Flood Drying Calculator — Dehumidifier Size & Drying Time

Drying time depends primarily on wall construction: timber frame walls dry in about 4 weeks, cavity brick in 8 weeks, solid 225mm brick in 12 weeks, and thick stone walls can take 6 months or more. Concrete floors typically take 4-8 weeks. Professional dehumidification speeds up the process significantly.

As a rule of thumb, allow 10 litres/day extraction capacity per 30m³ of room volume. A typical room (4×5×2.4m = 48m³) needs a 20L/day commercial dehumidifier. For severe flooding, multiple units may be needed. Domestic dehumidifiers (5-10L/day) are too small for flood drying.

Yes, maintaining a temperature of 18-22°C significantly improves drying rate. Warm air holds more moisture, which the dehumidifier can then extract. However, do not exceed 30°C as this can cause salts to crystallise and damage plaster. A combination of heat + dehumidification + air movement is most effective.

Replaster only once the masonry is genuinely dry. The reliable test is gravimetric moisture content — drill samples and weigh wet then oven-dry, the method set out in BRE Digest 245 — expressed as a percentage of dry mass; readings should be low and stable before relining. A surface protimeter is useful for tracking the trend but its "WME" (%) scale is calibrated for wood and only gives qualitative comparisons on masonry, so do not rely on a single WME number as a go/no-go limit. Replastering too early traps moisture and salts behind the new coat and causes ongoing damp; after flood damage use a salt-resistant renovating plaster (e.g. a sand/cement renovating render system) rather than ordinary gypsum.

Yes. Air movement fans are essential to distribute air across wet surfaces, accelerating evaporation. Without air movement, a stagnant boundary layer of humid air forms against the wet surface, slowing evaporation. Place fans to direct air across wet walls and floors, not directly at the dehumidifier.