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- the exterior top covering of a building, the internal covering being the ceiling. Roofs are usually described in one of two ways, either by external shape eg, gambrel roof, or by the principal structural component eg hammer beam roof. When a roof is described by exterior shape, it implies a plain interior. Open timber roofs evolved from the early use of crucks to encompass superb design and construction as spectacular as the best vaulting.

Glossary - external shape
Gabled - see gable. A ridged roof which finishes at each end on a gable.
Gambrel - a ridged roof finishing at each end on a small vertical gable below which the roof slopes to meet the gable wall.
Half-hipped - a ridged roof finishing at a gable of which a small section of the top part is angled or hipped, the rest vertical. In effect the opposite of gambrel. See jerkin head
Helm - a four sided roof, rising to a point, and sitting on four gabled walls.
Hipped - where the ends of the roof are sloped rather than vertical. see piended
Lean to - a roof with one slope only, built against a vertical wall.
Mansard - a roof with a double slope in which the top part is shallower.
Pavillion roof - a roof that is hipped at both ends.
Pitched roof - refers to almost any roof built at an angle, the pitch refers to the slope of the roof.

Structural components
Ashlar piece - a short vertical timber which runs down the inner face of the wallhead. It is morticed into a rafter at a point near its foot, and is also usually connected to the rafter by a timber spanning the wallhead, which effectively forms a timber triangle. The ashler post, in pressing against the wallhead, prevents the roof from spreading outwards.(rather confusingly it has nothing to do with ashlar stonework) (illustration).
Braces - pieces of reinforcing timbers set diagonally between structural members. An arched brace comprises two curved timbers forming an arch.
Collar beam - a horizontal timber which ties rafters together at a height above the wall plate, ie above the level of a tie beam.
Crown post - a vertical post standing on the centre of a tie beam and supporting a collar beam.
Hammerbeam roof - a hammerbeam is a horizontal timber projecting at wall head level towards the centre of the roofspace, like a tie beam without a central section. Supported on corbels, they carry a vertical timber, the hammer post, which supports a purlin, and is usually braced to a collar beam.
King post - a vertical timber rising from the centre of a tie beam to support the ridge.
Principals - the main inclined timbers in a roof.
Purlin - a continuous horizontal timber running parallel to the ridge, sometimes referred to as side timbers.
Queen post - a pair of vertical timbers rising from a tie beam to support purlins.
Rafter - a timber sloping from the ridge to the wall head and supporting the roof coverings. An angle rafter supports hip ends.(illustration).
Ridge - the line formed where two sides of a sloping or pitched roof meet. The ridge piece is a main timber running along the apex of the roof.
Sprocket - a short timber placed at the foot of a rafter to project over the wall head.
Tie beam - a horizontal beam which joins the feet of the principals at wall head level.
Truss - name given to rigid frame of principals and tie beams or collar beams.
Wall plate - a timber laid along the top of a wall, rafter ends and studs and posts from below may be joined to it.