Movement joints
Movement joint layout for a floor: bays and joint lengths
- where
- number of bays along the length and across the width
- floor length (m)
- maximum joint spacing (m)
- floor width (m)
- maximum bay area (m²)
- number of bays
- area of each bay (m²)
- total length of intermediate joints (m)
- perimeter joint length (m)
Explain this equation
Divides a floor into equal bays separated by movement joints and tells you how much joint profile or sealant you need. Use it at setting out, especially on heated floors and large open-plan areas.
S is the longest distance allowed between joints (about 10 m on internal unheated floors, 8 m on heated floors, 3–5 m outside). Amax is the bay area limit, 40 m² on heated floors. nL and nW are how many bays fit each way; Lint counts the joints between bays and Lper the joint round the edge.
Rounding up (the ceiling brackets) makes sure no bay edge is longer than S. If the bays are still too big in area, the longer bay side is split again until the area limit is met. Each line of joints runs the full width or length of the floor.
Sense check: a 9 × 6 m heated kitchen needs one joint across the middle, giving two 27 m² bays. A 12 × 10 m floor needs four bays of 30 m².
Watch out: joints in the screed or structure below must be carried through the tiling at the same place, whatever this layout says.
Splits a rectangular floor into equal bays so no bay edge exceeds the joint spacing and no bay exceeds the area limit (nL and nW are the smallest bay counts, at least L/S and W/S rounded up, that keep each bay within the area limit), and gives the length of joint profile or sealant to order.
Enter your values to see the result.
Good to know
Typical spacings: internal unheated floors, bays up to about 10 m each way (NHBC: 8–10 m centres); external floors, intermediate joints at 3–5 m; heated floors, bays no larger than 40 m² (25 m² for resin agglomerate) with no edge longer than 8 m. Perimeter joints are needed against walls, columns and upstands (NHBC: where the tiled area is wider than 2 m), and every structural joint in the base must be carried through the tiling. On heated floors make perimeter joints at least 6 mm wide.
Common mistake
Placing intermediate joints to suit the tile layout but not over the screed bay joints or structural joints below. Joints in the base must be continued through the tiling at the same position.
Limits
Rectangular floors only. L-shaped rooms, doorways and re-entrant corners need extra joints where the shape changes. The tile or adhesive maker may require closer spacing, especially for large-format tiles.
Sources
- BS 5385-3:2024 Wall and floor tiling – Design and installation of ceramic, mosaic, natural stone, agglomerated stone and terrazzo flooring – Code of practice (BSI)
- NHBC Standards 9.3.5 Ceramic, concrete, terrazzo and similar tile finishes (movement joints, screed drying, 75% RH)
- The Tile Association: Tiling to heated floors (2013/2014) – bay sizes, movement joints, thermal expansion coefficients, screed drying, floor loads
- The Tile Association: External floor tiling incorporating movement joints (intermediate joints at 3–5 m)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the tile, adhesive and grout manufacturers’ instructions, BS 5385 and the Tile Association guidance. Coverage rates vary by product and substrate. Check prices and VAT rules with your own records and HMRC.