What is the effective coverage width of T&G boards?

The effective (face) coverage width is the nominal board width minus the tongue overlap. For a standard 94mm T&G board, the tongue is approximately 12mm, giving an effective coverage of 82mm per board. For 119mm boards, coverage is 107mm. For 144mm boards, coverage is 132mm. Always use the coverage width — not the nominal width — when calculating how many boards you need.

Do I need battens for tongue and groove wall cladding?

Yes. When fitting T&G boards to walls or ceilings, you must first fix battens (25×50mm or 25×38mm timber) to the wall at 400mm centres. The battens run perpendicular to the direction of the boards and provide a fixing point for each board. This also creates an air gap behind the cladding, which helps with moisture management. On solid masonry walls, use screws and wall plugs; on stud walls, screw into the studs.

Should T&G boards run horizontally or vertically?

Either direction works, but each has advantages. Horizontal boards make a room feel wider and are easier to fit — start from the bottom with the groove facing down. Vertical boards make a room feel taller and are traditional in many period properties. For ceilings, boards typically run perpendicular to the joists. For floors, boards run perpendicular to the joists for structural support.

How do I fix tongue and groove boards?

Use 'secret nailing' — drive a panel pin or brad nail at 45 degrees through the tongue of each board before sliding the next board's groove over it. This hides all fixings. Alternatively, use T&G clips that slide into the groove and nail to the batten. For floors, use lost-head nails or ring-shank nails driven through the tongue at each joist crossing.

What wastage allowance should I add for T&G?

Allow 10% wastage for straightforward rectangular rooms. Increase to 15% for rooms with alcoves, windows or complex shapes. The tongue-and-groove joint itself doesn't create waste (it's already factored into the coverage width), but cutting boards to length at each end of a row always produces offcuts. Longer offcuts can start the next row to reduce waste.

Can I use tongue and groove in a bathroom?

Yes, but choose your material carefully. Standard softwood (pine) must be treated or sealed to prevent moisture damage. Moisture-resistant MDF T&G works well in bathrooms. For the best results, ensure good ventilation (an extractor fan compliant with Approved Document F) and seal all cut edges and backs of boards before fitting. Avoid T&G directly behind a shower or bath — use proper tanking and tiling in wet zones.

Tongue and Groove Calculator — How Many T&G Boards Do I Need?

Tongue and Groove Calculator

Calculate T&G boards, battens, clips and fixings for wall cladding, ceilings and floors. Accounts for tongue overlap and wastage.

Area Length

Length of wall, ceiling or floor

Area Width

Height (wall), width (ceiling/floor)

Board Type

Wall/ceiling need battens; floor fixes to joists

Board Width

Nominal width (coverage is less due to tongue)

Board Thickness

Direction of Run

Direction boards run across the area

Include wastage allowance

Adds 10% extra for cuts and waste

Wastage Percentage

10% standard; increase for complex layouts

How We Calculate This

This calculator determines how many tongue and groove boards you need to cover a wall, ceiling or floor area, plus the battens and fixings required.

Board coverage

The effective coverage width is the nominal board width minus the tongue overlap (~12mm):

94mm board → 82mm effective coverage
119mm board → 107mm effective coverage
144mm board → 132mm effective coverage

Number of boards

Boards = ceil(Span to fill / Effective coverage width), where "span to fill" is the dimension perpendicular to the board direction.

Battens (wall and ceiling)

For wall cladding and ceiling applications, battens are fixed at 400mm centres perpendicular to the boards. The number of battens depends on the length of the board run.

Fixings

Walls/ceilings: T&G clips or panel pins, 1 per batten crossing per board
Floors: Lost-head nails through the tongue at each joist (assumed 400mm centres)

Standards

T&G flooring should comply with BS 1297 for tongued and grooved softwood flooring. Wall and ceiling cladding should be fixed to battens per good carpentry practice, with adequate ventilation behind the boards.

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Tongue and Groove Calculator

How We Calculate This

Board coverage

Number of boards

Battens (wall and ceiling)

Fixings

Standards

Frequently Asked Questions

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