Tongue and groove is a profile joint in which one board has a protruding ridge (tongue) that fits into a groove in the adjacent board. This allows the boards to connect seamlessly without gaps. It is the standard joint for floorboards, wall and ceiling panelling, and roof boarding.
How does it work?
Each board has a tongue (protruding lip) on one long edge and a groove (slot) on the other. When fitting, you slide the tongue of one board into the groove of the next.
The result:
No visible seams
Boards cannot tilt
Blind fixing possible (screws through the tongue, covered by the next board)
Boards can expand and contract without gaps
Where is tongue and groove used?
Application
Example
Floorboards
Solid timber floor, pine or oak
Panelling
Wall and ceiling panelling
Roof boarding
Timber boards on the inside of the roof
Cladding
Some weatherboard profiles with tongue and groove joint
Sauna/bathroom
Timber wall finish
Tongue and groove in DIY project The five working steps for laying a tongue and groove floor, in order. s
Laying a floor
1. Start at the wall with the groove side facing the wall
2. Leave 10-15 mm expansion gap along the walls
3. Slide the next board with the tongue into the groove
4. Tap together with a tapping block (never strike the tongue directly)
5. Fix blind by screwing or nailing at an angle through the tongue
Panelling (wall/ceiling)
1. Start at a straight wall or corner
2. First panel with groove facing the wall, screw to the battens
3. Slide in subsequent panels, fix blind through the tongue
4. Cut the last panel to size and secure
Profile variations
Square tongue — Standard, straight lip
Rounded tongue — With a rounded profile, creating a visible groove line (V-groove)
Double-sided tongue — Tongue on both sides, groove in the middle (for thick boards)
Common mistakes
Too tight a fit — Always leave a fraction of space. Wood swells with moisture. If the boards are too tight, they will buckle.
Striking the tongue directly — The tongue breaks off. Always use a tapping block or an offcut as a buffer.
No expansion gap — Leave at least 10 mm free along walls and obstacles.
