How to Diagnose a Sagging Sofa Seat in Three Steps — and Fix It Right

A sagging sofa seat is one of the most common upholstery repair jobs in the UK — and one of the most frequently misdiagnosed. Clients describe the symptom (sinking too far, uneven seat height, cushions sliding forward) and assume they know the cause (worn-out springs, collapsed foam). Sometimes they are right. More often they are not. Getting the diagnosis right before quoting is the difference between a job that takes three hours and one that takes seven.

The Three-Step Diagnosis

Remove all cushions from the seat and press firmly on the seat deck with both hands in the centre, then at each corner, then mid-way along each edge. You are feeling for three distinct things.

Step 1: Assess the Support Platform

Press down firmly on the seat deck with the flat of your hand. If the base gives significantly — more than 3–4cm before resistance — the primary support system has failed. This is either the webbing (if the sofa uses elasticated or jute webbing stretched between the front and back rails) or the sinuous springs (if the sofa uses zig-zag wire spring units). Both feel similar under hand pressure: the base sags and then rebounds sluggishly rather than returning crisply.

Turn the sofa upside down and remove the dust cover if there is one. Failed elasticated webbing is immediately visible — stretched, loose, or broken straps are obvious. Failed sinuous springs will show as wires that have detatched from their clips at the front or rear rail, or wires that have broken mid-span (less common but it happens).

Step 2: Assess the Foam

If the support platform feels solid and correctly tensioned, the issue is almost certainly in the seat cushions. Remove a cushion and compress it firmly with both hands. Good foam springs back promptly and completely. Degraded foam shows characteristic signs: it compresses with little resistance, recovers slowly, or has begun to break down at the edges and corners while the centre remains relatively firm.

Check the foam grade label if one exists (often stitched into the inner cover). Standard domestic sofas typically use foam in the HR30–HR35 range for seat cushions with a reflex or fibre wrap. Quality varies enormously — budget sofas use foam grades that can begin to break down within three to five years of regular use.

Step 3: Assess the Seat Deck Padding

The seat deck — the platform that the cushions rest on — often has its own layer of padding: typically dacron or a bonded fibre quilt stapled or tacked over the base. If this layer has shifted, bunched or thinned, cushions will rock or sit unevenly even if the support structure and foam are both intact. Press the seat deck and look for areas where cushion contact is uneven.

Quoting the Job

The diagnosis determines the quote, and they are very different jobs:

• Webbing re-strapping (elasticated): typically 1–2 hours labour plus webbing materials. The sofa base must be accessible, which on some modern sofas requires partial frame disassembly.
• Sinuous spring repair/replacement: typically 2–3 hours, depending on the number of failed units and clip condition. Spring sets are relatively low cost (£40–£90 for a standard 3-seat sofa) but the labour is the significant element.
• Seat cushion foam replacement: quote per cushion. A standard seat cushion in HR35 CMHR foam typically costs £35–£60 in materials; cutting, wrapping in dacron and re-covering the inner in calico adds 45–60 minutes per cushion.
• Seat deck padding refresh: typically 1–1.5 hours plus materials. Often a worthwhile addition when re-doing either of the above.

A Note on Combination Failures

Older sofas — particularly those more than ten years old — frequently have multiple concurrent failures. The webbing has stretched and the foam has degraded and the seat deck padding has shifted. When this happens, quoting the repair correctly requires identifying all three issues and including them in the estimate. A client who was told 'just the springs' and then finds the cushions still feel soft after the repair is unlikely to recommend you to anyone.