Fix your mattress · Updated September 2026
Should I Put Plywood Under My Mattress?
Plywood under a mattress is the internet's favourite firming hack, and it genuinely does firm up a bed with a weak base. It also voids some warranties and can trap moisture under the mattress if done carelessly. Here is when it helps, when it hurts, and what to use instead.
When plywood actually helps
Plywood is a fix for a foundation problem: slats that are too far apart, a sagging box spring, or a frame with no proper support. A solid board over a weak base gives the mattress an even, rigid surface, and the whole bed feels firmer because the mattress is finally supported the way its maker intended.
It is also a legitimate stopgap — cheap, same-day, and reversible — while you decide on a proper base.
The two catches
First, warranties. Many mattress warranties specify the foundation the mattress must sit on, and some treat plywood as non-compliant or as evidence of improper support. If your mattress is in warranty, read the foundation clause before reaching for the board.
Second, moisture. A mattress needs airflow underneath — you perspire into it nightly, and that moisture exits through the bottom. A solid, unpierced sheet of plywood seals the underside against a solid surface, which can trap moisture and invite mould, especially in humid rooms. If you use plywood, drill ventilation holes through it or accept it as a short-term measure.
What plywood cannot fix
Plywood firms the base, not the mattress. If the mattress itself is worn out or sagging internally, a board underneath changes nothing you can feel — the foam above it is still compressed. Confirm where the problem lives by putting the mattress on the floor first; if it still sags there, plywood was never the answer.
Better alternatives
A bunkie board is the purpose-built version of the plywood trick: a thin, rigid, upholstered board designed for exactly this job, sized to the mattress, and usually accepted by manufacturers as a compliant foundation.
Better still is fixing the base itself — adding slats to close up gaps, or replacing a tired box spring with a platform or foundation that meets the mattress maker's specification. It costs more than a sheet of plywood and solves the problem properly.
- Bunkie board: the purpose-built, warranty-friendlier version.
- Extra slats: closes gaps on an otherwise sound frame.
- New foundation or platform: the permanent fix for a worn base.
- Check the mattress warranty's foundation requirements before choosing.