Butt joint
Also called: butt, square butt, glued butt, rubbed joint
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A butt joint is the simplest joint in woodworking: one piece's end or side is placed flush against the end of the other, the two pieces meeting square to their length with nothing cut into either one to lock them together.
Names and variants
The butt joint is also known as the rubbed joint or the slipped joint, and in some regions, as Fairham (1920) noted, it goes by "slaped", pronounced "slayped". The straight end butt is the simplest form of the joint. In one variant, the end-butt joint's faces are cut at an oblique angle instead of square, which greatly increases the contact area, and a larger glued area makes the joint stronger; a further variant adds square-shouldered tenons, short projecting tongues with flat shoulders, to this oblique form, increasing the contact area further and resisting shifting and slippage. In Japanese joinery, the butt joint is classified as a type of scarf joint, a joint that lengthens two timbers by lapping their ends together.
Why it needs help, and how it gets it
Because end grain, the surface left where wood is cut across its fibres, glues poorly, a plain butt joint has nothing to lock the pieces together mechanically and relies on the glue alone; most butt joints, in fact, join a long-grain surface, running along the fibres, to an end-grain one, which is part of why the joint is weak unassisted. A cleat, a strip fixed across the joint to reinforce it, can be added; metal fish plates, flat metal strips spanning the joint, or nail-plate connectors, thin toothed metal plates, can sometimes replace timber cleats in a single-cleat butt joint. In a double-cleat butt joint, reinforced with two cleats set opposite each other, a cleat's length depends on its fixing type and the stress the joint carries. In one example of butt-jointing the ends of boards, three cleats were each trenched, cut with a shallow channel, roughly halfway through their own depth. Short splines, thin strips let into matching grooves in both pieces, spaced along a butt joint can replace one continuous spline. In a doweled butt joint, angling the dowels adds strength. Butt bolts, originally made to secure handrail butt joints, are now used generally to pull any butt joint tight and hold it there, as Ellis (1908) put it.
Making and using a butt joint
Making a well-fitting butt joint by hand requires skilled use of hand planes. Gluing the pieces together works with or without cramps, clamps that hold the joint shut, or other applied pressure, as Fairham (1920) recorded. To check a hand-planed joint for straightness, one board is turned against its mate while both sit in a vice, revealing any crowning, a convex high spot, or hollowing along the edge. In timber flooring, a tongue-and-groove joint avoids the leakage problem of a butt-jointed single floor. In sash-window casing, where a lining, the boards framing the window opening, extends beyond the head and pulley stile, the grooved upright that guides a sliding sash's counterweight, its edge is usually bevelled and mitred into the butt joint. In timber framing, fasteners alone must resist the thrust load in a butt joint, since nothing is cut to lock the pieces together; the drop-in pocket, also called a butt cog, is both the easiest to make and the most widely used joint of this kind, and a basic butt joint cut this way, level, may also include an added tail for a better grip. In an older, now-superseded view of riveted metal butt joints under compression, it was once thought that few rivets were needed, since the load passed directly from the end of one plate to the other, and the rivets served only to hold the plates in position.