Treenails

Treenails, pronounced 'trennels' and called 'chevilles bois' or 'chevilles charpente' in French, are a key part of traditional carpentry, joinery and boat construction. Structural beams, rafters and braces are linked using mortise and tenon joints. That is where one element has a 'tongue' (tenon), which fits into a slot (mortise) in the other element. The joint is then fastened with a treenail, which is pushed perpendicularly through all the layers of wood by way of carefully drilled rabbet holes.

The grain of the treenail must be at right angles to the grain of the beam with the mortise, and the diameter of the treenail needs to be fractionally bigger than the hole into which it will be forced. In France both ends of the treenail generally protrude from the joint ie they are usually not cut flush. They are not glued. Treenails are structurally sound in spans up to 9 metres, but cannot be guaranteed beyond that.

A hard but flexible wood such as Robinia (Fr. Acacia) is traditionally used, but Oak and Chestnut can also be found (and would have been used before Robinia arrived in Europe in the 17th century). Treenails are subjected to tremendous force and must be able to withstand the natural movement in the carpentry joint as the Oak beams dry and twist. As a result they can't generally be removed, except by drilling out, and they certainly can't be reused, as they will have deformed in situ.

Before mechanisation treenails were made by splitting the wood then shaping with a drawknife. As a result they will be slightly faceted. Modern ones are about 35 mm in diameter and 24 cm long. The old ones in our house are variable and between 15 and 25 mm in diameter, and 21 to 24 cm long.

The smaller versions of treenails used in furniture making are known as dowels (tourillons in French) and are generally glued.

 

A broken treenail.

The photos are all of treenails in our attic, so are 19th century and handmade.