Joint types in furniture and joinery

At a glance

Strength statements are the Wood Handbook's general ones (US Forest Products Laboratory, 2021). Where the cell says 'no figure read', none was found.

What a joint has to do

A joint has three jobs: carry the load the part sees, resist racking (the twisting of a frame out of square), and survive the timber's movement with humidity. Which joint does that best depends on which faces of the timber meet, because wood glues well along its length and badly across its ends. The US Forest Products Laboratory puts the point plainly in the Wood Handbook (chapter 10): end-grain surfaces are many times more porous than the sides, adhesive soaks in, and that is a main reason it is so hard to make a strong, load-bearing butt joint.

This sheet describes six joints, says where each is normally used, and lists what to put on a drawing or in a specification so a partner workshop quotes and builds the joint you intend. It does not rank joints by strength: apart from the general statements below, no published strength figure for a decorative joint was found, and the choice is a matter for the workshop's judgement and the part's load. See the drawer construction sheet and the furniture tolerances sheet for neighbouring detail.

Grain decides: end grain against long grain

The Wood Handbook gives two figures worth carrying into a specification. An end-grain butt joint (end of one piece glued to the end of another) reaches only about 25 per cent of the tensile strength of the wood parallel to the grain, even with special techniques. A well-made scarf or finger joint, where the faces are cut long so the glue line lies mostly along the grain, can reach up to 90 per cent, if the slope is 12 in 1 or flatter (chapter 10, p. 10-16 and 10-17). These are structural, tension figures for adhesive-bonded lumber: they show the principle, and they are not design values for a table leg.

For joints where one piece meets the edge of another, as in a frame or carcass, the Handbook says a plain end-to-edge joint is difficult to design to carry appreciable load, and that strong connections interlock the surfaces so that edge grain bonds to edge grain. It names dowels, mortise and tenons and rabbets as examples and lists the dovetail in the same figure (Fig. 10-8). Because wood swells so much more across the grain than along it, moisture changes in these joints produce large internal stresses, so all end-to-edge joints should be protected from changes in moisture content in service (p. 10-17).

The six joints

What follows is how each joint is normally used in furniture and joinery, written to help you brief it. Drawn proportions in the figure are not a design.

Dovetail. Wedge-shaped tails interlock with pins so that the joint resists being pulled apart in one direction. Used for drawer boxes and fine carcass corners. It can be cut by hand or machine, and the choice shows: hand-cut is slower and reads as such. Decide whether the joint is to be seen. A lapped (half-blind) dovetail hides the tails behind a drawer front.

Mortise and tenon. A tongue (the tenon) on one piece enters a socket (the mortise) in another. It is the joint for frames: legs and rails, and door stiles and rails. The Architectural Woodwork Standards (2009, US) illustrate haunched and slot versions in door joinery. A haunch is a short extension of the tenon that fills the groove at the end of a stile.

Dowel. Round pins in matching holes across a joint. Common in carcasses and for edge joints. The main risk is alignment: two holes must meet exactly. Ask for the dowel diameter, length, number and spacing. The AWS shows dowels in door, frame and handrail joinery.

Loose tenon. A separate rectangular tenon set into a mortise cut in each piece. It does the work of a mortise and tenon without cutting a tenon on the rail, so it suits repeated frames and heavier carcass work. The AWS shows it in door joinery.

Biscuit. A thin lens-shaped compressed wood chip in a slot across a joint. It is used to line up panel edges and carcass corners while the glue sets. Treat it as an aid to alignment, not as a source of strength, unless the workshop gives you a reason. Water in the glue can swell it.

Screw. A screw is a fastener, not a glued joint: it is right for knock-down parts and hidden fixings. The Wood Handbook says screws in end grain hold on average about 75 per cent of the load of screws in side grain, when splitting is avoided (ch. 8, p. 8-11), and that screws should be turned in, never driven with a hammer. Where a part must come apart, say so.

Which joint where

These are starting points to brief a workshop with, drawn from common practice and the sources above. They are not a ruleset, and a good workshop may propose an alternative.

What to put on the drawing and the schedule

A specification does not need to name every joint. It needs the ones that matter, and the evidence that they were made as drawn.

