Purlin roofs

The majority of houses with traditional roofs are too deep for simple closed couple or collar roofs. Houses that are two rooms deep normally require roofs which can span at least 7m. The load from the roof coverings, snow and wind will cause the rafters to deflect, placing extra strain on the nails securing the rafter feet. In theory, it is possible to increase the span of a roof and prevent deflection by using ever-increasing rafter sizes. However, the use of very deep sections is both uneconomic and also creates handling problems on site due to their weight. In traditional construction this problem was solved by the introduction of purlins. Purlins are large timber beams which support the rafters’ span, thus preventing deflection. Purlins span from gable to gable (sometimes with intermediate support) and, besides supporting the rafters, they provide lateral stability to the gable walls and, in terraced housing, to the party walls.

The purlin roof is a very common type of construction, particularly in its simplest form, where it is used extensively in terraced housing.

As in an earlier example, some form of support will be required for the ceiling joists. This can be in the form of hangers, although it is more likely to be in the form of an internal loadbearing wall, which divides the front and rear bedrooms. As with timber floors, the ceiling joists are often in two pieces because long lengths of timber were not (and are not) readily available. This type of purlin roof is suitable for spans up to 8m or so.