Rafter

A rafter is one of the inclined timbers (or steel or engineered-wood members) that form the sloping planes of a pitched roof. Each rafter runs from the top of the roof, at a ridge or hip, down to the wall plate at the eaves, and together the rafters carry the sheathing or battens, the roof covering, and loads such as snow and wind. A matched pair of rafters meeting at the top is traditionally called a couple.

Rafters are cut and installed on site, which distinguishes a "stick-framed" or "cut" roof from one built with factory-made trusses. In houses they are usually softwood.

Types of rafter

• Common rafter: the ordinary, full-length rafter repeated along the roof. In historic "double" roofs it is lighter than the principal rafters that carry the purlins.
• Principal rafter: a heavier rafter, usually part of a truss, that supports purlins, which in turn support common rafters.
• Hip rafter: runs diagonally at the external corner where two roof slopes meet in a hip.
• Valley rafter: runs diagonally along an internal corner where two slopes meet in a valley.
• Jack rafter: a shortened rafter that ends on a hip or valley rafter, or is cut short by an opening such as a dormer.

Thrust and restraint

A pair of rafters loaded from above tends to push the tops of the walls outward. In the simplest historic roofs a tie beam across the bottom of the rafters resists that spread; in modern houses ceiling joists or rafter ties usually do the same job. A collar higher up can stiffen the pair. Where neither is present, a structural ridge beam must carry the upper ends of the rafters instead.

Historical and regional notes

The earliest surviving European roofs use common rafters on a tie beam, an assembly known as a closed couple. A short extra piece at the foot of a rafter, used to create a kick in the eaves or change the pitch, is called a sprocket in British usage (coyau in French). Part of a cruck frame that acts as a rafter is called a cruck blade.

Historically, many rafters tapered toward the top, a practice period writers argued saved timber.