A guide to choosing the right roofing systems

A sloping tiled roof is the most common style of construction, whether your house is four years old or four hundred years old. The covering may be roof slates or tiles, and the shape could be a simple up and down slope or a more elaborate combination of pitches, valleys and hips. Here is a guide to choosing the right roofing systems.

The anatomy of a pitched roof

A roof that is pitched means simply that it slopes. A single pitch rests on a wall at one end and slopes down to the eaves, but most pitched roofs are the duo-pitch type with two slopes that meet in a ridge. The ends may be flat (a gable end) or sloping (a hipped end). A valley is formed at the place where two sections of roof change direction.

Purlin roof

On this type of system, a horizontal beam called a purlin runs between the gable ends found along each slope of the roof, midway between the ridge board and the eaves. This system effectively provides extra support for the rafters, and so it should be used in house constructions where extreme pressures may be placed on the rafters. However, by using this purlin beam, the unsupported span of the roof increases to about 7 m.

Prefabricated roofs

Since the 1960s, almost all modern roofing solutions have used prefabricated roof trusses in their construction. Each truss combines rafters, joists and struts, constructed in a W-pattern in order to create an extremely strong frame that can span the external walls of the building with no need for internal
load-bearing walls. The triangular structure is strong enough to support the roof covering. So, the timbers used can be slimmer in cross-section than those in a traditional purlin roof reducing cost and weight, and making them easier to fit. Construction The trusses are positioned 600mm apart, and are nailed into place. There is no ridge board, but horizontal braces are fitted where the W-shaped internal supports meet the rafters and the ceiling joists. Diagonal braces are added across the underside of each section of the roof slope, running from the ridge to the eaves. These brace the structure to keep it from collapsing sideways. The roof structure is tied to the house walls with galvanised steel straps so as to prevent the roof from lifting, or gable walls collapsing in high winds.