Joinery
Joinery cuts interlocking geometry into timber so the parts locate and hold each other, with or without adhesive or fasteners.
- Part
- Joining
- Revised
- 2026-08-11
At a glance
- Family
- Mechanical
- Typical tolerances
- Glue line of 0.002–0.006 in (0.05–0.15 mm) for PVA; joints fitted to firm hand pressure rather than to a numeric tolerance
- Typical volumes
- 1 to a few thousand — hand work for one-offs, jigged or CNC-cut joinery for batch production
- Lead time
- Minutes to an hour per joint depending on method; 30–60 minutes clamp time and 24 hours to full adhesive cure
- Materials
- Wood
What it is
Joinery cuts interlocking geometry into timber so the parts locate and hold each other, with adhesive and fasteners as reinforcement rather than as the joint itself. The reason it exists is a material fact: an end-grain glue joint develops only a fraction of the strength of a long-grain one, so a plain butt joint between two pieces of wood is structurally worthless. Joinery converts that end-grain butt into long-grain glue area plus a mechanical interlock.
Properly executed, the result is stronger than the wood: a long-grain glued joint tested in block shear fails in the timber rather than at the glue line, and acceptance is generally stated as a percentage of wood failure rather than a stress.
The dominant families are mortise and tenon (frames), dovetail and box joints (carcase corners), lap and bridle joints (crossing members), dado and rabbet (shelving and casework), and loose-tenon systems — dowels, biscuits and domino tenons — which trade some strength for repeatability and speed.
How it works
- Mill the stock true. Every joint dimension is referenced from flat, square, parallel faces. Timber that is not four-square first will not produce a tight joint no matter how carefully the joint is cut.
- Lay out from a face side and face edge. All measurements come from the same two reference surfaces so cumulative error does not accumulate around a frame.
- Cut to a push fit. A mortise and tenon should assemble under firm hand pressure. A joint that needs a mallet is too tight: it squeezes the adhesive out of the joint line and starves it, and it can split the mortise cheek.
- Glue and clamp. PVA and aliphatic resin adhesives work best in a thin, close-fitting glue line of roughly 0.002–0.006 in (0.05–0.15 mm) and have poor gap-filling ability beyond that. Clamp at 100–150 psi for softwoods and 175–250 psi for hardwoods. Open assembly time is typically 5–10 minutes, clamp time 30–60 minutes, and full cure 24 hours. Where gaps are unavoidable, epoxy tolerates them and PVA does not.
- Peg or wedge where appropriate. Draw-boring a pegged mortise and tenon — offsetting the peg hole in the tenon by 1/16–1/8 in (1.5–3 mm) toward the shoulder — pulls the joint tight mechanically and holds it without adhesive at all.
Classic proportions
Tenon thickness is traditionally about one third of the stock thickness, rounded to the nearest available chisel or router bit size, with tenon length two thirds to three quarters of the mortised member's width. Dovetails are cut at a 1:6 slope for softwood and 1:8 for hardwood — roughly 9.5° and 7° — the shallower angle reflecting hardwood's greater resistance to short-grain failure at the pin tips.
Design guidelines
Design around wood movement, not against it
Wood moves across the grain and essentially not along it. In a typical interior environment a wide board will move roughly 1/8–1/4 in per 12 in (3–6 mm per 300 mm) of width between the humid and dry seasons, and no joint will stop it. The design rules follow directly: never glue a wide panel cross-grain to a rail, let tabletops float on slotted fasteners or buttons, size breadboard ends so only the center is pinned, and orient panels in frames so they can expand into their grooves.
Maximize long-grain glue area
The joint's strength comes from long-grain-to-long-grain contact. A mortise and tenon works because the tenon cheeks present long grain to the mortise walls; a dovetail works because the interlocking geometry adds mechanical resistance where glue area is limited. When choosing between joints, count the long-grain area, not the total surface area.
