Tenon, dowel, screw: what wood joints reveal about quality

Where the forces act

A joint is loaded in four ways: in compression, when weight pushes it together. In tension, when something pulls it apart. In shear, when two parts are displaced sideways against each other. And in bending, when a long lever acts on a short connection.

The lever is the underestimated case. A chair leg is long, the joint to the rail short. Every time someone leans back, the force acting at the top is multiplied down at the joint. That is exactly why chairs fail more often than shelves, even though more weight rests on a shelf: shelves are loaded almost only in compression, chairs in every way at once.

In addition, wood moves. It absorbs moisture and releases it again, swelling and shrinking considerably more across the grain than along it. A good joint allows this movement instead of blocking it. Where it is blocked, either the wood or the joint cracks.

Mortise and tenon: the most durable

A tenon is a projection cut at the end of a component that fits into a matching opening in the other part. The joint does not hold through glue alone but through its shape – known in the trade as a form-fit. The mating surfaces are generous, and the geometry prevents the parts from twisting against each other.

What matters here is the glue surface. With a tenon, two long-grain surfaces meet, and this glued joint is the strongest there is in woodworking – often stronger than the wood next to it. If a tenoned chair does break, it is usually the wood next to the joint that gives way and not the joint itself.

That is why old chairs that are still standing are almost always tenoned. It takes a lot of work, and the joint is practically permanent – which is also its drawback: taking it apart and shipping it flat is not possible. Related forms of the same principle are the finger joint at drawer corners and the dovetail, whose wedge-shaped geometry also stops it being pulled apart.

Dowels: the compromise

Round wooden pins in drilled holes, glued. Considerably quicker to make than a tenon and can be done by machine, because two drill holes are enough where otherwise chiselling and sawing would be needed.

Dowels hold well in compression and moderately in tension. The reason lies in the glue line: the dowel's long grain sits in a hole whose wall consists partly of end grain, and end grain soaks up glue without forming a particularly strong bond. The effective bonding surface is smaller than it looks.

In a chair, where leaning back pulls on the back legs, this is the weak point. In carcass furniture, where mainly flat surfaces meet and a back panel set into a groove provides additional bracing, dowels are entirely sufficient. Two dowels per joint instead of one are not a detail here, but the difference between a joint that can twist and one that cannot.

The ribbed surface of many dowels is not decoration either. The grooves take up excess glue and ensure that a film of glue remains between dowel and hole instead of being pressed out completely when the dowel is driven in.

Screws: reversible, but limited

Screws are the only joint that can be opened again. That is exactly why they are found in every piece of furniture that is delivered in a box and only assembled where it is to stand.

The problem is the thread. A screw holds by its threads gripping fibres behind them. In solid wood across the grain, it grips many times over and along the entire screw-in depth. In end grain, that is, into the fibre, it grips poorly: there the thread runs parallel to the fibres and tends to push them apart rather than grip them. A screw into end grain therefore holds, depending on the species, only a fraction of what the same screw holds across the grain. Where such a screw joint cannot be avoided, it helps to insert a wooden dowel crosswise, so that the screw grips across the grain again.

In MDF and chipboard a different limitation applies. These materials have no continuous fibres; they consist of compressed chips or wood fibres with a binder. The thread forms its own bed when first screwed in, and in doing so destroys exactly the material it is meant to hold on to. If the screw is taken out and put back in, it finds a widened hole and holds considerably less. In the edge of a board, where the structure is loosest, it holds worst. Carefully designed board furniture therefore uses recessed threaded sleeves, cam fittings or metal threaded inserts there – components that spread the load over a larger area and can be loosened as often as needed.

And anyone who keeps turning during assembly until it "feels tight" destroys the thread completely. A stripped thread in a fibreboard cannot be restored. The rule is: tighten until the parts lie against each other without play, and then stop. Electric screwdrivers with high torque are more of a risk than a help when assembling furniture, because they reach the point at which the thread gives way faster than you can react.

Screw joints need aftercare in return. Because wood moves and every load tugs slightly at the joint, screws loosen over the years. Checking all accessible screw joints once a year and tightening them only until they are firm again considerably extends the life of a piece of furniture that can be taken apart. A piece that wobbles slightly is often stable again after this alone. If the wobbling returns after a short time, however, that points to a worn thread and not to a loose screw.

What visible screws reveal

Visible screw heads are not a quality defect – in many designs they are deliberately part of the design. What is revealing is whether their position looks planned or left over: even spacing, uniform alignment of the heads and clean countersinks point to the former, an irregular pattern in odd places to the latter.

Concealed screw joints are the more elaborate route, because the construction then has to be thought out from the inside instead of being screwed from the outside. The screw sits in a pocket, behind a fitting or on the back, and the parts must go together in an order in which every screw can still be reached. This costs design time before the first piece of furniture is made.

Covered screws are something different again from concealed ones. Stuck-on plastic caps hide only the head and change nothing about the joint beneath. They are a cosmetic component, not a construction feature – and they are usually the first thing to fall off.

Why metal should be welded

With metal frames, the same logic applies in a different form. A screwed frame can loosen over the years, because every load works on it slightly and because there is always some play between screw and hole. A cleanly welded frame cannot: the parts are fused into one component, and there is no joint that could widen.

The catch lies in the workmanship. A weld is only as good as its penetration. A seam that only sits on the surface or contains pores can crack under alternating loads, and then the damage is harder to repair than with a screwed joint. This can be seen in the seam itself: even, continuous and without holes is good; interrupted or lumpy is not.

Geometry counts too. A frame made from one continuous bent profile or from a U-shaped welded frame has fewer junctions than one assembled from many short pieces. Every joint is a potential weak point, and the most stable joint is the one that was never needed. Where a type of construction is described at all, it is therefore worth paying attention to exactly this difference.

The invisible support

The most revealing details sit where nobody sees them. With seats of rattan, Viennese cane, cane weave or rope, carefully built furniture has an additional seat support beneath the woven surface – a frame, webbing or a thin board that takes the weight.

Without it, weaving sags after a few years, because it cannot carry weight over time without giving. The fibres lengthen under constant load, and this stretch does not fully recover. Natural weaving also reacts to the indoor climate: in dry air it shrinks and becomes more brittle; in humid air it expands. A support beneath takes over the load and leaves the weaving mainly to form the surface.

A manufacturer who builds in such a component even though it cannot be seen and can hardly be reflected in the price has probably thought ahead elsewhere too. The same applies to other invisible components: corner blocks in the rail corners of a chair, a continuous centre rail under a wide top, back panels set into grooves instead of nailed on. None of them can be seen; each extends the lifespan.

How to recognise joints

On the finished piece of furniture, the joint itself is usually hidden, but it leaves traces. A through tenon shows as an end-grain surface on the outside, often lighter or darker than the surrounding wood. A finger joint is immediately visible at the corner of a drawer. Dowels give themselves away as round dots where they are not covered.

A second clue is how it behaves under load. Press a piece of furniture lightly on the diagonal and you will feel the difference between a construction that responds as a whole and one in which individual joints move. A faint creak almost always comes from a joint and not from the surface.

Where a construction is not described at all and only material and dimensions are given, it is usually unspectacular. That need not rule it out – for a piece of furniture that is loaded mainly in compression and rarely moves house, a simple construction is entirely sufficient. But it puts the price into perspective, and it says something about how many moves and rearrangements a piece is likely to survive.