Does putting plywood under a mattress make it firmer?

Yes, placing plywood under a mattress makes it feel firmer by eliminating flex and sag in the support surface beneath it. The effect is most noticeable on slatted bed frames with wide gaps or on worn box springs.

Plywood works by creating a rigid, continuous surface that prevents the mattress from partially sinking through slat gaps or soft spots in a deteriorating foundation. Without that give underneath, the mattress compresses only against a solid plane, which registers as firmer to the sleeper. The difference in feel depends on how much the original base was flexing: a frame with 4-inch slat gaps will show a dramatic change, while a solid platform base may show almost none.

Recommended plywood thickness for under-mattress use: ¾ inch (19 mm) to prevent bowing under body weight.
Standard bed slat spacing that benefits most from plywood: gaps wider than 3 inches.
Plywood under a mattress can reduce airflow, potentially increasing heat retention at the sleep surface.
A full-coverage plywood board adds roughly 20–40 lbs to the bed frame load depending on size.
Plywood firms the feel without changing the mattress materials themselves; the foam or coil layers remain unchanged.