Tongue and Groove Joint: Reference Guide

A tongue and groove joint is an edge joint where a protruding tongue on one board fits a matching groove cut into the edge of the next. The joint aligns the faces flush, resists vertical movement between boards, and leaves a small gap at the bottom of the groove so the tongue does not bottom out. It is the standard edge joint for flooring, panelling, wainscoting, and any run of boards that needs to stay flat while moving as a single surface.

The joint is not a structural connection in the way a mortise and tenon is. Its job is registration and surface continuity. Tongue and groove wood is used where a flat plane has to be built from narrow stock: floorboards, ceiling boards, beadboard, cabinet backs, and sometimes tabletops and drawer bottoms. In each case the boards move across their width with humidity, and the joint has to allow that movement without opening a gap or cupping.

How the joint works

The tongue is cut on one edge, the groove on the other. The tongue is slightly thinner than the groove is deep, and the groove is slightly deeper than the tongue is long. That clearance is deliberate. If the tongue bottoms in the groove, the joint cannot close fully, and any seasonal swelling drives the boards apart at the face.

For flooring and panelling, the groove is usually centred on the edge and the tongue is centred on the mating edge. For edge joints in solid stock, the tongue is often offset so the groove sits closer to one face, which keeps more material around the groove on the show face.

Wood movement is the reason the joint exists in this form. A board moves across its width, not along its length. A tongue and groove run lets each board swell and shrink independently while the tongue keeps the faces aligned. Gluing the tongue into the groove locks the boards together and defeats that allowance. Panelling and flooring are therefore usually left unglued at the tongue, or glued only at the ends, so the run can move as a unit. For more on the underlying behaviour, see the wood movement page.

Cutting by hand

Hand cutting is slow but reliable for short runs and for reproduction work where the profile has to match an existing board.

1. Mark the groove position on the end grain of both edges. For centred work, gauge from both faces and split the difference. For offset work, gauge from the show face.
2. Cut the groove first. Use a plough plane or a router plane with a fence, working from one end. Take light passes and check the width with the tongue stock as you go.
3. Cut the tongue second. Use a shoulder plane or a rabbet plane to remove the waste on both faces of the edge, leaving a tongue that fits the groove with light hand pressure.
4. Fit and adjust. A tongue that needs a mallet is too tight. A tongue that drops in with no contact is too loose. The target is a fit that holds by friction and can be pushed home by hand.

A rebate plane and a plough plane are the traditional pair. A router plane is useful for cleaning the bottom of the groove flat.

Cutting by machine

The two common machine methods are a router bit set and a table saw with a dado stack or a slot cutter.

Router bit set

A tongue and groove router bit set cuts both profiles with matched cutters. The groove cutter is usually a straight bit with a bearing or a fence, and the tongue cutter is a slot cutter or a matched pair of cutters. Set the groove depth first, then set the tongue height to match. Run test pieces from the same stock and the same thickness as the finished work. The tongue should enter the groove with light pressure and sit flush at the face.

For long runs, use a router table with a fence and featherboards. Feeding a long board past a router bit by hand invites the board to tilt, which rounds the tongue or bellies the groove.

Table saw

A table saw with a dado stack cuts the groove in one or two passes. The tongue is cut by removing the waste on both faces with the blade tilted or with the board on edge against the fence. A slot cutter in a router table is another option for the tongue.

The table saw method is faster for long runs and easier to keep consistent, but the tongue has to be cut in two passes, one from each face, and any difference in the two setups shows as an off-centre tongue. Cut the groove first, then sneak up on the tongue with test fits.

Ripping narrow stock for tongue and groove work is where most accidents happen. Use a push stick, keep the blade guard in place where the cut allows, and never reach over the blade to retrieve an offcut.

The loose tongue (spline)

A loose tongue, also called a spline, is a separate strip of wood glued or fitted into grooves cut in both mating edges. The grooves are cut the same way as a standard groove, but no tongue is cut on either board. The spline is sized to fit both grooves with clearance at the bottom.

The loose tongue is useful when the boards are already finished on both edges, when the stock is too thin to cut a reliable integral tongue, or when the joint has to be assembled dry and taken apart. It is also the standard approach for joining sheet goods, where an integral tongue would be too weak.

Spline thickness is usually about one third of the board thickness. For a 19 mm (3/4 in) board, a 6 mm (1/4 in) spline is typical. The spline should be slightly thinner than the combined groove depth so it does not bottom out and hold the faces apart.

Judging the fit

A good tongue and groove joint closes by hand, sits flush at the face, and shows no gap along the seam. The tongue should not bottom in the groove. There should be a small clearance at the bottom, and the faces should touch before the tongue reaches the end of the groove.

A bad fit shows in three ways:

• The tongue bottoms out and the faces stand open. The groove is too shallow or the tongue is too long. Deepen the groove or shorten the tongue.
• The tongue is too thin and the joint racks. The tongue should fill the groove in width, not just in depth. A thin tongue lets the boards shift vertically under load.
• The groove is off-centre and the faces are not flush. This usually comes from cutting the groove from one face only. Gauge from both faces and check with a square.

For dry-fitted work, the joint should hold together when lifted by one board. If it falls apart, the fit is too loose.

Where it fails

Tongue and groove fails in three predictable places.

At the tongue root. The tongue is the thinnest part of the edge and the most likely to snap when the board is forced. A tongue that is too tight, or a board that is twisted, puts the root under tension. Cut the tongue slightly thicker than the groove is wide and ease the fit with a shoulder plane rather than forcing it.

At the groove wall. A groove cut too close to the face leaves a thin wall that blows out under clamping pressure. This is common in thin stock and in flooring where the wear layer is shallow. Offset the groove toward the thicker side of the board.

At the seam. In panelling and flooring, the seam opens when the boards are glued across the tongue and cannot move. The joint is designed to allow movement, not to resist it. Leave the tongue unglued, or glue only at the ends, and let the run float.

Comparison of methods

The most common mistake

The most common mistake is cutting the groove too shallow and the tongue too long, then forcing the joint closed with clamps. The tongue bottoms in the groove, the faces stand open, and the clamps pull the boards out of flat. The reason it happens is that the groove depth is set by the cutter or the plane, and the tongue length is set by the same setup, so a small error in either doubles. The fix is to cut the groove deeper than the tongue is long by a clear margin, and to test the fit with the tongue dry before any glue is applied. If the joint closes by hand and the faces touch, the clearance is correct. If it needs clamps to close, the tongue is too long or the groove is too shallow.