Underpurlin
An underpurlin is a horizontal timber beam under the rafters at mid-span, transferring load to a strutting beam or wall. Sized under AS 1684 span tables.
Ask Chalkline about this →An underpurlin is a horizontal timber framing member that sits below the rafters at mid-span, transferring load from the rafters down to a strutting beam, internal wall, or other supporting member below. It shortens the effective rafter span, so a smaller rafter section can carry the same roof load over the same horizontal distance.
Underpurlins are sized under AS 1684.2:2021 (non-cyclonic) or AS 1684.3:2021 (cyclonic), with span-table selection by rafter span, roof type (sheet or tile), wind classification, and the spacing of the supports the underpurlin itself bears on. The standard gives the size and stress grade combination directly; the chippy reads the table for the specific rafter span and load case.
An underpurlin is distinct from a purlin: a purlin sits above the rafters (or replaces rafters in a steel-framed shed) and supports roof cladding directly. An underpurlin sits below the rafters as an intermediate structural support. The names are similar; the structural roles are opposite.
Underpurlins exist in conventional cut roofs, where rafters span between the wall plate and the ridge with intermediate support. In trussed roofs there is no underpurlin: the prefabricated truss is engineered to span without mid-span support, and the underpurlin is replaced by the truss web members.
Also known as: under purlin, rafter support beam (informal).
Category: Roof framing / structural timber.
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Last updated: 2026-05-14. Verified: 2026-05-14.