1. The joint at each position, by drawing reference, not a general statement: "drawer corners dovetailed" is not the same as "all corners".
2. Visible or hidden: a joint on show is a design decision, so draw it.
3. Size: tenon thickness, width and length; dowel diameter, length and spacing; tail and pin proportions: as dimensions on the shop drawing, which the workshop proposes and you approve.
4. Adhesive: type and grade, stated by the workshop, with its suitability for the humidity of the room (see finishes on timber joinery and timber movement).
5. Moisture content of the timber at assembly, and the room it is going into (see timber species for joinery).
6. No load-bearing butt joints, no staples as a structural fixing, no brackets covering for a weak joint.
7. Evidence: a section or a dismantled sample, and a drawing sign-off (see the shop drawing checklist).

How to check one. Pull a drawer fully out and turn it over; look at the corners. Look under a table or chair to see how the legs meet the frame. Sight along a shelf under load. A visible, well-cut joint is a good sign. A metal bracket hiding the connection often covers a weak one (our journal makes the same point in its guide to types of furniture joints).

Joints between [legs and rails / door stiles and rails] to be [haunched mortise and tenon / loose tenon], tenon [x] mm thick x [x] mm wide x [x] mm long, glued with [adhesive and grade stated by the workshop] to joint reference [drawing ref]. No load-bearing butt joints. Timber at [8] per cent moisture content (+/- [ ]) at assembly.

Drawer box corners to be [lapped / through] dovetails, [hand-cut / machine-cut], at [front / back] corners, as drawing [ref]. The joint is [visible / hidden] when the drawer is open. Glued with [adhesive]; the drawer bottom let into a groove, not fixed to the underside.

Carcass joints to be [dowelled / loose tenon / housed], dowel [8] mm diameter x [30] mm long at [x] mm centres, glued. Biscuits, if used, are for alignment only and are not to be counted as the structural joint. Joint set-out to be shown on the shop drawings for approval before production.

Parts marked [KD] on drawing [ref] to be assembled with [screws / steel fittings] and not glued; fixings [type and length], lead holes [as the supplier states]. Screws are not to be driven into end grain for load unless the workshop confirms the detail.

Standards and sources

• USDA Forest Products Laboratory, Wood Handbook: Wood as an Engineering Material, FPL-GTR-282 (2021): ch. 10 pp. 10-16 and 10-17 (end-grain and end-to-edge joints); ch. 8 p. 8-11 (screws). US publication; general principles only.
• Architectural Woodwork Standards, 1st edition (2009), AWI, AWMAC and Woodwork Institute: Sections 6, 7 and 9: joint illustrations for frames, handrails and doors. US document, free section PDFs; grade rules excluded. Not read for strength.
• Library sheets: drawer construction; furniture tolerances; timber movement: neighbouring detail.

Questions we are asked

Is a dovetail stronger than a mortise and tenon?

They do different jobs. A dovetail resists being pulled apart in one direction and suits drawer and box corners. A mortise and tenon suits frames that see racking. No published strength comparison for decorative joints was found, so ask the workshop which suits the part and approve it on a drawing.

Is a biscuit joint strong?

Treat it as an aid to alignment. It lines up panel edges and corners while the glue sets. If the joint must carry load, ask the workshop what carries it besides the biscuit.

What is the difference between a dowel and a loose tenon?

A dowel is a round pin in two round holes. A loose tenon is a flat rectangular tongue in two matching mortises, so it has a larger glue area and resists twisting better, at the cost of a more involved cut. Both are separate pieces glued into both parts.

Why do end-grain joints fail?

The Wood Handbook says end grain is many times more porous than the sides, so adhesive soaks in and leaves a weak line, and that butt joints reach about 25 per cent of the wood's tensile strength. Joints are designed so that long grain meets long grain.

Can I screw into end grain?

It holds, but less: the Wood Handbook says about 75 per cent of the holding of a screw in side grain, if splitting is avoided. For load, use a different joint or a through fixing.

Should I specify the joint or let the workshop choose?

Specify the ones on show and the ones that carry load, with a drawing reference. Leave hidden joints to the workshop, but ask for the choice to be shown on the shop drawing.

How can I check joints without taking a piece apart?

Look at drawer corners, the underside of tables and chairs, and the junction of door stiles and rails. A mock-up or a sectioned sample is the clearest evidence.