Tongue and Groove

Part of Woodworking

When you need a wide panel — a tabletop, a floor, a wall — you must join narrow boards edge-to-edge. The tongue-and-groove joint does this while allowing each board to expand and contract with humidity changes. Without this joint, wide surfaces would crack, buckle, or gap open with the seasons.

Why Tongue and Groove

Wood moves. Every board swells when wet and shrinks when dry. A wide solid panel (if you could find one) would crack. Multiple narrow boards joined edge-to-edge can each move independently — if the joint allows it.

Tongue and groove provides:

• Alignment: The tongue keeps boards flush on the face — no stepping or offset between boards
• Mechanical connection: The interlocking profile resists forces perpendicular to the joint line
• Movement tolerance: In flooring and wall applications, the joint can be left unglued so boards slide freely as they expand and contract
• Weatherproofing: The interlocking profile blocks air and water from passing through the joint gap

Applications

Application	Glued?	Why
Tabletop panel	Yes	Needs to act as one rigid surface
Flooring	No	Must allow seasonal movement across many boards
Wall sheathing	No	Movement expected, weatherproofing needed
Ceiling planks	No	Gravity and nails hold; movement expected
Door panel in frame	No	Panel floats in groove, free to expand

Tongue Sizing

The universal rule: the tongue is 1/3 of the board thickness.

Board Thickness	Tongue Thickness	Tongue Length (depth)	Groove Depth
15 mm	5 mm	8-10 mm	10-12 mm (slightly deeper than tongue)
20 mm	6-7 mm	10-12 mm	12-14 mm
25 mm	8 mm	12-15 mm	14-16 mm
30 mm	10 mm	15-18 mm	17-20 mm

Warning

The groove must always be 1-2 mm deeper than the tongue is long. If the tongue bottoms out before the shoulder seats, the joint has a gap at the face. Leave room for glue squeeze-out too.

Cutting the Groove

The groove is the easier half. You are removing a centered channel from the edge of a board.

Method 1 — Plow Plane

A plow plane (also called a grooving plane) is the traditional tool. It has:

• A narrow iron (blade) the width of the groove
• An adjustable fence that rides along the board face
• A depth stop

Procedure:

1. Set the fence to center the groove (1/3 from each face)
2. Set the depth stop to the desired groove depth
3. Clamp the board edge-up in a vise
4. Run the plane along the edge, taking multiple passes until the depth stop bottoms out
5. The groove will be clean, straight, and consistent

Method 2 — Chisel and Gauge

Without a plow plane:

1. Set a marking gauge to 1/3 of the board thickness. Scribe a line on both faces along the edge.
2. Set the gauge to 2/3 of the board thickness. Scribe a second line on both faces. You now have two parallel lines defining the groove walls.
3. Score the groove walls: Run a marking knife along both gauge lines to sever the surface fibers
4. Chisel the waste: Working from the end of the board, chop downward between the scored lines. Work in short sections, removing waste chips. A chisel exactly matching the groove width speeds this enormously.
5. Clean the groove bottom: Pare with a narrow chisel held flat, checking depth with a small ruler

Method 3 — Saw and Chisel

1. Saw two kerfs along the gauge lines to the desired depth (use a small backsaw)
2. Chisel out the waste between the kerfs
3. Clean up the bottom

Tip

If you have a drill, bore a series of overlapping holes between the gauge lines at the correct depth, then clean with a chisel. This removes most waste quickly.

Cutting the Tongue

The tongue is formed by cutting a rabbet (step) on each face of the board edge, leaving the centered tongue standing.

Method 1 — Rabbet Plane

A rabbet plane cuts a step along the edge. Make two passes — one from each face — to leave the tongue.

1. Set the plane to cut 1/3 of the board thickness deep (matching the tongue position)
2. Set the depth stop to the tongue length
3. Plane the rabbet on one face
4. Flip the board and plane the rabbet on the other face
5. The remaining center strip is the tongue

Method 2 — Saw and Chisel

1. Gauge lines on both faces at 1/3 thickness (same as the groove walls)
2. Gauge the tongue length on the end grain
3. Saw the cheeks: With the board edge-up, saw along the gauge lines on each face down to the tongue length
4. Remove waste: Lay the board flat, saw or chisel along the end-grain line to release the cheek waste
5. Test fit: The tongue should slide into the groove with hand pressure — snug but not forced

Fitting

• Too tight: Pare the tongue cheeks with a chisel or plane
• Too loose: You cannot add material. If slightly loose, glue will fill a small gap. If very loose, re-cut the tongue on a new board.
• Test with a scrap piece that has a groove cut to the same dimension — do not rely on the actual workpiece until the fit is right.

Loose Tongue (Spline) Joint

An alternative that is often easier: instead of shaping a tongue on the board edge, cut a groove in both boards and insert a separate strip of wood (the spline or loose tongue).

Advantages

• Both operations are identical — groove cutting, which you already know
• No risk of breaking a thin integral tongue during handling
• The spline can be made from plywood or cross-grained material for extra strength
• Easier to fit — just plane the spline to thickness

Procedure

1. Cut a centered groove in both board edges (same as above)
2. Make a spline strip from a thin piece of wood, planed to fit snugly in both grooves
3. The spline width (grain direction) should match the combined depth of both grooves minus 2 mm for clearance
4. Apply glue to the grooves (for panels) or leave dry (for flooring)
5. Insert the spline and push the boards together

Tip

For the strongest spline, orient the grain across the joint (perpendicular to the board length). This resists the opening force better than long-grain splines. However, cross-grain splines do not work for flooring — they break when boards move. Use long-grain splines for unglued applications.

