Shelf
Also called: shelving, bookcase
One page. Other spellings and regional names stay on this URL.
A shelf is a flat, thin board fixed horizontally to carry things resting on it, usually held between the sides of a cabinet or bookcase, or fixed straight to a wall.
How a shelf is housed into its sides
The commonest way to hold a shelf between two sides is a housing, also called a trench: a wide groove cut across the grain that takes the shelf's full thickness. A stopped housing is the same groove cut so that it stops short of the front edge instead of running the shelf's whole width; Fairham (1920) shows the two side by side, one 'running the whole width of the shelf', the other a 'stopped housing, the groove coming to within' a set distance of the front. Most housings of this kind, including the common T-shaped shelf dado, the American term for a housing, and some rabbet housings, an L-shaped version cut at the end of a board, give glue nothing to bond to but end grain and cross grain, since there is no long-grain surface, the strongest kind of glued joint, anywhere in the joint. That is not usually a problem for holding the shelf up: a housed joint's resistance to being sheared, pushed sideways along the groove, suits the job of carrying a shelf's load well. It is a problem in the other direction: on its own, a housed joint has no mechanical resistance to being pulled straight apart, and needs some other feature, or a cabinet back fixed across the whole carcase, to add that stability. A router-built cabinet project illustrates the method directly: two housings are routed into a side wall, one for the bottom shelf and one for the centre shelf; once the cabinet is knocked apart again, glue is spread into both grooves before it is reassembled, and the lower shelf and its support are further fixed with countersunk screws driven through the cabinet sides.
Supports that avoid a housing
Not every shelf is let into a groove. A central shelf can instead sit on shaped angle supports screwed up from underneath into the shelf, with small spandrels, shaped filler brackets, fitted between the angle supports, glued, and likewise screwed up from below; or a shelf can simply be glued and fixed with screws driven in at a slant from underneath. A shelf bracket can be made to grip without any groove at all: a small spur on its front end bites into the underside of the shelf to stop it slipping forward, while a shallow mortise cut behind each slot in the wall strip lets the bracket's lug, the tongue that engages the slot, swing into position as it is fitted. A traditional alternative uses sawtooth strips, timber strips notched like a row of saw teeth, fixed to the sides, with splayed-end bearers that push into any tooth and so let the shelf height be reset wherever wanted. A lighter shelf can be fixed to a wall with nothing more than two brass plates of the kind used to hang mirrors.
Shelves that rest loose or slide
The simplest shelf of all is not fixed at all: it rests in grooves cut straight across the end pieces, and a groove is needed to hold a shelf like this in place rather than let it slide off. A sliding shelf works on a similar idea but tapered, and the taper can run its full length, though too tight a fit will stop the shelf sliding the whole distance it is meant to travel. For a bookcase built with a run of shelves at different heights, bookcase studs, a row of fixed pins, have one advantage over adjustable slips, a strip of holes taking a moveable peg: a stud does not require the thickness of both bookcase ends to be lined up with each other, as fitting adjustable slips does.
Setting out and finishing the edge
Good practice is to mark out the end pieces first, and then take the shelf's own size from the actual carcase rather than from a plan. Where a shelf's ends are tongued into its sides, the tongue is typically cut to about a third of the material's own thickness. A technique applied to the front edge can let a thinner board be used for the shelf while it still reads as a thicker one, and a small strip glued along a shelf's rear edge can close a gap that would otherwise be left showing behind a bearer. Where a shelf is joined with biscuits, thin oval wafers glued into matching slots cut in each piece, the shelf's face position is marked on the carcase side first, the biscuit centres are marked out from that, and the biscuit positions are then marked onto the shelf itself to match.
Choosing the material
Material affects how far apart the supports need to be. Particle board creeps, meaning it sags permanently under a load carried for a long time, more than solid timber does, so particle-board shelving needs its supports spaced closer together than solid boards of the same thickness would need.