The Common Rafter and Purlin Roof

As a civil engineer or roof contractor, you should aware of the construction of a roof and trussed roofs also types of roof construction in the industry. So, here our new article about roof rafter and purlin roof related to roof construction.
The structure is most commonly used where there is a gable at both ends of the roof and is frequently to be found on terraced houses, as indicated in the below picture.

This simple form of roof is illustrated in below picture.

Its construction has been included here because of the now very common refurbishment of such houses.
The wall plates are often simply bedded on mortar on either the inner skin of a cavity wall or, as is often the case with older terraced houses, on the inside edge of a solid 9 in. brick wall. Wall plates should be half lapped where they meet, and should not be less than 75 mm wide and 50 mm thick. They should be treated with preservative. Below picture shows typical plate connections.

Purlins In some of the older houses purlins were placed at right angles to the rafter. A more effective construction results with the purlins truly vertical, for three reasons,

(1) The purlin is easier built-in or set in hangers at the gable walls.

(2) The purlins allow the rafter to be birds mouthed over them, thus avoiding the tendency for the rafter to slide off the roof. A notch in the rafter can be used on sloping purlins but a birds mouth is easier to locate and a quicker joint to cut on site.

(3) The sloping purlin has a tendency to sag down the roof slope thus necessitating a much thicker timber to maintain a true line. Following picture illustrates this point.

purlin deflection

A common problem with this type of roof is the tendency to stretch the purlins structurally close to their design limit, so achieving maximum economy on the section of the purlin to be used. This sometimes can result in roof sag caused by deflection of the purlin, although the deflection may be within design tolerances. There are two ways of overcoming this problem.

One is to design a stiffer purlin, i.e. probably one or two sizes up from the design table solution, the other is to stiffen the purlin using purlin struts, as illustrated in below picture. The latter is to be preferred for, although slightly more labour intensive, it does allow ultimate economy in timber section and the struts give a stabilising effect to the walls supporting the purlins.

One final point on purlins: care should be taken with regard to the Fire Regulations when building purlins in to dividing or party walls between terraced dwellings. Unfortunately the approving authorities vary somewhat from area to area in their approach to timber built in to what are essentially fire walls between the dwellings, some allowing timber to be built in provided there is a positive fire break between the ends of the purlins, whilst others simply do not allow timber to be built in at all.

In such cases built-in steel shoes will be necessary as indicated in above picture. On longer spans of purlins it may be necessary to use prefabricated beams, these being dealt with in more detail under the attic roof solution later in this chapter, the beams themselves being illustrated in below picture. An alternative solution on long spans is of course to provide an intermediate support for the purlin by means of a post which, in turn, is directly supported from a structural wall below.

alternative purlin beam constructions

Common Roof Rafters

Little needs to be said about the common rafters as these can be simply designed from the span tables. However on a very long roof slope it may not be practical to obtain timbers in one continuous length. On roof slopes in excess of 4.8 m a second purlin should be considered as illustrated in below picture. A collar should be fitted on every pair of rafters immediately beneath the ridge, and a further collar should be fitted to every other pair of rafters immediately above the purlin position.