How to calculate roof timber volume

For rectangular members, multiply the cross-sectional area by the member length and quantity. Using metres throughout: volume (m³) = width (m) × depth (m) × length (m) × quantity. Repeat for each distinct member and sum the results. This is a geometric volume calculation, not a structural sizing check.

Worked example: common rafters

For 20 rafters, each 45 × 195 mm and 4.8 m long: 0.045 × 0.195 × 4.8 × 20 = 0.8424 m³. Other members, including ridge, hip or jack rafters, wall plates, purlins and braces, must be calculated separately.

Cut list versus purchase list

A cutting list records the members you intend to produce. A purchase list records available timber stock and must consider lengths offered by the supplier, saw kerf, trimming, defects and handling allowances. A universal waste percentage cannot replace an actual stock-length optimization.

Data to collect before ordering

• Individual element identifiers and member types.
• Cross-section and grade as specified by the structural design.
• Finished length versus stock length.
• Quantity of each distinct geometry.
• Additional allowance for fabrication and rejected pieces.

Why hip roofs need a complete element model

Hip and jack rafters have different lengths and cuts. Estimating the whole roof using one average rafter length can understate stock requirements. Start with hip rafter geometry, then create a cutting list with IDs.

Note: Timber volumes here are geometric estimates, not structural design, strength verification or a supplier quote.

Continue with related roof calculations

Roof framing calculator · Rafter calculator · Hip roof calculator · Roof framing cutting list

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