Movement joints
Thermal movement of a tiled field and minimum joint width
- where
- thermal movement of the tiled field (mm)
- coefficient of linear expansion (×10⁻⁶ per K)
- length of tiled field between joints (m)
- temperature range (°C)
- minimum sealant joint width (mm)
- sealant movement accommodation factor (%)
Explain this equation
Works out how much a run of tiles grows and shrinks with temperature, and how wide the movement joint must be for the sealant to cope. Use it on heated floors, conservatories and outdoor areas.
α is how much the material grows per metre per degree, in millionths: about 6–7 for porcelain, 11–13 for a cement screed. L is the run between joints and ΔT the temperature swing it sees. MAF is how far the sealant can stretch or squash as a percentage of its width, 25% for a class 25 sealant.
Thermal expansion is proportional to length and temperature change, so the equation is a straight product. The 10⁻⁶ and 1000 handle the units. Dividing by the MAF sizes the joint so the movement uses no more than the sealant’s allowed share of its width.
Sense check: 5 m of porcelain heated through 30 °C grows about 1 mm, so a 25% sealant needs a joint at least 4 mm wide. A 1 m tile warmed 20 °C grows only 0.14 mm.
Watch out: new screeds also shrink as they dry, and the screed moves more than the tiles. Respect the minimum joint widths in the standard and the maker’s data even if this gives less.
Works out how much a tiled area grows or shrinks with temperature (ΔL = α·L·ΔT) and how wide a sealant movement joint must be to take that movement.
Enter your values to see the result.
Good to know
Published α (×10⁻⁶/K): porcelain (BIa) 6–7; other dry-pressed floor tiles (BIb) 5–9; granite 6–9; marble 3.5–7; limestone 6–10; sandstone 11–12; slate 3–9; cement:sand screed 11–13; calcium sulphate screed 10–12; resin agglomerate 20–38. Use the higher end for safety. A heated floor typically sees 20–30 K; an external south-facing terrace can see more. The movement accommodation factor is the sealant’s movement class: 25% for class 25 sealants, 20% for class 20 (BS EN ISO 11600). Also observe the minimum widths in BS 5385 and the tile maker’s data, whichever is larger.
Common mistake
Ignoring the screed. The base often moves more than the tiles (cement screed about 12 × 10⁻⁶/K against 6–7 for porcelain), and that difference is what shears the adhesive bond.
Limits
Free thermal movement only. It ignores drying shrinkage of new screeds, moisture expansion of tiles and structural movement, which can each be larger than the thermal movement.
Sources
- The Tile Association: Tiling to heated floors (2013/2014) – bay sizes, movement joints, thermal expansion coefficients, screed drying, floor loads
- BS EN ISO 10545-8:2014 Ceramic tiles – Determination of linear thermal expansion (test method summary, Lucideon)
- BS EN ISO 11600 Building construction – Jointing products – Classification and requirements for sealants (movement classes 20, 25)
- 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)
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.