Purlin Spacing Calculator

Purlin spacing calculator

A purlin spacing calculator counts how many purlins fit along a length at a set spacing. The count is ceiling(length in inches ÷ spacing in inches) + 1. A 40 ft run with purlins at 24 inches on center has 20 gaps, so it needs 21 purlin lines.

The result is a material count. It is not a structural design, and the spacing must come from your plans.

What a purlin line is

A purlin is a horizontal member that runs across the rafters or trusses and holds the roof sheeting. In most buildings the purlins run the length of the building. They are spaced up the slope, one row at a time, from eave to ridge.

So the calculator’s length is the distance over which you are spacing them. For one roof face, that is the slope length from eave to ridge. Each purlin line then runs the building length, in as many pieces as you need.

The formula in plain words

1. Convert length to inches: feet × 12.
2. Divide by the spacing in inches.
3. Round any fraction up.
4. Add 1 for the starting purlin.

The calculator rounds a leftover fraction up. When there is a leftover, the count includes a purlin at the far end, and the last gap is narrower than the rest.

Worked examples

A 40 ft run at 24 in, the default:

• 40 × 12 = 480 in
• 480 ÷ 24 = 20 gaps
• 20 + 1 = 21 purlin lines

A 14 ft slope length (eave to ridge) at 24 in on center:

• 14 × 12 = 168 in
• 168 ÷ 24 = 7 gaps, so 8 purlin lines per side
• A gable roof has two sides, so 16 lines in all

A 30 ft run at 16 in:

• 360 ÷ 16 = 22.5, rounded up to 23 gaps
• 23 + 1 = 24 lines, with the last gap about 8 in narrower than the rest, so check the end detail

Lines by length and spacing

Length	16 in	24 in	36 in	48 in
20 ft	16	11	8	6
30 ft	24	16	11	9
40 ft	31	21	15	11
50 ft	39	26	18	14

Where the division leaves a remainder, such as 40 ft at 36 in (13.3 gaps), the count rounds up to 14 gaps and 15 lines, so the last gap is narrower than the rest.

Turning lines into pieces

If each line runs the full building length, the number of pieces depends on the length you buy.

• A 40 ft building with 12 ft purlins: 40 ÷ 12 = 3.33, so 4 pieces per line (with splices at supports).
• A 40 ft building with 20 ft purlins: 2 pieces per line.
• Multiply pieces per line by the number of lines.

Purlins typically need to lap or splice over a support, so a design may add length. Check with your engineer or supplier.

Spacing and roof panels

The panel decides the maximum purlin spacing. Closer spacing supports thin panels and heavy snow, while wider spacing works with stiffer panels and lighter loads. Look at the manufacturer’s span table for your panel profile, and cross-check with local code. The metal roofing calculator counts panels once the framing is set.

Common mistakes

• Entering the footprint width when you want the slope length. A sloped roof is longer than it looks from above.
• Forgetting a purlin at both ridge and eave.
• Counting one roof face only for a gable roof.
• Using the spacing for roof sheathing when the plans call for something else.
• Skipping the engineer. Snow, wind and span all change the right spacing.

Related calculators

The roof truss calculator counts trusses with the same formula. The rafter calculator gives rafter length from run and pitch, which helps with slope length. Beam sizes are covered by the beam span calculator. A licensed engineer or the building supplier should confirm every structural member.