Roof Coating Dry Time and Recoat Window Calculator
Estimate the touch-dry time, through-dry time, and safe recoat window for common roof coatings based on environmental conditions and coating type.
Fill in the fields above and click Calculate.
Formulas Used
Adjusted Dry Time:
T_adjusted = T_base × F_temp × F_humidity × F_thickness × F_wind × F_sun
Temperature Factor (evaporative coatings — acrylic, elastomeric, butyl):
F_temp = 2 ^ ((25 - T_°C) / 10)
Based on Arrhenius approximation: drying rate doubles per 10°C rise.
Temperature Factor (moisture-cure — silicone, polyurethane):
F_temp = 2 ^ ((25 - T_°C) / 15)
Humidity Factor (evaporative):
F_humidity = (RH / 50) ^ 0.5
Higher humidity slows evaporation of water-based coatings.
Humidity Factor (moisture-cure):
F_humidity = (50 / RH) ^ 0.4
Higher humidity accelerates moisture-cure reactions.
Thickness Factor:
F_thickness = (WFT / 20) [evaporative] F_thickness = (WFT / 20) ^ 0.7 [moisture-cure]
Wind Factor:
F_wind = 1 / (0.7 + 0.06 × min(v_mph, 30))
Sun Factor: Full Sun = 0.75×, Partial = 1.0×, Shade = 1.35×
Evaporation Rate (ASTM E96 / Carrier equation simplified):
E = 0.00267 × (P_sat − P_a) × (0.089 + 0.0782 × v_m/s) [kg/m²·hr]
P_sat = 6.1078 × 10 ^ (7.5 × T_°C / (237.3 + T_°C)) [hPa, Antoine eq.] P_a = P_sat × (RH / 100)
Assumptions & References
- Base dry times are averages derived from manufacturer Technical Data Sheets (TDS) for Tremco, Henry, GacoFlex, Karnak, and Sherwin-Williams roof coatings at 77°F / 50% RH / 20 mil WFT.
- Temperature factor uses an Arrhenius-type approximation (Q₁₀ rule): reaction/evaporation rate doubles per 10°C for evaporative coatings, per 15°C for moisture-cure coatings.
- Humidity factor for evaporative coatings follows a power-law relationship with relative humidity based on vapor pressure deficit principles.
- Moisture-cure coatings (silicone, polyurethane) require ambient moisture; very low RH (<20%) can significantly slow cure despite favorable temperature.
- Thickness factor assumes linear scaling for evaporative coatings and sub-linear (power 0.7) for moisture-cure coatings where surface cure precedes bulk cure.
- Wind factor is based on convective mass transfer principles; effect diminishes above ~15–30 mph due to boundary layer saturation.
- Evaporation rate uses the Carrier/ASTM E96 simplified equation with Antoine equation for saturation vapor pressure.
- Results are estimates only. Always consult the specific product TDS and follow manufacturer guidelines. Do not apply most coatings below 50°F or above 100°F surface temperature.
- Recoat window maximum is critical: exceeding it may require surface preparation (sanding, priming) to restore adhesion.
- References: ASTM E96 (water vapor transmission), ASTM D1640 (dry/cure times), manufacturer TDS for Tremco AlphaGuard, Henry 887, GacoFlex S2000, Karnak 97AF.