Drying and site conditions
Drying time estimate before decorating
- where
- drying allowance for the backing coat (days)
- backing or undercoat thickness (mm)
- drying allowance per mm (days/mm)
- total drying allowance before decorating (days)
- allowance for the finish coat (days)
Explain this equation
Gives a planning figure for how long new plaster needs before you can seal, paint, paper or tile it. Use it when telling a customer or the next trade when the room will be ready.
Tb is the time allowed for the backing or undercoat, found by multiplying its thickness t by a drying allowance r in days per millimetre. Tf is a separate allowance for the thin finish coat, which dries quickly because it is only about 2 mm thick. T is the total.
Water has to travel out through the full thickness of plaster before it can evaporate, so thicker coats take proportionally longer. The default of 1 day per mm comes from the trade rule borrowed from BS 8203 for sand–cement screeds; manufacturers themselves only say plaster must be thoroughly dry.
Sense check: 11 mm of backing plus a skim gives 11 + 3 = 14 days, about two weeks, which matches typical guidance for two-coat work. A skim on board alone is usually two to four days. Double the backing thickness and the backing allowance doubles.
Watch out: cold, damp rooms take far longer. Check with a moisture meter before tiling.
A planning estimate of how long new plasterwork needs before it can take decoration, from the backing thickness and a drying allowance per millimetre.
Enter your values to see the result.
Good to know
Manufacturers give setting times, not drying times: British Gypsum only says gypsum plaster must be thoroughly dry before decorating, and that tiles and other impermeable finishes must wait until both background and plaster are dry. The default of 1 day per mm is the long-standing trade rule of thumb, borrowed from the BS 8203 estimate for sand–cement screeds (one day per mm up to 50 mm). A 2 mm skim on board is usually dry in two to four days in good conditions. Ventilate and use gentle background heat; a mist coat of permeable emulsion can go on before the plaster is fully dry.
Common mistake
Treating the estimate as a guarantee. Cold, damp, unventilated rooms, solid masonry that was wet before plastering and cement-based backings can take far longer. Check colour (an even pale pink for gypsum) and use a moisture meter before tiling or applying vinyl paper.
Limits
Rule-of-thumb estimate only, not a manufacturer figure. Assumes about 15–20 °C, normal ventilation and a background that was itself dry. Does not model humidity or air movement. For cement renders and lime work follow the product data sheet curing times.
Sources
- 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)
- The Concrete Centre: Floor screeds (BS 8203 drying estimate of one day per mm of thickness up to 50 mm, for sand–cement screeds)
- Homebuilding & Renovating: How long does plaster take to dry? (Saint-Gobain/British Gypsum technical adviser: skim usually dry in two to four days)
- Screwfix guide: How long does plaster take to dry? (skim 3–7 days; multi-coat work up to four weeks)
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.