frequently Asked Questions

What is the standard truss spacing?: 24 inches on-center (o.c.) is the residential standard. 16" o.c. is used for higher load requirements.
How many trusses do I need?: Divide building length by spacing, round up, and add 1. For a 40' building at 2' spacing: (40/2)+1 = 21 trusses.
Can trusses span any distance?: Engineered wood trusses can commonly span up to 60-80 feet without interior supports, making them ideal for open-plan homes.