Roof Framing Detail

Roof framing detail for a building regulation application

The term pitched roof includes any roof whose angle of slope to the horizontal lies between 10 and 70 degrees, below this range it would be called a flat roof and above 70 it would be classified as a wall.

The pitch is generally determined by the covering that is to be placed over the timber carcass whereas the basic form is governed by the load and span. The terminology used in timber roof work and the basic members for various spans are shown in the detail.

The cost of constructing a pitched roof is greater than for a flat roof, but the pitch will create a useful void for locating cold water storage cisterns and storage. The timber used in roof work is structural softwood, the members being joined together with nails.

The sloping components or rafters are used to transfer the covering wind rain and snow loads to the load bearing walls on which they rest. The rafters are sometimes assisted in this function by struts and purlins in what is called a purlin or double roof. As with other forms of roof the spacing of the rafters and consequently the ceiling boards that are to be fixed on the underside of the joists.

Roof Framing Detail Terminology

Common rafters

Common rafters are the main load bearing members of a roof they span between a wall plate at eaves level and the ridge.

Rafters have a tendency to thrust out the walls on which they rest and this must be resisted by the walls and the ceiling joists.

Wall plates

These provide the bearing and fixing medium for the various roof members and distribute the loads evenly over the supporting walls.

Ceiling joists

Ceiling joists fulfill the dual function of acting as ties to the rafters and provide a surface on which to attach a plaster ceiling to.

Purlins

These provide additional support o the common rafters.

Where large spans are required purlins structural engineers will specify a beam to provide extra rigidity.

Collars are placed at purlin level to assist also.

Eaves

There are two basic types of eaves finish: open eaves and closed eaves.

The former is less expensive, with rafters left exposed on the underside treated with preservative.

With both, space between the rafters and the roof covering receives insulation over the top of the wall.

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Roof material details

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