Loose Tenons attaching Butt Purlins

A ‘Loose’ tenon is used to attach in-line side purlins onto a roof truss principal rafter. It is also often referred to as a ‘Slip’ or ‘Free’ tenon.

One end of the tenon is attached to the end of a purlin, generally using wooden pegs. The tenon then passes through a mortice in the principal rafter and then attaches to a second purlin on the other side of the principal rafter, with a similar fixing. The tenon is not usually pegged or otherwise fastened to the principal rafter, and so is regarded ‘loose’ within the truss, or a ‘slip’ or ‘free’ tenon.  The ends of the purlins are usually housed within shallow recess in the principal rafter. This is enables the roof loads bearing on the purlin to be directly transferred to the principal rafter, rather than imposing any sheer load on the tenon itself. The only planned load applied to the tenon is tension as it hold the two purlins within the recesses, therby preventing the purlins from pulling away from the roof truss.

The map below shows the distribution of this type of loose tenon in Oxfordshire roofs surveyed by the OBR

A gazetteer of the buildings shown on the map is available in the members area of the website