Board-and-Batten

The simplest alternative to tongue and groove. Boards are placed edge-to-edge (or with small gaps) and a narrower board (the batten) is nailed or pegged across the back to hold them together.

When to Use

• Doors, gates, shutters
• Quick wall sheathing
• When boards are rough or inconsistent in width
• When no specialized planes are available

Construction

1. Lay boards side by side, face down
2. Place battens across the back, perpendicular to the boards
3. Peg or nail each batten to each board
4. Use at least two battens (top and bottom); three is better (add a diagonal for rigidity)

Warning

Board-and-batten is not weatherproof — gaps open between boards as they dry. For exterior use, overlap boards (like clapboard siding) or cover gaps with thin cover strips.

Gluing Panels

When joining boards into a solid panel (tabletop, door panel), gluing technique matters.

Board Selection and Arrangement

1. Alternate growth ring direction: Look at the end grain of each board. If one curves up, the next should curve down. This balances warping forces so the panel stays flat.
2. Match grain and color: Arrange boards so the visual pattern flows naturally
3. Joint the edges: Each edge must be dead straight and square. Test by holding two boards together against a light source — no light should pass through.

Glue-Up Procedure

1. Dry-clamp first: Assemble without glue to verify fit and prepare clamp positions
2. Apply glue to one edge of each joint — a thin, even coat
3. Assemble and clamp: Use bar clamps or pipe clamps alternating above and below the panel to equalize pressure and prevent bowing
4. Check for flat: Place a straight edge across the panel during clamping — adjust clamp pressure if the panel is cupping
5. Clean squeeze-out: Scrape excess glue after it firms but before it fully hardens (about 30 minutes for most wood glue)
6. Let cure: Minimum 4 hours under clamps; overnight is better

Clamping Pressure

Panel Width	Number of Clamps	Spacing
Under 300 mm	2-3	Every 150-200 mm
300-600 mm	3-4	Every 150-200 mm
Over 600 mm	4+	Every 150-200 mm, alternating above/below

Tip

If you lack clamps, use wedges between the panel and a fixed object (like a wall or stout board clamped to the bench). Two opposing wedges create adjustable pressure. Tap them together to tighten.

Flooring Installation

Tongue-and-groove flooring is one of the most common applications. The principles are simple but execution requires care.

Subfloor Preparation

The subfloor (joists and any rough boards below) must be:

• Flat within 3 mm over 2 meters
• Dry — moisture from below ruins flooring
• Ventilated underneath if over a crawl space

Installation Steps

1. Start at one wall: Place the first board with the groove toward the wall and the tongue facing out. Leave a 10 mm expansion gap between the board and the wall.
2. Nail through the tongue: Angle a nail through the tongue at 45 degrees into the subfloor. The next board’s groove will hide the nail head. This is called “blind nailing.”
3. Fit the next board: Place the groove of the next board over the tongue of the first. Tap the board home with a mallet against a scrap offcut (to protect the tongue edge).
4. Continue across the room: Stagger end joints between rows by at least 300 mm — never align end joints in adjacent rows.
5. Final board: Rip the last board to width, leaving a 10 mm gap at the wall. This board must be face-nailed since there is no room for blind nailing.
6. Cover expansion gaps with baseboard trim.

Warning

Never glue flooring tongues and grooves. The boards must be free to expand and contract. A glued floor will buckle in humid weather or gap in dry weather. Nails through the tongue provide all the hold needed.

Shiplap and Other Profiles

Tongue and groove is not the only interlocking edge profile.

Profile	Description	Best For
Tongue and groove	Centered tongue fits into a groove	Flooring, panels, precision work
Shiplap	Overlapping rabbets on each edge	Exterior siding, barns, quick construction
V-joint tongue and groove	Tongue and groove with a V-shaped chamfer at the joint line	Ceilings, wainscoting (the V creates a decorative shadow line)
Channel groove	Tongue and groove with a bead profile at the joint	Decorative wall paneling

Cutting Shiplap

Shiplap is the simplest interlocking profile:

1. Cut a rabbet on the top face of one edge (half the board thickness, 15-20 mm wide)
2. Cut a rabbet on the bottom face of the opposite edge (same dimensions)
3. The two rabbets overlap when boards are placed side by side
4. No precise fitting needed — some gap is acceptable and even desirable for drainage on exterior walls

Tongue and Groove — At a Glance

Tongue-and-groove joints let you build wide panels, floors, and walls from narrow boards while accommodating wood movement. Size the tongue at 1/3 of the board thickness and make the groove 1-2 mm deeper than the tongue length. Cut grooves with a plow plane, chisel, or saw; form tongues by cutting rabbets on both faces. For an easier approach, use a loose tongue (spline) in two identical grooves. Glue panels with alternating ring direction and clamp every 150-200 mm. Leave flooring unglued with 10 mm expansion gaps at walls. For quick work, shiplap provides overlap without precision fitting.