Carpentry calculator

Enter roof dimensions. Calculate rafter length and see what to measure.

Run R is half the span for a symmetrical roof, or the full span for a mono-pitch roof. Measure H from the support reference level to the ridge and E horizontally beyond the support. Length is theoretical without ridge correction. Birdsmouth includes S, P, N and HAP. Cut line angles are measured to the long rafter edge.

Solve a right triangle or convert roof pitch.

A is the horizontal leg, B the vertical leg and C the hypotenuse. Angle α is between A and C; β is the other acute angle. Percentage pitch = 100 × B / A. A 100% pitch equals 45°, not 90°.

Calculate the number of elements, gaps and positions from the start.

Measure D between the centres of the outermost elements. There is one more element than interval. Clear gap = centre spacing minus element width. For battens, enter the spacing range specified by the roofing manufacturer and measure the first and last positions from the same origin. This tool does not select structural spacing or a roofing system. In between-post modes, D is the clear opening between inner post edges. For n elements of width W, equal gap = (D − n × W) / (n + 1), including both ends. Centre positions start at the left opening edge. Other modes measure between outermost element centres.

Add different timber sizes. Total the volume and order cost.

Enter actual dimensions and piece count for each batch. Prices can be per m³, piece or linear metre, even when dimensions use inches. Cost totals only priced batches. Allowance increases estimated ordering volume, not finished piece volume. Currency labels your price without exchange conversion.

Measure roof planes, subtract openings and add an allowance.

In roof-plane mode, measure length and width along the sloped surface. The number of planes multiplies the area of one rectangle. In projection mode, enter the full horizontal rectangular projection of the roof including overhangs; all planes must have the same pitch. Area = projection area / cos(α). Calculate different pitches separately. Enter openings in m² of sloped surface, not projection. The section list totals different roof planes and pitches. Add each section once. Surface mode uses sloped dimensions; projection mode uses horizontal dimensions. Triangle and trapezoid heights are perpendicular to their bases in the same plane. Openings always use m² of sloped surface. Sections must not overlap.