Fit is a strength variable
Aim for a joint that assembles under firm hand pressure. Too loose and PVA cannot bridge the gap; too tight and the adhesive is squeezed out and the joint is starved, with the additional risk of splitting the mortise. Where a gap is unavoidable — a repair, a hand-cut joint that opened up — switch adhesives rather than adding clamping force.
| Joint | Typical proportion | Use | Why |
|---|---|---|---|
| Mortise and tenon | Tenon 1/3 stock thickness, length 2/3–3/4 of member width | Frames, doors, chairs, tables | Maximum long-grain area in a right-angle frame joint |
| Through dovetail | 1:6 softwood, 1:8 hardwood | Carcase corners, drawers | Mechanically resists pull-out even without adhesive |
| Box / finger joint | Fingers roughly equal to stock thickness | Boxes, drawers, machine-cut casework | Large glue area, simple to jig |
| Half lap | Half thickness removed from each member | Crossing rails, frames | Fast; the weakest of the frame joints |
| Dado / housing | 1/3 stock thickness deep | Shelving, casework dividers | Supports load without relying on the glue line |
| Loose tenon (domino, dowel) | Tenon 1/3 stock thickness | Repeat production frames | Mortise and tenon strength with machine repeatability |
| Butt joint, end grain | — | Avoid | End-grain glue joints carry a fraction of long-grain strength |
Mortise widths and dowel holes are cut with standard bit sizes — see the drill size chart.
Inspection
Visual first: gap-free shoulders, continuous glue squeeze-out along the whole joint line, and shoulders that pull tight without clamp force fighting a badly cut joint. Beyond that, block shear testing to ASTM D905 on sample joints, where acceptance is generally stated as a minimum percentage of wood failure rather than a stress value, and moisture content verification with a meter — the timber and the shop should be at the same equilibrium moisture content before any joint is cut.
Cost drivers
Joinery is labor. Hand-cut dovetails in a drawer take an experienced maker a substantial fraction of an hour per corner; the same corner cut on a jig takes minutes, and a machine-cut loose tenon takes seconds. The cost question is therefore always which joint, cut by what method, rather than which material.
Machine setup is the countervailing cost. Router jigs, dovetail templates, hollow-chisel mortisers and domino machines all pay for themselves over a batch and cost time on a one-off. Timber itself is a real input cost, and joinery consumes length — tenons, laps and dovetails all require stock beyond the finished dimension.
Clamp time occupies fixtures and floor space. A 30–60 minute clamp cycle at 24 hours to full cure sets how many assemblies can move through a bench per day, and it is usually the binding constraint in a small shop.
- Use loose tenons for repeat work. Domino and dowel joinery gives most of the strength of a cut mortise and tenon at a fraction of the machine time.
- Jig anything you make more than twice. The jig pays for itself on the second batch and improves consistency on the first.
- Batch the same joint across an assembly so one setup cuts every mortise before the machine is reconfigured.
- Buy timber at the right moisture content and let it acclimate — a joint cut in wet stock will open regardless of how well it was made.
- Reserve hand-cut joinery for what is seen. Through dovetails on a visible drawer front, machine joints everywhere else.
Questions
5 questionsWhy can't you just glue and butt two pieces of wood together?
Because an end-grain glue joint develops only a fraction of the strength of a long-grain one — the open cell ends absorb adhesive and there is nothing for it to key into. Joinery exists to convert an end-grain butt into long-grain glue area plus a mechanical interlock. A properly glued long-grain joint, by contrast, is stronger than the surrounding timber.
How thick should a tenon be?
About one third of the stock thickness, rounded to the nearest available chisel or router bit size, with a length of two thirds to three quarters of the mortised member's width. Thinner and the tenon itself becomes the weak point; thicker and the mortise cheeks are left too thin and split. The one-third rule balances the two failure modes.
What dovetail angle should I use?
1:6 for softwoods, about 9.5°, and 1:8 for hardwoods, about 7°. The steeper angle in softwood provides more mechanical interlock in a material that compresses easily; the shallower angle in hardwood avoids leaving fragile short grain at the pin tips, which would break out when the joint is assembled.
How tight should a mortise and tenon fit?
It should assemble under firm hand pressure. A joint driven home with a mallet is too tight: it squeezes the adhesive out of the glue line and starves the joint, and it can split the mortise cheek. A joint that falls together is too loose for PVA, which has almost no gap-filling ability beyond about 0.006 in (0.15 mm) — use epoxy if a gap cannot be avoided.
How much does wood move seasonally, and how do I design for it?
Across the grain, roughly 1/8–1/4 in per 12 in (3–6 mm per 300 mm) of width in a typical interior environment; along the grain, effectively nothing. Design so the movement is unrestrained: float tabletops on slotted fasteners or buttons, leave panels loose in their frame grooves, and never glue a wide panel cross-grain to a rail.