Shelf span and the sag you can avoid

Every sagging bookshelf was built to a length that looked reasonable and was never checked against the load. Books are heavy, roughly twenty kilos to the running metre for hardbacks packed tight, and a shelf carries that weight continuously for years.

Sag increases very sharply with span. Doubling the distance between supports does not double the deflection, it multiplies it many times over, which is why a shelf that is fine at eight hundred millimetres becomes a hammock at a metre and a half.

Rules of thumb worth keeping

For books, a support every 750 to 800 millimetres is a safe figure for solid timber around twenty millimetres thick. Shelving grade plywood of the same thickness behaves similarly. Particle board with a melamine face is much weaker and wants a support every 600 millimetres or a good deal less load.

Making a long shelf stiff

Depth does almost nothing for sag. Thickness does a great deal, and adding a strip along the front edge does more still. A batten of twenty by forty millimetres glued and screwed under the front lip turns a floppy shelf into a rigid one, because the effective depth of the beam is now the shelf plus the batten.

The same trick works at the back against the wall, and it can be hidden completely. If the front edge must stay slim for the look of the piece, a hardwood lipping glued on edge to the front of a plywood shelf gives a good deal of the stiffness while reading as a plain board.

One last detail: a shelf that has already sagged will not recover if you simply unload it. Turn it over and reload it, and the timber will slowly bend back the other way.