Edge joint
Also called: edge-to-edge joint, rubbed joint, slot screwed joint, planed edge joint, jointing boards
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An edge joint joins two boards along their edges, usually with glue, to make a single wider board or panel. Because the boards meet long grain to long grain, the face of the wood running along its fibres rather than across them, the glued joint can be very strong on its own, without added reinforcement.
Preparing the edges
Preparation begins by truing the edges of both boards, planing them straight and square before glue-up. Traditionally this was done with the jointer, an extra-long trying plane, a hand plane long enough to true boards over their full length; Hasluck (1907) notes it was 'seldom used' once machinery took over the job. On a power jointer, alternating which face of each board rests against the fence as it passes through helps keep the finished panel flat once the boards are edge-glued. For hand-planed edges, twisting one shot board, a board whose edge has just been planed straight, against its mate in a vise reveals any crown or hollow, a slight convex or concave curve, left in the edge.
The rubbed joint
The simplest way to glue an edge joint is the rub joint: the two edges are coated with glue and rubbed together without any clamping. Fairham (1920) lists the joint's other names: 'the butt joint, the rubbed joint, the slipped joint, whilst in certain localities it is known as the slaped (pronounced slayped) joint.' A plain rubbed joint holds well on timber that glues easily and will not be put under much strain, but modern PVA and synthetic adhesives do not suit the traditional rubbed-joint technique.
Reinforcing the joint
Because a plain glued edge joint can already be strong on its own, reinforcement is optional rather than essential. Dowels are occasionally added, and woodworkers sometimes fit splines or biscuits, thin wafers or tongues of wood let into matching slots in both boards, to help line up the mating boards across the joint. A spline offers a fast, simple way to strengthen a glued edge joint, and laying one out from the marked reference face, the face used as the datum during preparation, needs only minimal measuring when the boards are already referenced from that face. A full tongue-and-groove, a continuous ridge on one edge fitting a matching groove on the other, strengthens the joint further and adds gluing area. A lock-miter joint, which interlocks two edges by cutting a matching stepped profile into each, is another option for joining flat panels specifically. Drawer slips, the small grooved strips fitted inside a drawer to hold its bottom panel, are an exception: with modern glue they must be clamped rather than rub-jointed.
Faults and finishing
Planing glued-up stock too soon after bonding can leave what USDA (2021) defines as a sunken joint: 'depression in wood surface at a joint, usually an edge joint, caused by surfacing material too soon after bonding.' Where the joint will be visible, one edge is commonly beaded, run with a small rounded moulding, to disguise the glue line. A board-jointing jig used to guide this work should be built from hardwood rather than softwood.