Is the spacing set by the purlin or by the sheet?

By the sheet, far more often than people expect. A cold-formed purlin is cheap and strong for its weight, and on an ordinary building it is rarely the member that runs out of capacity first — what runs out is the cladding’s ability to span between supports without deflecting, oil-canning, or letting a fixing work loose under wind suction. That is why the profile manufacturer publishes a span table and why it, rather than the frame, usually decides the spacing. Two loads then pull in opposite directions at the edges: snow gathers in valleys and against parapets, while wind UPLIFT is fiercest at eaves, corners and verges, so the same roof may need purlins closer together at its perimeter for a reason that has nothing to do with weight bearing down.

Why is the row count one more than the number of spaces?

Because the rows are the lines and the spacing is the gap between them: five gaps take six lines. In practice the more useful direction is backwards. Purlin rows have to sit on the rafters of frames that were fabricated and priced weeks before anyone thinks about sheeting, so the spacing is normally settled by dividing the fixed frame centres down until the result clears the sheet's span rating — rather than choosing a spacing first and hoping the last row lands on steel.

Where does the maximum purlin spacing value come from?

It comes from the roof panel manufacturer's span table for the applied design load (snow, wind, live load) and panel gauge — it is not a fixed universal number, so confirm it against your specific panel product before using this calculator.

Does this account for purlin bending strength or lap requirements?

No — this only lays out how many purlin rows fit within your maximum allowable spacing. Purlin section selection (Z or C size and gauge) and lap/overlap design still need to be checked separately against the actual loads.

Materials & Quantities

Steel Z/C Purlin Spacing Calculator

Calculate the number of steel Z or C purlin rows needed along a metal building roof run.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Roof Run Length

The total roof run (eave-to-ridge or slope length) the purlins span across.

The distance the purlins are spaced ALONG, which is the slope length from eave to ridge and not the plan run — on a 6/12 roof those differ by 12%. If the drawing gives a plan dimension, convert it before entering. Both the eave and the ridge usually carry a purlin whatever the spacing works out to, which is why the count is one more than the divisions.

Maximum Purlin Spacing

The maximum allowable spacing between purlin rows, from the roof panel manufacturer's span table.

Check the metal roof panel's rated maximum support spacing for the applied load — this sets the maximum purlin spacing.

Purlin rows needed

15 rows

High confidence

Bays from eave to ridge: 14
Purlin row centres, eave to ridge: 4.71 ft

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Purlin rows = ceil(roof run length ÷ maximum purlin spacing) + 1, the standard layout method for spacing intermediate roof support members

Inputs used

Roof Run Length: 66 ft
Maximum Purlin Spacing: 5 ft

Intermediate steps

Bays from eave to ridge: 14
Purlin row centres, eave to ridge: 4.71 ft

Final result15 rows

What this calculation does not cover

The top and bottom rows in this count are not ordinary purlins. At the eaves the member is normally an eave strut or eave beam — a different section, supplied with the frame, carrying the gutter and the head of the side cladding — and at the ridge most systems need a PAIR of purlins, one either side of the apex, to land the sheet end on both slopes and fix the ridge cap. A single row at the top and a purlin priced at the eaves both leave the order wrong.
Where the sheeting is made up of more than one length up the slope, the end lap has to sit on a purlin and be fixed through it, so the lap line dictates a row position that an even division of the run will not produce. On a long rafter that means either an extra purlin or the whole layout reset from the lap downwards.
Rows are counted and nothing that holds them straight is. Z and C purlins are slender and torsionally weak, and they need sag rods, bridging or anti-sag ties between rows — commonly one line at mid-span and two over longer spans — to stop them rolling during sheeting and under uplift. The cleats, sleeves and bolts at every rafter sit outside this figure as well.

Estimated cost — your price

Price per row ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Purlin rows = ceil(roof run length ÷ maximum purlin spacing) + 1, the standard layout method for spacing intermediate roof support members

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Cladding a Portal Frame: Built-Up or Composite Paneluses this calculator
Twin skin built up on site against factory composite panel, and the spacer bar that quietly caps the U-value whatever thickness of quilt you buy.
Fitting a Radiant Barrier Under a Metal Roofuses this calculator
Foil lands in the gap between the steel gang and the sheeters, and the roll count, the lap allowance and the condensation risk are all settled before it does.
Spacing Purlins and Deck Supportsuses this calculator
Secondary framing set out so purlin rows, deck spans and sheet end laps land on frame lines the primary steel order fixed weeks ago.

Called something else where you work? Purlin — the term in each market, how close the equivalence really is, and the standard that governs it.

How to calculate steel Z/C purlin spacing in 3 steps

Roof Run LengthThe total roof run (eave-to-ridge or slope length) the purlins span across.
Maximum Purlin SpacingThe maximum allowable spacing between purlin rows, from the roof panel manufacturer's span table.
Purlin rows neededThe tool computes the purlin rows needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Purlin rows needed by roof run length

Page defaults, not your figures above.

Roof Run Length	Purlin rows needed (rows)
40 ft	10
60 ft	14
80 ft	18
100 ft	22
120 ft	26

Frequently asked questions

Is the spacing set by the purlin or by the sheet?: By the sheet, far more often than people expect. A cold-formed purlin is cheap and strong for its weight, and on an ordinary building it is rarely the member that runs out of capacity first — what runs out is the cladding’s ability to span between supports without deflecting, oil-canning, or letting a fixing work loose under wind suction. That is why the profile manufacturer publishes a span table and why it, rather than the frame, usually decides the spacing. Two loads then pull in opposite directions at the edges: snow gathers in valleys and against parapets, while wind UPLIFT is fiercest at eaves, corners and verges, so the same roof may need purlins closer together at its perimeter for a reason that has nothing to do with weight bearing down.
Why is the row count one more than the number of spaces?: Because the rows are the lines and the spacing is the gap between them: five gaps take six lines. In practice the more useful direction is backwards. Purlin rows have to sit on the rafters of frames that were fabricated and priced weeks before anyone thinks about sheeting, so the spacing is normally settled by dividing the fixed frame centres down until the result clears the sheet's span rating — rather than choosing a spacing first and hoping the last row lands on steel.
Where does the maximum purlin spacing value come from?: It comes from the roof panel manufacturer's span table for the applied design load (snow, wind, live load) and panel gauge — it is not a fixed universal number, so confirm it against your specific panel product before using this calculator.
Does this account for purlin bending strength or lap requirements?: No — this only lays out how many purlin rows fit within your maximum allowable spacing. Purlin section selection (Z or C size and gauge) and lap/overlap design still need to be checked separately against the actual loads.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.