Drying and site conditions
Minimum application temperature check
- where
- lowest relevant temperature (°C)
- air temperature now (°C)
- background surface temperature now (°C)
- lowest forecast temperature until dry (°C)
- margin above the product minimum (°C)
- product minimum application temperature (°C)
Explain this equation
A go/no-go check before mixing: is everything warm enough, now and until the material has dried or cured? Use it in autumn, winter and early spring.
Ta is the air temperature, Ts the background surface temperature and Tn the lowest temperature forecast while the work dries, usually overnight. Tlow is the lowest of the three. Tmin is the product minimum from the data sheet, and M is the margin above it.
The equation takes the minimum because the coldest of the three governs: cement renders stop hydrating and can be damaged by frost, and gypsum plaster must not freeze before it is dry. A negative M means you should not apply without heating and protection.
Typical minimums: cement-based renders 5 °C and rising; Thistle gypsum plasters 2 °C at the background until dry. A morning reading of 6 °C with a 3 °C overnight forecast fails for render.
Watch out: a 5 °C air reading can sit above a background that is still frozen from the night before. Measure the wall, not just the air.
A go/no-go check: air, background and forecast overnight temperature must all stay at or above the product minimum.
Enter your values to see the result.
Good to know
Typical published minimums: cement-based renders and plasters 5 °C and rising (Weber); Thistle gypsum plasters 2 °C at the background until dry, ThistlePro DuraFinish 5 °C (British Gypsum). Never apply to frozen or frosted backgrounds, or with frosted materials. Bagged gypsum plaster is not harmed by cold storage, but the applied plaster must not freeze. Low temperatures lengthen setting and drying times.
Common mistake
Starting at 5 °C on a falling thermometer. The rule is 5 °C and rising, and the material must stay above the minimum while it sets and cures, so the overnight forecast matters as much as the morning reading.
Limits
Simple comparison against the data sheet figure you enter. Does not account for wind chill on render, direct sun or rapid drying at high temperatures (Thistle plasters should not be used where they will exceed 49 °C).
Sources
- Saint-Gobain Weber: Working in winter – potential pitfalls (do not apply render below 5 °C; no frosted materials or substrates)
- Saint-Gobain Weber: weber.pral M monocouche render application guide (5 °C and rising; movement joints up to 6 m in unreinforced blockwork; 3:1 panel ratio; ±2 mm thickness tolerance)
- British Gypsum White Book: Thistle plaster finishes (coat thicknesses, coverage per 25 kg bag, dry set weights, minimum temperature 2 °C until dry, plaster must be thoroughly dry before decorating)
- British Gypsum technical FAQs: minimum temperature for gypsum products (2 °C until dry; ThistlePro DuraFinish 5 °C)
- BS EN 13914-1:2016 Design, preparation and application of external rendering and internal plastering – Part 1: External rendering
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.