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A sag is not one fault. It is the softest of three layers, and your palm can name it.

You sink into one seat and not the other · You can feel the frame under the cushion · It comes back up when you stand · It does not come back at all · One hollow, and the rest of the seat is fine · The whole sofa rocks when you lean on an arm · It started after one person always sat in the same place · A board under the cushion made it better for a month
FreePress it with your palm and feel how far down it goes
CheapWebbing: a few pounds of band and a staple gun
BoughtFoam, and only when it is genuinely the foam
Press it first Why the foam is last
Easy · 40 min · The cheapest fix is the one people do last

A sofa head on with its back, two arms, two seat cushions and a seat rail, and a dent drawn into one cushion; below left, the seat in section — foam on top, a row of webbing bands under it and the timber rail at the bottom, with two arrows pressing down and a dashed line showing a palm that bottoms out; below right, three webbing bands between two rails with the middle one broken and red load arrows on the two neighbours

diagnose
What failed
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Where
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what to touch fault correct Click the sofa, a cushion, the layers or the webbing

Three layers in series, and the softest one decides

Everything you sit on passes through the same stack. Foam on top, webbing or springs under it, a timber rail at the bottom. They are in series, which means the load goes through all three and the softest one governs the result. So a sag is never simply "the foam has gone" — it is whichever layer has lost its stiffness, and the three feel completely different under a palm.

Press the seat with the flat of your hand, hard. If you go down a long way and stop on something hard and unyielding, you are touching the rail: the webbing has gone. If it goes down and comes back with a slow springy resistance, the support is intact and it is the foam that has softened. If one place sinks and fifty millimetres to the side is firm, it is a local failure in the webbing, not a general ageing. That single press separates three different jobs, and it costs nothing.

Then the economics, which are the reason this page exists. Webbing is a few pounds of elastic band and a staple gun; foam is the most expensive part of the sofa, and replacing it means stripping the cover. And webbing fails in a chain: one band carries the load, breaks, and the two either side of it now carry half again as much — so they are next. Repair it while it is one band and it is an afternoon. Repair it at five and you are re-webbing the whole seat.

Three faults, three presses

Bottoming out onto the rail is the webbing, and it is the cheap one. Turn the sofa over, open the dust cover underneath — stapled on most sofas, zipped on some — pull the broken band out and prise out every old staple, because a stray staple will cut the new band. Fit the new band in the same direction as its neighbours, stretch it with a webbing stretcher until it rings when you tap it, and staple it in a zig-zag of at least three staples per end. Where bands cross, interlace them rather than laying one flat on another; parallel bands all carry load in one direction and sag as a set.

Soft, springy, and slow to come back is the foam. The test is a squeeze and a clock: press a cushion to half its thickness, let go, and see whether it returns to ninety per cent within ten seconds. If it stays squashed, the cells have collapsed and no amount of turning the cushion will bring it back. Buy high-density foam — 35 kg/m³ rather than the 25 that failed — cut it five millimetres oversize, and photograph the old one's shape before you cut, because seat foams are usually tapered and wrapped in a bonded wadding.

The whole thing rocks when you press one arm is the frame, not the seat. Lean on one front corner and watch the opposite rear corner: if it lifts, a joint has opened. Most frames are stapled and glued blocks, and the repair is to clean the old glue out, glue and clamp, and add a screw or a corner brace across the joint. Do this before re-webbing anything, because a frame that moves will pull the new staples straight out.

Three rules, and the first one is the mistake that turns a ten-pound job into a hundred-pound one. Never put a board under the cushion. It feels like a fix for a week: the load now lands on the two bands either side of the board's edges, at several times the stress they were designed for, and they break in a line. Do not stuff a pillow into the hollow either. It hides the sag, so you keep sitting there, and the neighbouring bands keep loading up until the whole seat has to be done. Do not re-web over old staples. They sit proud of the timber, they cut the new band from underneath, and it fails in the same place within a year. Prise them out, and if the timber is chipboard that has crumbled around the old staples, that rail needs a batten screwed alongside it, or the staples will never hold.

Faults next to this one

Carpet dents Carpet spring comes from crimped fibre, so a dent is flattened yarn — until you press it… Buckle pops open A side-release buckle is two prongs with tabs that sit in windows. Belt too long or short Belt holes are a standard pitch, which means there is no fine adjustment, only a position.