Render movement
Thermal movement of a render panel (ΔL = α · L · ΔT)
- where
- change in length (mm)
- coefficient of thermal expansion (× 10⁻⁶ /K)
- panel length between joints (m)
- converts m to mm
- temperature change of the render (°C)
Explain this equation
Works out how much a render panel expands or contracts as its temperature changes. Use it to judge the movement a joint or bead must take.
α is how much the material grows per degree, entered in millionths per kelvin, so it is multiplied by 10⁻⁶. L is the panel length between joints in metres, the 1000 turns the answer into millimetres, and ΔT is the change in render temperature.
Most building materials expand by a fixed fraction for each degree of warming, so the change in length is proportional to both length and temperature change.
Sense check: a 6 m panel of cement render (α about 10 × 10⁻⁶) warming by 40 °C grows about 2.4 mm. Double the panel length and the movement doubles.
Watch out: dark render in direct sun gets much hotter than the air, and new render also shrinks as it dries, which this equation does not include.
Works out how much a render panel grows or shrinks with temperature, to judge the movement a joint or bead has to take.
Enter your values to see the result.
Good to know
Cementitious materials expand by roughly 10 × 10⁻⁶ per kelvin (Eurocode 2 value for concrete); sand–cement renders and mortars are typically in the 10–13 × 10⁻⁶ /K range, so use the product figure if published. Dark render in sun runs much hotter than the air, so allow a larger temperature range for dark colours. This reversible movement is in addition to the one-off drying shrinkage of new render, which is usually larger.
Common mistake
Using air temperature range instead of the render surface temperature range, which underestimates movement on south and west elevations with dark finishes.
Limits
Free (unrestrained) linear movement only. Real render is partly restrained by the background, so stress and cracking depend on bond, strength and joint layout. Ignores moisture movement and shrinkage.
Sources
- BS EN 1992-1-1 Eurocode 2, clause 3.1.3(5): coefficient of thermal expansion of concrete 10 × 10⁻⁶ /K (summarised by Structville)
- BS EN 13914-1:2016 Design, preparation and application of external rendering and internal plastering – Part 1: External rendering
- NHBC Standards 6.11.5 Render – accommodation of movement (Table 3: movement joint spacing by blockwork type, half spacing from corners; metal lath bays ≤ 5 m)
Results are calculations from the published methods and manufacturer figures shown, and depend on the data you enter. Always follow the manufacturers’ system and product data, BS 8212, BS EN 13914 and Building Regulations. Coverage rates vary by product, background and thickness. Check prices, VAT and CIS rules with your own records and HMRC.