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Purlin Spacing Calculator

Work out purlin spacing instantly with clear inputs, formula shown and shareable results.

About Purlin Spacing Calculator

Purlin spacing is set by how far the roof sheeting can span. Dividing the rafter length by the maximum permitted sheeting span and rounding up gives a whole number of equal spans, and the number of purlin lines is one more than the number of spans because both eaves and ridge need support.

Formulas

Spacing

Spans = ceil(rafter length / maximum sheeting span); spacing = rafter length / spans

Purlin quantity

Total length = (spans + 1) x building length x 2 slopes

Worked example

With the default values below, Purlin Spacing Calculator returns Purlin spacing to adopt of 1.250. Change any input above and every figure here is recalculated in your browser.

Inputs used

Rafter length up the slopeminimum 0.1 m
5 m
Maximum sheeting spanminimum 0.1 m
1.4 m
Building lengthminimum 0.1 m
20 m

Results

Purlin spacing to adopt
1.250 m
Purlin lines per slope
5
Number of sheeting spans
4
Total purlin length both slopes
200.0 m

Step by step

  1. 1Values used
  2. 2Spacing
  3. 3Purlin quantity
  4. 4Purlin spacing to adopt= 1.250 m
  5. 5Purlin lines per slope= 5
  6. 6Number of sheeting spans= 4
  7. 7Total purlin length both slopes= 200.0 m

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Purlin and sheeting spans must be confirmed against the manufacturer's load-span tables and checked by a qualified engineer for wind uplift.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Purlin and sheeting spans must be confirmed against the manufacturer's load-span tables and checked by a qualified engineer for wind uplift.

Frequently Asked Questions

How is the Purlin Spacing figure worked out?

Spans = ceil(rafter length / maximum sheeting span); spacing = rafter length / spans. Purlin spacing is set by how far the roof sheeting can span.

What limits the sheeting span?

Deflection and, for walkable roofs, point load from foot traffic. Manufacturers publish span tables by profile, gauge and whether the span is single, double or continuous.

What values do I need to enter?

This calculator takes 3 inputs: Rafter length up the slope, Maximum sheeting span, Building length. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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