The scarf joint was born of necessity when carpenters needed longer timbers than their forests could provide. The first documented example comes from the remains of a 7th-century Anglo-Saxon burial ship that was discovered in an English riverbank. A scarf was used to join stem and stern timbers to a center keel. Dozens of scarf joints have been documented over time. Some have failed miserably, while others have survived centuries. The double-bladed scarf, first introduced in the 16th century, is the strongest joint for joining two timbers. Commonly used in a horizontal application (over a post, for instance), it also can be used vertically.
This particular timber-frame joint has long been a favorite of mine because of its decorative as well as its structural power. To make it stand out more, I increase the length of the blades from the usual 1-1/2 in. to 5 in.
Accurate layout is critical if you don't want to spend all day fitting the four face cuts on each half of the joint. It is a time-consuming process, but my speed picks up on the second and third joints. In a completed frame, the scarf is an eloquent representation of the time spent and the level of my craft.
Layout is based on a centerline
Typically 20 in. to 30 in. long, the completed scarf consists of two identical halves. Each has a 5-in. by 1-1/2 -in. blade, a 14-in. space, and a corresponding 5-in. by 1-1/2 -in. housing. Once assembled, the joint can be pegged through the blades.
Mark the end of the cut to reduce tearout
The joinery is as decorative as it is structural, so it’s important to have clean cuts. With that in mind, use a chisel to score the end grain at the end of the housing cut. Next, rip the centerline from both sides; the mortise will clean out anything left by the stopped cuts.