Graphical Construction Glossary >> Roofs and roofing. >> Roof Framing >> timber purlin

Timber purlin
A horizontal timber or lumber roof member that supports the common rafters, or horizontal roof member to which roofing is fixed, either on the rafters or on trusses etc.

timber under purlins

Seldom seen in new work but there are many great historical roofs far grander that the sketch above that rely on timber under purlins.

under purlins

In standard house roof framing under purlins are commonly placed at mid span of the rafters and propped or strutted off internal walls.

lumber and steel

Before the introduction of cheap cold rolled roofing sections came on the market it was common to use a combination of steel beams and lumber purlins. In the above sketch the joints of the purlins is shown in two ways.

  • The double cleat with the simple joint is all to often seen but it is a poor choice.
  • The bolts through the timber should have minimum edge distances, say 12mm bolts in 50 thick hardwood purlins should be at least 80 from the end, making of quite a big double cleat.
  • It was soon found that lapping the purlins at the joint was far better and stronger, with the single cleat shown.
  • However timber being in finite lengths it is better to make the most of the lengths available and splice a shorter piece of the same material across the joint.
  • The splice timber is bolted through the joint withe two bolts towards each end and the lot is then bolted though a double cleat.

This hassle of short lengths of timber and joining them to the steelwork must have been the driving force for the development of lightweight engineered steel purlins.

There are a whole range of timber engineered laminated and hybrid steel and timber beams made that are very suitable for rafter support purlins provided that there is enough space available.

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"When we build, let us think that we build for ever."John Ruskin 1819-1900

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