Scarf Joint for Wood: Elegance and Strength

Scarf joint with a wedge, in wood.

Scarf joint with a wedge, in wood.

I have seen this joint in old wooden barns and houses in Britain on TV, and always wondered how and why it was done. Was it difficult to do? A random youtube video showed me how. While I do not build houses, from an engineering viewpoint it is educational to understand why it is so strong. No sources state the origins of this joint.

The scarf joint is elegant and also strong. The joint can be planed down flush. There is a lot of long grain glue surface area.

The scarf or scarph joint os described in Wikipedia. The joint is mirror image on each side, with a middle sliding double wedge. As the wedge is hammered in it pushes one wood beam against the other, compressing both sides, locking the two beams together. It is very elegant and relatively simple to create. Here is a diagram, exactly as in the youtube video

The scarf joint joins two beams in an elegant and simple way. A middle double sliding shim puts both beams in compression, locking the joint into place. The glue surface is also quite large. The joint can then be reinforced with pegs or bolts, going through both beams.

The scarf joint joins two beams in an elegant and simple way. A middle double sliding shim puts both beams in compression, locking the joint into place. The glue surface is also quite large. The joint can then be reinforced with pegs or bolts, going through both beams.

The video instructs to make the length of the scarf 3 times the width of the beam. He used a 1×8 on long edge as the guide, using both sides of the board. The joint was marked out and cut on one beam, the second beam aligned straight below, and then marked, after which he did the second beam cut. In the first beam he cut out the middle plug for the added double sliding wedge. The double sliding wedges are simply angled wood wedges, similar to the ones you use when aligning a door frame, pushing against each other, one on each side. Interestingly he screwed one wedge into one side and then just slid the wedge into the other side. Here’s a scarf joint in wood.

Scarf joint with a wedge, in wood.

Scarf joint with a wedge, in wood.

Scarf joint with a wedge, French source

Scarf joint with a wedge, French source

The guy in the video used a circular saw to do the cuts, up to the end lines, and then used a hand saw to square up the circular cut.

This is one cool joint! I need to make one.

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