Why has my sofa gone saggy in the middle?

Blog · Problems & Repairs

Almost everyone diagnoses this wrongly, and understandably so. The cushion is the bit you sit on, the cushion is the bit that feels wrong, so the cushion must be the problem. New cushions get ordered, they go in, and six weeks later it feels exactly the same.

That is because the cushion was sitting on something that had already failed. Put a new cushion on a collapsed deck and you have simply given the collapse a nicer covering.

Start by taking the cushions off

Everything follows from this. Strip the seat cushions off entirely and look at what they were sitting on — the deck, in trade terms.

Then press down firmly on the deck with the flat of your hand, working across the whole width. What you are feeling for is whether it supports your hand evenly or whether there is a hollow.

• The deck feels firm and even everywhere. The cushions are the problem, and that is the good news. Foam replacement, and you are done.
• The deck sinks in the middle, or your hand meets a hollow. The suspension has gone. This is the most common answer.
• The deck feels reasonable but the whole seat moves or the front rail dips when you press. The frame is flexing, and that is the serious one.

Three suspects, and a two-minute test that narrows it down. Now for what each one actually means.

Suspect one: the cushion foam has given up

Foam does not wear out so much as it takes a set. Every time you sit down, the cell structure is compressed; over years it loses the ability to push back to its original height. The trade term is compression set, and it is why a cushion can look full when you lift it out and feel like nothing when you sit on it.

The tell is that the sagging follows people. The seat somebody always uses is dead, the seat nobody uses is fine, and the cushions are visibly different heights when you stand them side by side.

This is the cheapest problem you can have. New foam of the right density, wrapped properly, transforms a sofa — and if the covers are sound, an upholsterer can often do it without touching the covering at all. Our foam and cushion spec tool will tell you what density you should be asking for, which is worth knowing before you ring anyone, because the single most common reason a foam replacement disappoints is that somebody fitted the cheap grade.

Suspect two: the suspension has failed

This is what is under the deck, holding everything up, and there are three kinds depending on how old and how good the sofa is.

Elastic webbing — wide black rubberised straps stretched across the frame, standard on most modern sofas. It is designed to stretch, which is also its weakness: after years under load it stops recovering and hangs slack. It also fails at its fixings, where staples or clips pull out of the rail. A slack strap in the middle of a run gives you exactly the dip you are feeling.

Sinuous springs — also called zig-zag or serpentine, the S-shaped steel wires running front to back under the deck. They fail in two ways: the clip holding one end lets go of the frame, or the wire itself snaps. Both give a sudden, localised collapse rather than a gradual sag, and there is often a noise involved — a twang or a clunk when somebody sits down. If your sofa developed a dip overnight rather than over years, this is almost certainly why.

Coil springs on jute webbing — traditional construction, older or better pieces. The springs themselves rarely fail, but the jute webbing carrying them perishes, stretches or tears at the tacks, and the twine tying the spring tops together rots. When it goes, the whole spring unit drops.

Getting at any of these means turning the sofa over and taking off the black cloth underneath, the cambric or dust cover. That is a legitimate thing to do yourself just to look — it is stapled on and it goes back with a staple gun.

If you are only investigating, cut the dust cover along one edge rather than tearing it off. It goes back with a dozen staples and nobody will ever see it.

Suspect three: the frame

This is the one that decides whether the sofa has a future.

The middle of a long seat is simply the furthest point from anything holding it up. On a properly built sofa there is a centre rail, or a centre leg, or both, carrying the load across the span. On a cheaply built one there often is not, and the front rail is a length of softwood expected to bridge two metres on its own. Over years, under people, it bows.

The tells are a front edge that is visibly lower in the middle than at the ends when you sight along it, a seat that moves or creaks as a whole when you sit, and a frame that flexes when you push down hard on the deck near the front.

Sometimes this is genuinely repairable — a new centre leg, a reinforcing rail, re-gluing a joint that has opened. Our chapter on frame repair and joint reinforcement covers what is possible. But it means stripping the sofa back to get at it, and on a stapled softwood frame you have to ask honestly whether it is worth doing.

The plywood board, and why I keep being asked about it

The advice you will find everywhere is to slide a sheet of plywood or a "sofa saver" board under the cushions. It is worth being straight about what that does.

It works, in the sense that the sagging stops. It stops because you have taken the suspension out of the equation entirely and are now sitting on a board. The seat becomes flat and hard, the cushion foam is being compressed against an unyielding surface rather than a sprung one, so it breaks down faster, and the load that was spread across the webbing is now concentrated on the frame rails at each end.

As a stopgap for a few weeks before a proper repair, or on a piece you are running to the end of its life deliberately, it is a perfectly reasonable thing to do. As a permanent answer it is masking the fault and quietly making the next repair bigger. Just know which one you are choosing.

So is it worth fixing?

Roughly, in ascending order of trouble:

• Foam only. Straightforward and inexpensive, often done without disturbing the covers. Almost always worth it.
• Elastic webbing. Straightforward for an upholsterer, needs the dust cover off and sometimes the deck lifted. Worth it on any decent sofa.
• Sinuous springs. More work, needs the right gauge spring and clips, but a proper repair that lasts. Worth it on a good frame.
• Traditional springs and jute webbing. A real job — but pieces built this way are usually worth the effort, because a frame from that era is generally hardwood and jointed rather than stapled.
• Frame. Depends entirely on what the frame is. Hardwood and jointed, fix it. Softwood, stapled, no centre rail, and already bowed — that sofa is telling you something.

The decision almost always comes down to the frame rather than the price of the repair, which is why an upholsterer will want to look underneath before quoting. If you want a figure to think with in the meantime, the cost estimator will give you a range.

What I say on the phone

Take the cushions off, press the middle of the deck, and tell me whether your hand goes down.

If it does not, we are talking about foam and it is a good day. If it does, we are talking about what is underneath and I need to see it. And if you can sight along the front edge and tell me it dips in the middle, I will want to know how old the sofa is and what it cost, because the answer to that decides everything else.

Nobody enjoys being told a three-year-old sofa is not worth repairing. But it is a better conversation than taking somebody's money to put new foam on a collapsed frame.