Tongue and Groove Calculator

The effective (face) coverage width is the nominal board width minus the width hidden by the tongue, and it varies by profile. A standard 94mm TGV (V-jointed) board typically covers about 84mm; redwood matchboard can cover 86–90mm; shiplap loses a little more. As a guide: 94mm board ≈84mm cover (TGV), 119mm ≈107–109mm, 144mm ≈134mm. The calculator defaults to a conservative ≈10mm tongue loss (so it never under-orders), and you can set the exact tongue overlap under Advanced Options. Always calculate from the cover width, not the nominal width, and confirm the cover figure against your supplier's datasheet.

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. For external cladding, see our Timber Cladding Calculator.

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.

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.

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.

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.