Traditional Purlin Roof Construction

There are two basic methods of pitched roof construction.

Traditional purlin roof construction. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and or the building walls steel beams etc. It is a very common form of construction. This is a large timber running at right angles to the rafters and usually positioned mid span. A cut roof this is the traditional method of cutting the timber on site and building up the roof using rafters ridge boards joists and purlins etc the exact details being determined by the size of roof size of timbers etc.

Traditional roofs work by distributing loads from the rafters down through to the wall plate and then through to the walls below. 3 lean to roofs less common. Roofs and not for traditional purlin and rafter roof construction. All the figures are based on roofing tiles or slates laid on.

A truss roof using factory made trusses which are delivered to site complete and just erected. Building control guidance note purlin and rafter roofs timber sizes and construction details. The use of purlins as opposed to closely spaced rafters is common in pre engineered metal building systems and both the ancient post and beam and newer pole building timber frame construction methodologies. The best way to transfer load is with a timber to timber joint a compression fixing.

Common purlins are typically trenched through the top sides backs of principal rafters and carry vertical roof sheathing the key to identifying this type of roof system. In architecture or structural engineering or building a purlin or purline is a horizontal structural member in a roof. Nails bolts and screws create a mechanical fixing to stop uplift through wind etc.