A shelf bends under its load, and the bend grows fast with length: doubling the span makes the sag about eight times greater. That is why the board-build designer refuses a 3/4 in pine shelf longer than about 32 in without a support.
Rules of thumb for a loaded shelf
Rough spans for a shelf of books (about 20-25 lb per running foot) with visible sag kept to about 1/8 in:
| Shelf | Span |
|---|---|
| 3/4 in pine or poplar | up to about 30-32 in |
| 3/4 in oak, maple or other dense hardwood | up to about 36 in |
| 3/4 in plywood | about 30 in |
| 3/4 in MDF or particleboard | about 24 in |
| 1-1/2 in (two-by) softwood | about 48 in |
Lighter loads, such as ornaments, allow longer spans; heavy loads such as records or paper need shorter ones.
Making a span stiffer
- Make it thicker. Stiffness rises with the cube of thickness: a 1 in shelf is about 2.4 times as stiff as a 3/4 in one.
- Add an edge strip. Gluing a 1-1/2 in wide strip of the same wood along the front edge, standing on edge, turns the shelf into a shallow beam and roughly doubles the load it takes.
- Add a support. A centre divider or a back rail screwed under the shelf halves the span. On the designer, add a divider part between the sides.
- Fix the shelf. A shelf glued and screwed into the sides or set into dadoes sags less than one resting on pins, because its ends are held.
Choosing the wood
The wood database lists hardness and a movement class; stiffness follows density fairly closely, so oak, ash, maple and beech make stiffer shelves than pine, poplar or cedar at the same thickness. Quartersawn boards stay flatter than flatsawn.
Sag that appears later
Wood creeps: a shelf that looks fine when loaded can bow more over a year. Design for the span in the table rather than for what looks all right on the day.