Ready to Calculate Trusses
Enter your building dimensions and roof specifications on the left. The calculator will determine truss geometry, spacing, and precise timber requirements.
Which Truss Type Should I Choose?
King Post
Simple pitched roof truss with one central vertical king post and diagonal struts. Best suited to smaller spans and simple roof structures.
Queen Post
Pitched roof truss with two vertical queen posts and supporting web members. Useful for wider spans than a basic king post truss.
Fink
Common W-shaped pitched roof truss. Very common for modern timber roof construction and domestic roof structures.
Fan
Pitched roof truss with multiple web members spreading from the centre area to distribute roof loads. Useful for wider pitched roof spans.
Howe
Pitched truss with vertical and diagonal web members. Used where a stronger engineered-style roof truss layout is required.
Truss Anatomy: Understanding the Parts
A roof truss is an engineered framework of timbers designed to bridge the space above a room and provide support for a roof. Understanding the terminology is essential for planning and ordering materials.
What is a roof truss calculator?
A roof truss calculator is a specialized construction tool used to determine the geometric dimensions, spacing, and material requirements for a pitched roof structure. Instead of manually calculating angles and hypotenuses using trigonometry, this tool instantly provides the top chord length, bottom chord length, and roof pitch based on your building's width and desired roof height.
It also helps builders plan the layout by calculating exactly how many trusses are needed to span the length of the building, and estimates the total linear length of timber required for the trusses and roof battens.
What measurements do you need?
- Building Width (Span): The total distance from the outside of one load-bearing wall to the outside of the opposite wall. This forms the bottom chord of the truss.
- Building Length: The total length of the building. This determines how many trusses you need.
- Roof Rise (Height): The vertical distance from the top of the wall plates to the peak of the roof (the ridge).
- Overhang: The horizontal distance the roof extends past the outside walls to protect the building from weather.
Roof sheets vs roof tiles and batten spacing
The type of roof covering you choose drastically affects the truss design and batten spacing:
Corrugated/IBR Sheets
Metal roof sheeting is relatively lightweight, but allowable truss and purlin/batten spacing depends on the sheet profile, thickness, support conditions, wind loads and manufacturer span tables. Use the calculator spacing as a planning input only and verify it against the roofing manufacturer's requirements and the final truss design.
Concrete/Clay Tiles
Concrete and clay tiles add substantial dead load. Truss spacing and tile batten gauge must come from the structural/truss design and the selected tile manufacturer's fixing and gauge requirements. Use the calculator defaults for planning only; do not treat them as structural limits.
Why truss design must be checked
While this calculator provides excellent geometric planning and material estimates, it does not perform structural engineering calculations.
Real-world truss design must account for:
- Dead loads (weight of timber, tiles, ceiling boards).
- Live loads (workers on the roof, snow loads).
- Wind uplift forces (requires specific tie-down brackets).
- Timber grading and structural integrity.
- Connector plate sizing and placement.
Always have your final truss design approved by a qualified structural engineer or manufactured by a certified truss plant before installation.
Frequently Asked Questions
Geometry verification and structural limits
Formula: for a symmetric roof, half-span = span/2, angle = atan(rise/half-span), and top chord to the wall line = √(half-span²+rise²). Truss count is based on ceiling(building length/target spacing)+1, with actual spacing recalculated across the resulting spaces.
Verified example: a 6,000 mm span with 1,500 mm rise has a 3,000 mm half-span, 26.565° pitch and 3,354.10 mm top chord before overhang. A 12,000 mm building at 900 mm target spacing requires 15 trusses and gives 857.14 mm actual spacing.
Safety: these are layout and quantity estimates only. Web arrangement, member grades and sizes, connector plates, bracing, uplift, snow/wind loads, bearing and tie-downs require sealed truss design drawings or approval from the truss manufacturer and the responsible engineer.
Authoritative references
Roof-truss input geometry
Use this schematic to identify the calculator inputs and how they relate. It is explanatory and not a fabrication, installation or lift-plan drawing.