Structural component
What is a purlin?
A word that means one thing in a wood-framed attic and another under a metal roof — in both cases a horizontal member that helps carry the roof.
Updated September 12, 2026 · Every figure links to its source · Sources
DefinitionComponentAlso called: roof purlin, purlin brace
Purlin — a horizontal secondary structural member of a roof that runs parallel to the ridge: in wood framing, a timber set beneath the rafters and braced down to a bearing wall to shorten the rafters' span, as described in the 2021 IRC, Section R802; in metal roofing and pre-engineered buildings, one of the members spanning between the main frames to which the roof panels are fastened, as the Metal Construction Association uses the term.
In a wood-framed roof
Rafters are sized for their span. When the span from ridge to wall is longer than the lumber can carry, the framer can use bigger rafters or support them partway: a purlin — a timber at least as deep as the rafters it supports — is run under the rafters, parallel to the ridge, and braced down to a bearing wall below with struts at an angle. The IRC, Section R802 sets the rules: the purlin sized no smaller than the rafters, the braces spaced along it and set at not less than 45 degrees to the horizontal, and the struts bearing on a wall or beam, not on a ceiling joist. The result is two shorter rafter spans in place of one long one.
In an older house you will see the purlin and its struts from the attic as a horizontal beam halfway up the underside of the roof with diagonal braces running down to a partition. Cutting or removing them to open up an attic is a structural change, not a carpentry tidy-up.
Under a metal roof
In a pre-engineered metal building the primary structure is a row of rigid steel frames; the purlins are the lighter steel members — usually cold-formed C or Z sections — that span from frame to frame across the roof, and the roof panels are screwed or clipped to them. On wood-framed barns and sheds with metal roofing, "purlins" are the horizontal boards nailed across the rafters that the panels fasten to, in place of solid sheathing. The MCA and the Metal Roofing Alliance cover both: the purlin spacing, the panel's span rating and the fastener pattern together decide whether the roof holds in wind.
On a house, metal panels are usually installed over solid sheathing and underlayment rather than open purlins; the IRC, Section R905.10 allows metal roof panels over either, to the manufacturer's instructions, and the directory's guide to metal roofing explains the difference for a homeowner.
Why it matters
A purlin is part of the structure, and structure is the one part of a roof a roofing estimate does not cover. If an inspector notes a sagging ridge, spread walls or a missing purlin brace, the fix is a carpenter's or an engineer's, and it should be done before the new covering goes on. Under a metal roof, purlins that have rusted, or panels fastened between purlins rather than into them, are why metal roofs on outbuildings lift in wind. The directory's guide to a new roof installation covers what a re-roof includes — and structural repair is usually an extra.
Frequently asked questions
Short answers; the sections above carry the detail and the sources.
What is the difference between a purlin and a rafter?
Can I remove a purlin to open up my attic?
Can a metal roof be installed on purlins instead of plywood?
Next step
Find a roofer who does this
Structural questions come before roofing questions, and a good roofer will say so. The directory lists roofing contractors by state and city.
Sources
- 2021 International Residential Code, Chapter 8: Roof-Ceiling Construction — International Code Council (ICC)
- Metal Construction Association — Metal Construction Association (MCA)
- Metal Roofing Alliance — Metal Roofing Alliance (MRA)
- 2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)