Two sofas can look identical and feel completely different. The difference is inside, in the seat construction, and it is the part buyers see least and notice most after a year of use.
Here is how a sofa seat is built, explained simply, so you can specify one that stays comfortable.
The three layers of a seat
Almost every sofa seat has three parts. The frame carries the load. The suspension spans the frame and provides the give. The cushion sits on top and provides the comfort you feel first.
All three have to be right. A good cushion on poor suspension feels soft at first and then bottoms out. Good suspension under a poor cushion feels firm and unwelcoming.
The frame
Kiln dried hardwood is the traditional standard, often beech or a similar dense timber, dried to a low moisture content so it does not move after manufacture. Plywood is used widely and performs well, since layered construction resists warping.
What matters as much as the material is the joinery. Corner blocks glued and screwed into the joints make a frame that stays square. A frame that is only stapled together will loosen, and that looseness is what people describe years later as the sofa feeling tired.
Ask what the frame is made from and whether corner blocks are fitted.
The suspension
Three systems are common.
Sinuous springs, sometimes called serpentine or zigzag springs, are S shaped steel wires running front to back across the frame. They are the most widely used system, they are reliable, and quality depends on the gauge of the wire and how closely the springs are spaced. Closer spacing gives more even support.
Eight way hand tied coil springs are individual coils tied in eight directions by hand. It is the traditional high end method, it gives a deep responsive sit, and it is labour intensive, which is reflected in the price.
Elastic webbing is woven strapping stretched across the frame. It is lighter and works well in contemporary designs with a lower profile. Quality depends on the rubber content of the webbing and the tension it is fitted at.
All three can produce an excellent sofa. What you want to avoid is a system that is undersized for the seat span.
Foam density, the number to ask for
Density describes how much material is in a given volume of foam, usually expressed in kilograms per cubic metre. It is the best single predictor of how long a cushion keeps its shape.
As a working guide, foam around 25 to 30 kilograms per cubic metre suits occasional use. Around 30 to 35 suits a normal home sofa. Around 35 to 45 suits daily family use and contract projects. Above that is used for heavy commercial seating.
Density is not the same as firmness. A dense foam can be soft, and a light foam can feel firm at first. Firmness is described separately, often by an indentation figure. Ask for both, and remember that density is the one that predicts life.
High resilience foam
High resilience foam has a more open and irregular cell structure, which makes it bounce back more readily and keep its shape longer. It costs more than standard foam and is worth it in a seat that is used every day.
For outdoor use, reticulated quick dry foam has open cells that let water pass straight through, so a cushion left in the rain is usable again quickly.
Cushion fillings
Solid foam gives a tidy shape that stays neat with no maintenance. It suits contemporary designs and commercial use where nobody will plump cushions daily.
Foam with a fibre wrap adds a soft layer around a foam core, giving a slightly relaxed look while keeping the shape. This is a good middle ground and very common.
Feather and down wrapped foam gives the deep inviting look of a luxurious sofa. It needs plumping regularly to look its best, so it suits a home more than a hotel lobby.
All foam softens a little in the first months. That is normal settling rather than a fault, and a good specification allows for it.
Seat depth and height
Comfort is also geometry. A seat depth around 55 to 60 centimetres suits most people sitting upright. Deeper seats of 65 centimetres and above are wonderfully relaxing and can be awkward for shorter people unless cushions are provided.
Seat height around 42 to 45 centimetres suits most adults for getting up and down easily. Lower seats look contemporary and are harder to rise from, which matters in hospitality.
What to ask before ordering
Five questions cover it. What is the frame material and are corner blocks fitted. What suspension system is used and at what spacing. What is the foam density in the seat and in the back. Is the foam high resilience. What are the seat depth and seat height.
A supplier who answers these clearly is one who knows their product, and the answers let you compare two sofas that look the same in a photograph.
If you would like help specifying seating for a home or a project, our team in Foshan works with these constructions every day.