Why Your Sofa Sags — and Why It Is Worth Fixing

A sofa that dips in the middle or feels as though it has swallowed you whole is rarely beyond saving. In most cases the frame, the foam and the fabric are all perfectly sound, and the problem lies in the suspension underneath. Springs lose their tension, webbing stretches, and hessian dust covers perish. All three are replaceable, and none of them require a workshop full of specialist machinery. With an afternoon, a helper and about £30–£50 of materials, most sagging sofas can be brought back to comfortable, supportive life.

This guide walks you through the job from underneath, which is where the answers always are. Work slowly, mind your hands, and you will finish with a sofa that behaves like new.

Setting Up Safely Before You Turn the Sofa Over

Springs under load store energy, and old ones can ping free the moment you cut a retaining tie. Treat this as a job with real hazards and dress accordingly.

• Wear safety glasses and cut-resistant gloves. Rusty spring ends, broken hog rings and sharp staples are the three most common causes of injury in this job.
• Put on a dust mask. Older upholstery hides decades of dust, horsehair and degraded foam particles. Shake the sofa out outdoors if you can.
• Clear a work area at least 2 metres by 2 metres and lay down a dust sheet. Work on a hard floor rather than carpet, so you can find dropped staples.
• Get help to flip the sofa. A two or three-seater is heavier than it looks, and twisting it alone is how backs get strained. Turn it upside down onto a padded surface or a pair of trestles.
• Check for staples and tacks protruding from the frame before you put your hands anywhere near the underside, and knock down any that stick out with a hammer.

Reading the Underside: Broken, Stretched or Loose Springs

With the sofa upside down, peel back or cut away the dust cover — usually black or grey non-woven fabric stapled to the frame. What lies beneath tells you exactly what has failed.

You are looking for one of three spring systems. Coil springs are individual cones or cylinders, tied in place with twine and usually sitting on a webbed base. Sinuous springs, also called zig-zag springs, are continuous S-shaped wires clipped to the frame front and back. Some sofas use webbing alone with no springs at all.

• Broken springs are obvious: snapped coils, a sinuous wire sheared at a clip, or a spring lying loose and rattling.
• Stretched springs look intact but feel soft. Push down on each one with the flat of your hand — a tired spring collapses easily and fails to push back.
• Loose ties are the sneakiest fault. The twine holding coils together has frayed or snapped, so springs lean sideways instead of standing upright.
• Perished webbing will be brittle, cracked or snapped, with the whole platform drooping between the rails.

Mark every faulty spring with a scrap of masking tape or chalk so you know which ones need attention once you start working.

Webbing: The Forgotten Backbone of a Comfortable Sofa

If the webbing has gone, nothing you do to the springs will hold. Webbing carries the load and gives springs something to push against, so replace it first and work upwards.

You will need 50mm jute webbing for a traditional interwoven base, or elasticated rubber webbing for a quicker, more forgiving repair. You will also need a webbing stretcher — a simple spiked tool that levers the webbing tight while you staple — and a heavy-duty staple gun loaded with 10mm staples.

• Run webbing strips front to back at roughly 75mm intervals, keeping them parallel and evenly spaced.
• Fold the cut end over by about 25mm to give the staples something to bite into, then staple across the fold in a zig-zag pattern, roughly every 20mm.
• Pull each strip tight with the stretcher before stapling the far end. Aim for taut but not drum-skin tight — rubber webbing should still have a little give.
• Weave the side-to-side strips over and under the first set, exactly as a basket weave, so the platform locks together.

Tap down any staples that stand proud. A single raised staple will eventually wear through the hessian above it.

Replacing and Retying Springs Properly

For a snapped coil or sinuous spring, buy a like-for-like replacement — measure the height and diameter before ordering, and match the gauge of wire. Sinuous springs are usually held by metal clips bent around the frame; lever the old clip open with side cutters and crimp a new one with spring pliers.

For loose or leaning coils, retying is the answer. Use heavy upholstery twine and tie each coil to its neighbours in four directions, front, back, left and right. Knot firmly with a double hitch and pull each tie so the coils stand upright and evenly spaced. Traditional practice is to tie the rows first, then the diagonals — the diagonal ties are what stop the whole unit from shearing sideways under weight.

Where the twine passes over a spring top, wrap it once around the coil before knotting so it cannot slip. Work methodically across the frame rather than jumping about; it is easier to keep the tension even.

If the hessian or calico cover that sits between springs and foam has torn, replace it now with new hessian stapled to the frame. This layer stops the foam dropping between coils and is often the real reason a sofa feels lumpy.

Reassembly, Testing and Knowing When to Stop

Staple a fresh dust cover back over the underside, pulling it reasonably taut so it does not flap. Leave the sofa the right way up for at least an hour before sitting on it, and ideally overnight, so the new webbing settles.

Then test it properly: press down firmly across the whole seat, sit on each cushion position in turn, and listen for creaks or rattles that suggest a tie has slipped. Small adjustments are normal, and tightening one or two ties is far easier than starting again.

If, once you are underneath, you find a cracked or splintered frame rail, rotten timber or a completely collapsed spring unit, it is worth pausing. Frame repairs need clamps, wood glue and occasionally new timber sections, and a sofa in that state may be better assessed by a professional upholsterer. For everything else — sagging seats, soft spots, a dip in the middle — the fix is well within reach of a careful DIYer with a staple gun and a free afternoon.