What holds a seat up, and why it sags

The support under a cushion decides whether a lounge chair feels right in month one or month eighteen. It is also the cheapest part of a chair to get wrong, because a webbing deck that has sagged is not repairable in the field, only replaced.

Published by JPChairs · Chair manufacturer in Foshan, China · 2026-10-23

Three ways to carry a seat

Under every upholstered seat there is one of three systems, and the right one depends on how deep the cushion is and how much it has to give.

• Solid ply deck, 12 to 15 mm: zero give, the stiffest and the heaviest at 2.2 to 3.0 kg for a 500 x 500 mm seat; use it under 80 mm or more of foam.
• Elastic webbing: 8 to 12 per cent working stretch, 0.9 to 1.4 kg a seat, and the reason a 100 mm cushion can still feel soft.
• Sinuous springs: 4.0 mm wire, 8 to 10 springs a seat, 1.8 to 2.4 kg, and the choice for anything rated over 120 kg.
• Rule of thumb: under 80 mm of foam, webbing feels better; over 80 mm, springs or a solid deck keep the shape.
• Mixed decks cost 8 to 12 per cent more and are worth it only above a 150 kg rating or on 24 hour use.
• Weight in the container: webbing saves 1.0 to 1.6 kg a seat against springs, which is 40 to 65 kg off a 40HC of 40 lounge chairs.

Rubber webbing: the numbers we buy to

Elastic webbing is sold by width, and the two figures that decide how long it lasts are usually the ones missing from a quotation.

• Width: 50 mm for dining seats, 65 mm for lounge and anything wider than 550 mm; narrow straps stretch the same but creep about 20 per cent faster.
• Thickness: 2.0 to 2.5 mm rubber with a woven jacket; below 2.0 mm the edge frays at the clip in two to three years.
• Working stretch: 8 to 12 per cent at a 25 kg pull; above 15 per cent the seat bottoms out under an 80 kg sitter.
• Break load: 700 to 1,000 N a strap, and with four to six straps a seat the deck is never the weak link at 120 kg.
• Spacing: 60 to 90 mm centres, and never more than 5 per cent of the seat width unfilled at the edge.
• Cost: about USD 0.85 to 1.40 a seat for rubber, against USD 3.20 to 4.50 for a fully sprung deck.

Sinuous springs against webbing

Springs cost more, weigh more and last longer. Where the seat is wide or the load is high, that trade is worth making.

• Gauge: 8 gauge (4.0 mm) for dining, 9 gauge (3.7 mm) under a soft lounge cushion; 10 gauge (3.4 mm) belongs only in domestic frames.
• Count: 8 springs across a 500 mm seat and 10 across 600 mm, clipped at both ends and linked at every second spring.
• Arch height: 25 to 35 mm unloaded, dropping to 10 to 15 mm under 75 kg.
• Border wire: 4.0 mm, clipped and not stapled, or the end spring tears its clip within three years.
• Travel: springs give 20 to 25 mm under 75 kg where webbing gives 10 to 15 mm, and that is the whole difference in feel.
• Freight: the extra 1.0 to 1.6 kg a seat adds about USD 6 to 9 of ocean freight per 40 lounge chairs.

Sag, and the 10 mm test

Every elastic system loses height. The questions are how much, and how fast.

• Target: no more than 10 mm of permanent set after 60,000 cycles at 100 kg, measured unloaded.
• Rubber break-in: 4 to 6 mm in the first three months, then it holds; that is settling, not a fault.
• Springs: 2 to 3 mm and then static; 15 mm of drop in year one means the clips, not the springs.
• Test method: 75 kg on a 200 mm disc for 60 seconds, measure, then 50,000 cycles on the rig and measure again.
• Comfort threshold: once the front edge sits 12 mm lower than the back, guests slide forward and complain about the back angle instead.
• Repair: a re-web is USD 9 to 15 in labour; a full deck swap on a lounge chair is USD 26 to 40.

Clips, staples and where webbing lets go

Webbing almost never breaks in the middle. It lets go at the fastening.

• At the clip: 50 mm webbing needs a 1.2 mm steel clip with four teeth, pressed and not hammered; staples pull through at about 60 N.
• At the cut end: cut with a hot knife, or it unravels 20 to 30 mm into the seat within two years.
• At the frame: straps must meet the rail at 90 degrees; a 5 degree angled pull roughly halves the clip's holding load.
• Pre-tension: 3 to 5 per cent at fitting, or the first sitter takes up the slack and the clip rocks itself loose.
• Cross-overs: weaving alternate straps adds about 30 per cent hold and stops the deck twisting under an off-centre sitter.
• Spares: ship 2 per cent of the webbing length loose; a 5 m roll re-strings six chairs.

Spec lines for the enquiry

Six lines, and the support cannot be quietly downgraded between sample and bulk.

• Support type: webbing, springs or ply deck, named, never the words elastic support.
• Webbing: 50 or 65 mm, 2.0 to 2.5 mm, 8 to 12 per cent working stretch, woven jacket.
• Springs: 8 gauge, 8 or 10 a seat, 4.0 mm border wire, clipped at both ends.
• Sag: 10 mm permanent set or less after 60,000 cycles at 100 kg.
• Fit: 60 to 90 mm centres, hot-knife cut, 3 to 5 per cent pre-tension, 1.2 mm four-tooth clip.
• Spares: 2 per cent of webbing length shipped loose with the order.

The lounge frames these decks sit in are on the upholstered lounge chairs; seat foam and cushion build-ups are covered on the OEM cushion specifications.