At a glance
| Ref | Part | Job | What to look for |
|---|---|---|---|
| 1 | Front leg | Carries the front of the seat and meets the seat rail. | Solid kiln-dried hardwood, grain running with the leg. |
| 2 | Seat rail | Ties the legs together and carries the webbing or suspension. | Joints at both ends; no splits at the ends. |
| 3 | Back post and back rail | Carries the back load; the post often continues the rear leg. | Grain running along the rake, not across it; the back joint is a stress point. |
| 4 | Stretcher | Stiffens the frame against racking. | Fixed with a joint, not only a screw. |
| 5 | Corner block | Reinforces a corner of the seat frame. | Glued and screwed; fitted at every corner of a seat frame. |
| 6 | Webbing or suspension rails | Support the seat springs or cushion. | Fixings into the rail, not into the end grain. |
Parts as shown in the drawing. Workshop practice, not a standard: the partner upholsterer states what is used.
Parts of a frame
An upholstered seat is built on a frame of rails, legs and posts, with corners reinforced and the seat supported by webbing or a rail system. The drawing names the parts in a chair; a sofa adds longer rails and a central rail. How the seat is then sprung is covered in seat suspension systems compared, and the layers above it in seat cushion constructions compared.
Timber and moisture
Frame timber is dried before it is cut, because wood that dries after assembly shrinks, and a joint that was tight opens. The usual specification is kiln-dried hardwood, with sheet material for panels. A US Forest Products Laboratory note is reported to put furniture wood at 5 to 8 per cent moisture content, but that was seen only in a search summary here, and UK interiors with underfloor heating can be drier. Rather than quote a figure, ask the partner upholsterer to state the moisture content the frame was made at and how it was checked, and see timber movement in panelling and veneer and timber species for joinery for the movement and species picture.
Grain direction is the other check. Legs and back posts that curve cut across the grain at the bend, so the frame maker keeps the grain along the part or uses laminated or bent sections. Say in the specification which parts are solid, which are laminated, and which are sheet.
Joints compared
A joint carries load through glue and fit, and the corner reinforcement beside it does the rest. The table lists the common joints; each is acceptable when the workshop uses it where it suits. For furniture and joinery more widely see joint types in furniture and joinery.
| Joint | What it is | Where it suits | What to ask or look for |
|---|---|---|---|
| Dowel | Round pegs glued into matching holes in both parts. | Rails into legs in many seat frames. | Number and diameter of dowels per joint; glue type; fit. |
| Mortise and tenon | A tongue cut on one part fits a slot cut in the other. | High-load joints such as leg to rail, and stretchers. | Tenon thickness and length; pegged or glued; shoulder fit. |
| Loose tenon | A separate tongue fits slots in both parts. | Frames where a cut tenon is awkward to make. | Same checks as mortise and tenon. |
| Corner block | A triangular or square block glued and screwed inside a corner. | A reinforcement beside the main joint, not a replacement for it. | Glue and screws together; no nails alone. |
| Metal bracket or knock-down fitting | A fitting that bolts parts together. | Pieces that must be taken apart for delivery. | Fitting type and rating; access for re-tightening. |
| Screw only | Parts fixed with screws and no joint. | Non-structural parts, or where a joint has been added. | Not accepted for a structural joint without evidence. |
Specify a joint as an outcome as well as a type: the part the joint joins, the glue, and what a corner is reinforced with. Nails and staples hold upholstery in place; they are not a structural joint.
What a test report covers
A strength test report to EN 16139 is evidence that a sample of the chair took defined loads and cycles at a stated level, and stood up in the stability tests. The three accredited reports read for this sheet list seat and back static loads, front-edge and durability cycles, leg-forward and leg-sideways loads, impact tests and the stability tests, each with the clause in EN 1728 or EN 1022. They are useful. They do not say what is inside the frame.
| An EN 16139 test report shows | It does not show |
|---|---|
| Static loads on seat, front edge and back, with cycles and loads recorded | How the upholstery looks or wears |
| Durability cycles on the seat and back | Fire performance of the cover, foam or filling |
| Leg-forward and leg-sideways loads, and impact tests | Whether the frame wood was dry or kiln-dried |
| Stability tests (forwards, sideways, rearwards) | Moisture movement or finish ageing over years |
| The level it was tested at (L1 or L2) and the edition | Mechanisms, castors or electrical parts |
So ask for the report and for a frame statement: timber, moisture, joints and reinforcement, and whether the tested sample is the same build as the one specified. See BS EN 16139 seating levels, furniture strength and durability standards map and FIRA Gold and furniture test reports for the standards and the reports.
Specification wording
Paste into your schedule or specification and replace anything in [square brackets] with your own figures. Wording is a starting point: your specification governs.
Frame to be [solid kiln-dried [beech / ash / oak] / laminated hardwood] with the moisture content at manufacture stated by the workshop as [ ] per cent and the method recorded. Sheet material only where shown on the shop drawing. Grain to run along legs, rails and back posts.
Legs to rails to be jointed by [mortise and tenon / dowels / loose tenons], glued, with [number] per joint. Each corner of the seat frame to be reinforced with a glued and screwed corner block. Screw-only joints are not accepted for structural connections. Stretchers to be jointed to legs.
The supplier is to provide a test report to [BS EN 16139 level [L1 / L2], edition [ ]] for a sample of the same construction, and a frame statement listing timber, moisture content, joints and reinforcement. Any change to the frame after sign-off is to be notified and approved.
Terms used on this page
- Kiln-dried
- Timber dried in a controlled kiln to a stated moisture content before it is cut and used.
- Corner block
- A block of wood glued and screwed into the inside of a frame corner to stiffen it.
- Racking
- The sideways distortion of a rectangular frame towards a parallelogram under a push along one side.
- Webbing
- Strong woven strips stretched across the seat rail to support springs or foam.
- Mortise and tenon
- A joint in which a tongue cut on one part fits a slot cut in the other.
- Moisture content
- The weight of water in wood as a percentage of its dry weight.
Standards and sources
- BS EN 16139:2025: Furniture. Strength, durability and safety. Requirements for non-domestic seating. Read through the test reports that cite it; the standard itself was not read. Current at the time of the library notes (BSI, 31 Aug 2025).
- BS EN 1728:2012: Domestic furniture. Seating. Test methods for strength and durability; clause numbers seen in test reports. Not read. Listed current in the library notes.
- BS EN 1022:2023: Furniture. Seating. Determination of stability; cited in a 2025 test report. Not read.
Questions we are asked
What should an upholstered frame be made from?
Usually kiln-dried hardwood, with sheet material for panels. The specification should state the timber, the moisture content at manufacture and how it was checked, rather than a general promise of quality.
Are corner blocks needed?
A glued and screwed corner block is a common reinforcement in seat frames. It supports the joint beside it; it does not replace it. Ask the workshop which corners have one.
Is a screwed frame acceptable?
Screws are fine for fixing and reinforcement. A structural connection should have a proper joint as well. Say so in the specification.
Does a test report prove the frame is well built?
It shows a sample took defined loads and cycles at a stated level. It does not show moisture content, joint type, or how the upholstery will wear. Ask for a frame statement as well.
What is racking?
The sideways distortion of a frame under a push. Stretchers, jointed corners and corner blocks stop it.
Download the factsheet
A two-page A4 PDF with the frame drawing, the joints table and three specification clauses, to keep with your project file.