lumbermadeHow far can a shelf span before it sags?
For a shelf loaded with books, keep 3/4 inch plywood to about 32 inches between supports, and MDF or particle board to about 24 inches. Go longer and the shelf will bow over time.
Maximum spans for a 3/4 inch shelf
These are conservative spans for shelves about 11 to 12 inches deep carrying books, which are one of the heaviest things people store. Lighter loads like folded clothes or decor can go a little further.
| Material | Books | Light items |
|---|---|---|
| Plywood (birch, pine) | 32 in (80 cm) | 36 in (90 cm) |
| Solid pine board | 28 in (70 cm) | 34 in (85 cm) |
| MDF | 24 in (60 cm) | 28 in (70 cm) |
| Particle board or melamine | 24 in (60 cm) | 28 in (70 cm) |
Thinner material spans much less. A 1/2 inch plywood shelf should stay under about 20 inches for books.
Why shelves sag
Wood and wood composites creep. A shelf that looks flat on day one slowly bends under a constant load, and MDF and particle board creep more than plywood or solid wood. The deflection grows quickly with span, so a shelf twice as long sags far more than twice as much.
Four ways to stop sag
- Add a divider. A vertical panel in the middle halves the span. This is the simplest fix and it also stiffens the whole unit.
- Glue a strip to the front edge. A 3/4 by 1 1/2 inch hardwood strip glued and screwed under the front edge makes the shelf far stiffer.
- Fix the back edge. Screwing the shelf to a back panel or a rail along the wall stops it bending at the back.
- Use fixed shelves. Shelves screwed into the sides are stiffer than shelves sitting loose on pins.
lumbermade checks every shelf in a design against these limits and warns you, or adds a divider, when a span is too long.
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