A purlin is a horizontal structural member running along the length of a roof slope, parallel to the ridge and the wall plate, positioned midway (or at one-third and two-third points) up the rafter. It provides intermediate support to the common rafters, reducing their effective span and allowing smaller rafter sections than would be needed for the full unbroken rafter length. Purlins are themselves supported by struts rising from internal load-bearing walls, by hangers from the ridge, or - in steel purlin systems - by the principal rafters or gable walls.

How is a purlin supported?

In a traditional domestic cut roof, timber purlins are typically supported by raking struts (inclined timbers) rising from a load-bearing internal wall or from a binder over the ceiling joists. The strut angles are typically 35-45 degrees to the horizontal. The load path runs from rafter to purlin to strut to internal wall or floor. Where no load-bearing internal wall exists below the roof, purlins may be supported by steel hangers from a ridge beam, or by a structural engineer-designed alternative. The purlin itself must be sized to carry the rafter loads over its own span between supports.

What size is a typical domestic purlin?

Typical domestic timber purlin sizes range from 75x175mm to 75x225mm, depending on the span between supports and the spacing and loading of the rafters bearing on it. For a simple semi-detached or terraced house with a central party wall providing purlin support, spans of 2-3m are common and 75x175mm or 75x200mm sections are typical. For larger or open-plan roofs with longer purlin spans, larger sections or steel purlins may be needed. Purlin sizes should be confirmed by reference to TRADA Span Tables or by a structural engineer.

Are purlins used in trussed rafter roofs?

Traditional purlins are not used in standard trussed rafter roofs - the engineered truss is designed as a self-contained unit carrying roof loads without intermediate support. However, purlins are used in attic truss roofs (where the central truss web is raised to create habitable space) and in hybrid roof systems. Purlins are also used on the underside of roof slopes to carry insulation between rafters in warm roof constructions. When converting a trussed rafter loft to habitable space, the trusses must be replaced with a purpose-designed structural system including purlins and ridge beams.

Structural noun

Purlin

/ ˈpɜːlɪn /

Also known as: roof purlin, common purlin, purlin plate, side purlin

Definition

A purlin is a horizontal structural member running along the length of a roof slope, parallel to the ridge and wall plate, providing intermediate support to common rafters and reducing their effective span. It is typically positioned midway up the rafter slope in a domestic cut roof. Purlins are supported by raking struts from internal load-bearing walls, by hangers from a ridge beam, or by the gable wall at each end. Without a purlin, rafters on longer slopes would require much deeper sections.

In practice

In a typical domestic semi-detached or terraced house, the purlin spans between the gable walls (or between a gable wall and the party wall), with a raking strut rising from the party wall or an internal load-bearing wall to support it at mid-span. The strut angle should be 35-45 degrees to the horizontal for efficiency. The purlin sits in a notch cut in the underside of the rafters (or is bolted to the side face) and is fixed with skew nails or proprietary joist hangers.

During loft conversions, the existing purlins and their struts are typically in the way of the new habitable space. They cannot simply be removed - the rafter loading they carry must be redistributed via new structural members (ridge beams, steel frames, or attic trusses) designed by a structural engineer. Altering or removing purlins without engineering input is one of the most common structural errors in domestic loft conversions, sometimes leading to roof spread and wall cracking years after completion.

Building Regulations

Relevant PartPart A - Structure

Span tablesTRADA Span Tables for Solid Timber Members in Roofs

Strut angle35-45 degrees to horizontal for maximum efficiency

Loft conversionStructural engineer required before altering any purlin

Purlins are structural elements covered by Approved Document A. Any alteration to a purlin - repositioning, removing, or upsizing - requires Building Regulations approval and, in practice, a structural engineer's design. Loft conversions that alter the roof structure must have full Building Regulations approval before work begins. The struts supporting the purlin transfer loads to the walls below - these walls must be capable of carrying the additional load, and the foundations below them must also be adequate for the increased load.

Full Building Regulations guidance

Purlin

Just so you know...