Frame and Upholstery: How Structure and Cover Work Together

When a sofa looks crisp on the first day and slack a year later, the cause is rarely the leather alone. A cover can only hold the shape that the frame and padding give it. The frame sets the lines, the springs and foam set the volume, and the leather or fabric shows how well the two were matched. This article explains how those layers depend on each other and what to look for when you compare models in our leather sofa collection or ask for custom dimensions.

Every Sitdesign piece is handmade in our own factory in Büyükçekmece, Istanbul, with upholstered frames of kiln-dried beech, sprung seating and high-resilience (HR) foam. The notes below use that build as the example, but the principles apply to any upholstered piece.

The frame sets the lines the cover must follow

An upholsterer cannot add a line that the frame does not have. A sharp, square arm needs a rail with a square edge underneath it; a rounded arm needs a shaped rail or a built-up profile. If a design calls for a slim, tailored arm of around 15 cm, the timber inside has to be close to that width, with only a few centimetres of padding. A deep, cushioned arm of 25–30 cm can be built from a narrower frame part wrapped in thicker foam.

This is one reason we use kiln-dried beech. Leather and fabric are pulled tight and stapled into the frame, so the timber has to hold staples firmly and stay dimensionally stable. If a rail twists slightly as it dries, the tension in the cover changes with it, and diagonal pulls or loose areas appear on the surface.

Springs and foam decide volume and recovery

Between frame and cover sits the seat build-up: springs below, HR foam above. The springs spread the load across the seat frame; the foam gives the cushion its outline and returns to shape after someone stands up. The cover lies on top of both, so it reflects every change in the layers beneath.

• Too little foam over a rail: you feel the edge of the frame through the cover, and the leather wears first along that line.
• Foam that is too soft: the cushion flattens, and the cover, cut for the original volume, shows loose folds.
• Foam cut slightly larger than the cover: a common technique that lets the cushion fill the cover fully, so corners stay sharp.

Seat dimensions follow from this build-up. A typical seat height of 42–46 cm and a depth of 55–60 cm are measured on an unused seat; once you sit, the foam compresses by a few centimetres, so the seat feels slightly lower and deeper than the figures suggest.

Leather and fabric behave differently on the same frame

The same frame can look quite different in leather and in fabric, because the two materials stretch and fold in different ways.

• Leather stretches little and has a natural grain direction. On tight curves it needs more seams or careful easing, and the cutter works around the natural marks of each hide. Leather also shows the shape underneath clearly, so a precise frame and even foam matter most here.
• Fabric usually stretches more and drapes more softly, which can soften a geometric frame. Patterned or striped fabrics need extra length so the pattern lines up across seat, back and arms.

A curved model such as the Janette curved sofa shows why: on a curve, the panel shapes, the seam positions and the frame radius have to be planned together. Our leather page shows the 76 colours in 12 colour families, and the post Leather vs Fabric Sofa Set compares the two covers in daily use. Fabric upholstery is also possible, and customer's own material (COM) is accepted.

Tight upholstery or loose cushions

How the cover is attached also depends on the frame. In tight upholstery, the leather or fabric is fixed directly over the foam on the back and seat, so the frame shape stays visible and the piece looks tailored. It needs very even foam, because any unevenness shows through. With loose cushions, the frame forms a platform and a box, and separate cushions sit inside it. The cushions can be turned and plumped, and the frame lines matter mainly at the arms and base.

Armchairs are a good place to compare the two, because the proportions are small and every detail is visible. Browse our armchair collection with this in mind: notice where the frame line stays visible and where cushions take over. A swivel armchair adds another demand. The swivel base is fixed into the seat frame, so the frame has to take a turning load as well as a sitting load, as in the swivel armchair in orange leather we showed at IIFF 2024.

What to check before you order

1. Look along the arm and back edges: lines should be straight or evenly curved, with no waves where the cover is stapled.
2. Press the front edge of the seat: you should feel foam, not the frame rail.
3. Follow the seams: they should run parallel to the frame lines and sit on the edges, not drift across flat panels.
4. Sit, stand up and watch the cushion: HR foam should return to its shape within a few seconds.
5. For custom dimensions, ask whether the frame is re-proportioned for the new size, since the cover is cut to fit the new frame.

If you are choosing between models or planning a piece with custom dimensions on request, tell us your room size and how you use your seating. We can send leather samples and prepare a quote through our contact page.