A rafter is an inclined timber that forms the sloping structure of a pitched roof, running from the wall plate (a horizontal timber on top of the wall) at the eaves up to the ridge board at the apex. Rafters carry the weight of the roof covering (tiles, slates, felt, battens) and any imposed loads such as snow or maintenance workers. In a traditional cut roof, rafters are cut and assembled on site from C16 or C24 graded softwood. In a modern trussed rafter roof, the rafters form part of a factory-made engineered truss.

What are the different types of rafter?

Common rafters run from wall plate to ridge at regular centres (typically 400mm or 600mm) and are the main sloping members of a simple gable roof. Hip rafters run diagonally from the corner of the wall plate to the end of the ridge on a hipped roof. Valley rafters run diagonally into internal corners (valleys) where two roof slopes meet. Jack rafters are shorter rafters that run from the wall plate to a hip rafter, or from a ridge to a valley rafter. Principal rafters are heavier, wider-spaced rafters in traditional cut roofs that carry a purlin.

What size timber is used for rafters?

Common rafter sizes in domestic construction depend on the span (rafter length along the slope), spacing, and roof load. Typical sizes are: 47x100mm at 400mm centres for spans up to about 2.1m; 47x125mm at 400mm centres for spans up to about 2.6m; 47x150mm at 400mm centres for spans up to about 3.1m; 47x175mm for longer spans. Rafter sizes are set out in the TRADA Span Tables for Solid Timber Members in Floors, Ceilings and Roofs or must be calculated by a structural engineer. All structural roof timber should be C16 minimum; C24 where greater strength is needed.

What is the difference between a cut roof and a trussed rafter roof?

A cut roof (or traditional roof or couple-close roof) is built on site from individual timbers - wall plate, rafters, ridge board, collar ties, purlins, and hangers - cut and assembled by a carpenter. It creates a usable loft space. A trussed rafter roof uses factory-manufactured engineered timber trusses, delivered to site and lifted into place at regular centres - typically every 600mm. Trussed roofs are quicker and cheaper to build and require no site cutting, but the triangulated truss web fills the roof space, making it unusable as habitable space. Converting a trussed rafter loft requires a full structural redesign.

Structural noun

Rafter

/ ˈrɑːftə /

Also known as: common rafter, hip rafter, valley rafter, jack rafter, principal rafter

Definition

A rafter is an inclined structural timber forming the slope of a pitched roof, running from the wall plate at eaves level up to the ridge board at the apex. Rafters carry the roof covering (tiles or slates, battens, roofing felt or underlay) and any imposed loads from snow or maintenance. In a traditional cut roof they are cut and fitted on site; in a modern roof they form part of factory-made trussed rafters. Standard sizes range from 47x100mm to 47x175mm depending on span and spacing.

In practice

Common rafters are set at 400mm or 600mm centres, bird's-mouth notched over the wall plate at the eaves, and nailed or skew-nailed to the ridge board at the apex. The bird's-mouth cut must not exceed one third of the rafter depth - a deeper notch significantly weakens the rafter at its most critical point. Ceiling joists (or collar ties) tie pairs of opposite rafters together to prevent the roof from spreading and pushing the walls outward - this horizontal thrust is one of the key loads a pitched roof transfers to the walls below.

In a hipped roof, hip rafters run diagonally from each corner of the wall plate to the end of the ridge. Jack rafters fill in between the hip rafter and the wall plate. Hip and valley rafters are typically 50mm deeper than the common rafters they support. Where rafters are long (over about 2.5m on the slope), a purlin provides intermediate support, reducing the effective span and allowing smaller rafter sections. A strutting beam or dragon tie transfers purlin loads back to the gable or a load-bearing internal wall.

Building Regulations

Relevant PartPart A - Structure

Span tablesTRADA Span Tables for Solid Timber Members in Roofs

Timber gradeC16 minimum; C24 for longer spans or heavier loads

Bird's-mouth depthMust not exceed one third of rafter depth

Approved Document A requires roof structures to be designed to carry all expected loads - dead load (roof covering), imposed load (snow to BS EN 1991-1-3), and wind load. Rafter sizes for simple domestic roofs can be selected from TRADA Span Tables without further calculation. For non-standard roofs, complex geometries, or heavier roof coverings (green roofs, concrete tiles on steep pitches), structural engineering calculations are required. Changes to an existing roof structure - including loft conversions, adding dormers, or replacing roof coverings with significantly heavier materials - require Building Regulations approval.

Full Building Regulations guidance

Rafter

Just so you know...