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What is a purlin?

A purlin is a horizontal roof framing member that supports rafters or roofing. In wood rafter construction, it can shorten the unsupported rafter span. The 2021 International Residential Code, section R802.4.5, sets a rafter-based sizing rule; include the purlin’s size and bracing in a roof-framing assessment.

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By InstaBids editorial team with AI research agents. Updated

What this means for your home project

  1. On the roof framing drawing, mark the horizontal member crossing rafters or trusses; identify whether that is the purlin named in the proposed work. Source: Hansen Pole Buildings, read 2026-11-03.
  2. Request a written size schedule showing both the required rafter size and the proposed supporting purlin size, with the applicable code edition identified. Source: International Code Council, read 2026-11-03.
  3. Ask the roof-framing assessment to mark purlin continuity, brace size, brace angle and the bearing wall receiving each brace. A useful written scope identifies each of those items on the drawing. Source: City of Ozark, read 2026-11-03.
  4. For a post-frame roof, compare the proposed purlin spacing with the project’s snow-load specification and note whether that loading is documented. Source: FBI Buildings, read 2026-11-03.
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What is the difference between a rafter and a purlin?

A rafter slopes with the roof; a purlin runs horizontally across rafters or trusses. Hansen’s glossary distinguishes the sloping rafter from the horizontal secondary member. In a wood roof, the purlin can support the rafters partway along their span; it is a different member, not another name for the rafter. “A secondary horizontal framing member attached (generally at a right angle) to roof rafters or trusses.” Source: Hansen Pole Buildings, read 2026-11-03.

What size timber do I need for roof purlins?

For purlins supporting wood rafters, 2021 International Residential Code (IRC) section R802.4.5 sets the minimum at the required rafter size. Ask for the required rafter size and proposed purlin size together in the written assessment. Eureka’s 2022 California reference guide describes this span-breaking use; its local checklist does not establish the rule for your jurisdiction. “Purlins shall be sized not less than the required size of the rafters that they support.” Source: International Code Council, read 2026-11-03. “Purlins may be installed as per above to break spans.” Source: City of Eureka, read 2026-11-03.

How far apart should purlin braces be spaced?

The City of Ozark’s 2018 guide specifies braces no more than 4 feet apart, measured center to center, with no more than 8 feet of unbraced length. It also calls for continuous purlins and 2-by-4 braces to bearing walls at least 45 degrees above horizontal. Those are separate checks: spacing, brace length, and where the brace lands. “Purlins shall be continuous and shall be supported by 2-inch by 4-inch braces installed to bearing walls at a slope not less than 45 degrees from the horizontal.” Source: City of Ozark, read 2026-11-03. “The braces shall be spaced not more than 4 feet on center and unbraced length of braces shall not exceed 8 feet.” Source: City of Ozark, read 2026-11-03.

How far apart should purlins be for a metal roof?

For its post-frame building guidance, FBI Buildings recommends purlins no farther than 24 inches apart, measured center to center. It also tells readers to check the snow-load specifications. Treat that as guidance for the named construction system; it does not establish a spacing for every metal panel or convert the wood-brace spacing above into metal-roof spacing. “Purlins should be spaced no farther than 24’’ on center.” Source: FBI Buildings, read 2026-11-03.

What is the difference between C purlin and Z purlin?

Mona Steel distinguishes the sections by their flanges: C-section edges face the same way, while Z-section edges face opposite ways. Its guide explains that Z sections can overlap at supports to form a continuous run. For a metal roof scope, identify the section shape and its connection layout rather than specifying only “purlins.” “The difference is the flanges.” Source: Mona Steel, read 2026-11-03. “That continuity is a large part of why metal building roofs use Z sections.” Source: Mona Steel, read 2026-11-03.

What factors affect C and Z purlin prices?

For metal purlins, Western States Metal Roofing identifies job size, steel thickness and section size as pricing factors. Request those details in the material scope so the quantities and sections are clear. This is a list of factors from the manufacturer’s 2021 article, not a current price quote. “Metal purlin pricing is based on the size of the job, thickness of steel, and the size of the purlin.” Source: Western States Metal Roofing, read 2026-11-03.

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