How do I fix a sofa seat that has sunk in the middle?

Sofas sag from the bottom up, and people fix them from the top down. Buying new cushions for a sofa whose webbing has stretched produces a firm cushion sitting in a hammock. Five minutes with the cushions off tells you which layer has actually failed, and only then is the repair obvious.

What you'll need

• Torch, for looking underneath
• Staple gun and staples, for webbing (optional)
• Replacement elasticated or jute webbing (optional)
• 6 to 9mm plywood, cut to the seat area (optional)
• Replacement foam insert or foam sheet (optional)
• Pliers and spring clips, for zig-zag springs (optional)
• Timber, screws and glue for splinting a rail (optional)

Step by step

1. Take the cushions off and press the deck with your hand.This is the whole diagnosis. A firm deck with a dished cushion is a foam problem. A deck that gives under hand pressure is a support problem underneath. A deck that gives on one side only usually means a broken rail or a detached spring.
2. Turn the sofa over and look at the underside.Peel back or unstaple the black dust cloth. You will see one of three things: interwoven webbing straps, zig-zag serpentine springs running front to back, or a solid or sprung platform. Photograph it before disturbing anything.
3. Fix collapsed foam by replacing the insert, not the cover.Most cushion covers unzip. Measure the cover and order a foam insert cut to size, typically 30 to 35kg per cubic metre density for a seat — anything softer collapses again within a year. A wrap of polyester fibre around the foam restores the plump look.
4. Replace stretched webbing rather than tightening it.Elasticated webbing loses its stretch permanently. Remove the old straps, and staple new ones across the frame under tension, interwoven front-to-back and side-to-side, with the ends folded double where they are stapled. A webbing stretcher makes this far easier but a helper and pliers will do.
5. Re-clip or replace a detached zig-zag spring.Serpentine springs sit in small clips nailed to the frame rails. A spring that has jumped its clip can often be levered back with pliers and the clip renailed. A snapped spring needs a matching replacement cut to length, which upholstery suppliers sell.
6. Add a plywood deck as the pragmatic fix.A sheet of 6 to 9mm plywood cut to the seat area and laid on top of the webbing or springs spreads the load and stops the dip immediately. It changes the feel — firmer, less bouncy — and it is by far the quickest repair. Round the edges and cover it with fabric or a blanket so it does not chafe the cushions.
7. Splint a cracked frame rail.A softwood or hardwood batten glued and screwed alongside the crack, overlapping at least 150mm each side. Clamp while the glue cures. This works well on a clean split and not at all on a shattered end.
8. Check the legs and the frame joints while it is upside down.Loose leg bolts and opened frame joints are common and cheap to fix. Tighten everything, and add glue blocks in the corners if the joints move.
9. Restaple the dust cloth when you are done.It keeps dust and small objects out of the frame and it takes two minutes.
10. Reassess before spending real money.New foam and new webbing on a decent frame is a genuinely good repair and can add years. The same work on a stapled softwood frame that is already cracking is money into something that will fail elsewhere.

Tips

• Rotating and flipping seat cushions every month or two is what stops this happening — foam collapses where it is always sat on.
• Foam density matters far more than thickness. A thick low-density foam feels good in the shop and is flat in a year; 30 to 35kg per cubic metre is the domestic seat standard.
• If you use a plywood deck, put it under the webbing rather than on top where the frame allows — it looks the same and it does not lift the seat height.

Common mistakes

• Buying new cushions for a sofa with failed webbing — The new cushion sits in the same hammock. Fix the support layer first and then judge whether the cushions need anything.
• Stapling new webbing without tension — Webbing supports by being tight. Slack webbing is doing nothing from day one.
• Putting a bare plywood sheet straight under the cushions — The edges chafe the cushion covers and the sharp corners eventually cut through. Round the edges and cover it.
• Ignoring a cracking noise when sitting down — That is a frame joint or a rail failing. It gets worse and it is much easier to splint a crack than to rebuild a broken end.

If it doesn't work

Questions people ask