Notched joint
Also called: notching, birdsmouth, single notch, double notch
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A notch joint is cut by removing a piece from one timber so that a second timber crossing it can sit flush against it, the notch sized to match the second timber's thickness.
The birdsmouth cut
The commonest notch in roof and stair work is the birdsmouth: an internal angle cut into the end of a timber so it fits over the salient angle, the outward-pointing corner, of a second timber it crosses. It lets a sloping rafter seat cleanly on the horizontal wall plate, the timber running along the top of a wall, that carries it. In large Dutch timber-frame barns the birdsmouth is commonly paired with a stub tenon, a short projecting peg that fits a matching socket, at the point where rafter meets plate. Sometimes the wall plate itself needs cutting back at a building's corner so the birdsmouth can seat square against it. In present-day light, stick-built roof framing it has become common practice to reverse the cut so it seats against the outer face of the plate rather than the traditional way round. A related framing joint, cut like a housed birdsmouth, differs from an ordinary mortise, the socket half of a peg-and-socket joint, in that its mortise can run across the grain rather than parallel to it. The same idea turns up in stair building, where a stair carriage's end can be birdsmouthed to seat over the floor plate, and the underside of a staircase's bottom tread may likewise be notched where it meets the riser, the upright board beneath the tread.
Notching as a shortcut
Notching often earns its keep by saving work elsewhere. In light wood-frame house construction, floor joists can be notched onto their support and nailed with enough strength that the supporting girder need not also be notched, and ceiling beams can be nailed to their supports just as securely without cutting a slip mortise and tenon, a stronger but slower joint with a projecting tongue let into a matching socket. A plain notch is not always the strongest choice, though: keeping the supported timber at its full depth over the point where it bears, rather than cutting it away, adds strength compared with a simple notch. A cog, a projecting block left standing on one timber that fits a matching notch cut in the other, can join two timbers about as effectively as a double notch does.
Notching in hipped roofs
Where hip rafters, the sloping corner rafters of a hipped roof, meet the wall plate, the usual solution is a dragon piece and angle tie: a diagonal timber, the dragon piece, let into an angled tie beam at the corner. Small hipped roofs are often built more simply, with a batten nailed diagonally across the two wall plates and the hip rafter notched onto it instead.
Notching in cabinet and shop work
Away from framing, notching does smaller jobs. A shelf's front corner can be trimmed and notched into a small shoulder that bears against the wood left standing by a stopped dado, a housing cut that stops short of the board's front edge rather than running clean through it. An end-lap joint, where two pieces cross and each is cut away by half its thickness, has a single shoulder formed by cutting a rabbet, a step-shaped recess, to half the stock's thickness. An edge lap, cut the same way but into the timber's width rather than its thickness, is weakened by that notching and offers little long-grain gluing surface, the strong glued bond that only forms along the direction of the wood fibres, so it is the weaker of the two. In egg-crate style divider joinery the interlocking notches can be sawn by hand and the waste chopped out with a chisel, or cut on a table saw fitted with a notching jig, a guide that positions each cut accurately. Pegs and a wedge fitted to a workbench can hold chair parts steady while they are drilled or notched, and power-saw blades and cutters can cut a birdsmouth just as readily as housing, halving, mitre and dovetail cuts, so the joint is as much at home on a machine as under a handsaw.
A joint with many hands on it
Notching has always left room for personal habit. Hasluck, writing in 1897, put it plainly: everyone who has taken any special interest in this branch of work develops peculiar notions of his own, and if notes were made of the different special methods of mitring, notching, dovetailing and the rest, a variety of curiosities would be discovered.