Bases, Slats and Divans: What Goes Under the Mattress
People spend weeks choosing a mattress and five minutes on what it sits on. The base changes the feel, decides the airflow, and is the most common reason a guarantee claim gets refused.
The 7 cm rule
If the gaps between your slats are wider than about 7 cm, the mattress will settle into them over a couple of years and develop a washboard underside. Every guarantee in the trade excludes that, and it is entirely preventable.
Go and measure yours now. If the gaps are too wide, the fix is a sheet of slats laid across, or thin battens, not a solid board.
The centre rail
Any bed a double or larger needs a centre support rail with at least one leg to the floor. Without it the slats bow in the middle under two people, and the mattress follows. A mattress sagging down the centre line after a year is almost always a missing or unsupported centre rail rather than a failed mattress.
The four kinds of base, and what each does to the feel
Rigid slats
Fixed timber battens. Neutral — the mattress feels the way it was built to feel. Good airflow. This is the reference case.
Sprung slats
Curved laminated slats that flex. They add give, so a firm mattress feels a step softer on them. If you buy a firm mattress and it feels medium, this is why.
Solid platform
A continuous board. Firmest feel, worst ventilation. Moisture from the sleepers has nowhere to go and condenses on the underside of the mattress. Workable in a dry, heated room; a problem in a cold or humid one. Stand the mattress on edge for an hour a few times a year.
Divan
An upholstered box, either solid-topped or sprung-edge. A sprung-edge divan adds give like sprung slats; a platform-top divan behaves like a solid base. Divans generally have the least airflow of all, which is why the good ones have vents.
Old iron and mesh bases
An original sprung mesh under an antique frame flexes a great deal. A firm mattress on one will feel soft, because the base is doing the moving. Go firmer than you would otherwise — see mattresses for antique beds.
Ventilation, plainly
Two adults lose a meaningful amount of moisture into a bed every night. On a slatted base it evaporates downward. On a solid one it collects. That is the entire mechanism behind mould on the underside of a mattress, and it is a base problem masquerading as a mattress problem.
If you have a solid base and a cold room, a spacer mat between base and mattress solves it for a few pounds.
When to replace the base
Buying a new mattress onto a fifteen-year-old base is a false economy: the new mattress inherits the old dip and the guarantee will not cover it. Check for a bowed centre, cracked slats, and legs that have loosened. If the base is tired, replace both.
Common questions
Can I put a mattress straight on the floor?
Short term yes, long term no. No airflow underneath, and it is the fastest route to mould.
Does the base affect the guarantee?
Yes, directly. Photograph yours when the mattress arrives.
Do I need a bed board?
Only as a temporary diagnostic. See how to firm up a soft mattress.