Do curved sprung slats make your bed dip?

Yes, curved sprung slats are a very well-documented cause of the dipping and rolling together you are describing, and the explanation for it is straightforward once you understand how sprung slats actually behave under load.

Why curved sprung slats cause dipping, particularly for couples

Sprung slats are designed to flex downward when weight is applied and spring back up when it is removed. The intention is to add a layer of cushioning beneath the mattress. In practice, for two people sharing a king size bed, this flexing is deeply uneven across the width of the base. The two sleepers occupy the left and right thirds of the mattress. The slats beneath those areas are pressed down under their combined weight. The slats in the central third of the bed, between the two sleeping zones, carry far less load and do not compress in the same way. The result is that the occupied sleeping areas sit lower than the unsupported centre of the bed. Both sleepers are gradually pulled toward the middle during the night as they unconsciously roll toward the lower points. This worsens progressively as the mattress upholstery settles into the shape the base has imposed on it.

Thanks for your question. The bed base can affect the mattress. If a sprung slatted base has a central bar running down the middle then the mattress will logically dip in the area that isn't being supported.

Sprung, or curved, slats are a bit more forgiving than solid slats but we're getting more and more recent feedback that dips are appearing in mattresses that are supported by one of these bases. If you prefer to replace them with solid slats then that should also be fine and this is down to your own preference. All I'd advise is to check the measurements of the gaps between the slats. If they are bigger than 3" then you will need to solidify the surface anyway. I hope this has answered your questions fully but if you do have any further queries please don't hesitate to contact our office on 0161 437 4419. Our office is open Monday to Friday 8am to 8pm and Saturday and Sunday 10am to 4pm. Kind Regards, Lee.

Should you return the sprung slats and buy flat ones instead?

Returning your curved sprung slats and replacing them with flat rigid slats is a legitimate solution and is perfectly fine to do. For a quality two-sided pocket sprung mattress, flat rigid slats with gaps of no more than 7.5cm (3 inches) are a better foundation than sprung curved slats. Flat slats provide a consistent, predictable surface that allows the pocket spring unit to work correctly across its full width. Sprung slats introduce the uneven flexing described above, which is why dipping problems on slatted bases are reported more often with sprung slats than with flat ones.

However, replacing the slats involves cost and effort. Boarding over your existing curved slats with MDF is quicker, cheaper, and resolves the dipping problem equally effectively without needing to change the slats at all.

The MDF boarding fix: exactly how to do it

Cut sheets of 3 to 5mm MDF to the full internal dimensions of your bed frame, covering the complete area from side rail to side rail and from head to foot. Lay these over the curved slats and the central support bar. The MDF bridges the bar and the arched slats, creating one flat and even surface for the mattress to sit on. At 3 to 5mm thickness, the MDF remains fractionally flexible and still allows the curved slats beneath it to flex very slightly under load, rather than pinning them completely flat. Most large DIY stores will cut MDF to specified measurements for a small charge if you give them the internal dimensions of your frame.

For the plywood option you mentioned in your question: yes, plywood at 3 to 5mm works equally well. It is slightly more rigid than MDF of the same thickness, which will make the mattress feel a fraction firmer. Whether MDF or plywood suits you better is a matter of preference.

After the MDF or plywood is down, lay an old duvet or blanket over it before you put the mattress back. This protects the underside fabric of the mattress from indentation and from contact with the hard surface.

At your body weights, is the dipping a spring tension issue or a base issue?

At the weights you describe (approximately 12.5 and 10.5 stone) the dipping you are experiencing is a base problem, not a spring tension problem. Medium spring tension in a pocket sprung mattress is appropriate for both of you. The curved sprung slats are imposing the shape on the mattress from below rather than the mattress lacking adequate support internally. Once you board over the slats, the dipping should resolve. If you find that after boarding the mattress still feels uneven, testing it flat on the floor will confirm whether the mattress itself has settled permanently into the arched shape from prolonged use on the sprung slats, in which case the mattress may need turning to allow the upholstery layers to even out.

Why the best mattress manufacturers do not use sprung slatted bases

If you look at the bed bases that Vispring, Savoir, Hastens and Harrisons pair with their high-end mattresses, they are almost always sprung edge divans. This is not coincidental. A sprung edge divan provides a flat, even, consistent surface with a secondary layer of suspension built into the base itself. It is the opposite of a sprung slatted frame, which provides an uneven, reactive surface that changes shape depending on where the load falls. For the very best performance from a quality pocket sprung mattress, a sprung edge divan or a well-specified platform top divan is the right choice. For those with a slatted bedstead frame they wish to keep, proper boarding is the next best option.

Our guide to mattresses for slatted bed bases covers all of this in detail. Our answers to whether sprung slats are better than rigid slats and how to stop a bed bowing in the middle address related aspects of the same problem.

Please do call us on 0161 437 4419 if you have any further questions. Our office hours are here.

Kind regards, Gary