Roof Truss Calculator

Ordering Roof Trusses: Planning Your Take-Off

Quoting a new roof frame? Guessing your truss count leads to short orders, site delays, or paying for expensive engineered timber you don't need. Our roof truss calculator takes the guesswork out of material take-offs by instantly computing standard and girder truss quantities based on building length, roof geometry, and spacing.

Ordering prefabricated timber trusses requires factoring in overall building dimensions, wind classifications, roof pitch, and cladding dead weight. For a standard gable roof, the math is straightforward. But once you throw in a hip or Dutch gable, truncated girder trusses are required at the ends to support the jack framing and hip corners.

Truss Spacing Standards: 600mm vs 900mm Centres

The total number of trusses required depends directly on your truss spacing (measured from the centre of one truss to the centre of the next). In Australia, spacing is primarily governed by the roofing cladding material and local wind loads:

For detailed roof area calculations and sheet estimates, pair your truss order with our Roof Area Calculator. If stick-framing rafters instead of ordering trusses, use our Roof Pitch & Rafter Calculator or check member allowable spans with our Timber Span Calculator.

Standard vs Girder & Truncated Trusses

Roof frames contain distinct truss configurations depending on structural responsibility:

• Standard (Common) Trusses: Classic A-frame trusses that span across the main rectangular width of the building, supporting purlins, battens, and ceiling joists.
• Girder (Truncated) Trusses: Heavy-duty, reinforced double or triple-ply trusses installed at hip corners or wall intersections. They act as main structural beams carrying the loads of intersecting jack trusses and creeper rafters.
• Jack Trusses & Creepers: Shorter, graduated trusses that frame the triangular sloping ends of hip roofs.

Wind Classifications & Tie-Down Hardware

Under AS 1684, every roof truss must be securely anchored to the top plate and wall framing to resist uplift forces. Non-cyclonic wind classifications range from N1 (sheltered) to N4 (exposed coastal).

While standard common trusses may require framing anchors or cyclone ties, heavy girder trusses carrying concentrated loads require engineered bracketry, M12 bolting, or multi-grip straps to comply with building certifier inspections.

Engineering Compliance & Site Certification

While this calculator provides rapid material take-off figures for quoting and budgeting, prefabricated timber trusses are engineered structural units. Prior to fabrication, your building plans must be submitted to an accredited truss plant. They utilize specialized software (such as Pryda Build or MiTek Pamir) to issue certified design layouts and Form 15 compliance documentation required by private certifiers.

Worked Example: 18m Gable Roof Calculation