Roof coating wash-off during rainstorm through drains into the parking lot

In the fall and winter, it’s not uncommon to hear of roof coating wash-offs or roof coating issues from heavy dew. Water-based coatings, such as acrylic coatings, are susceptible to washing off the roof if coming in contact with moisture before they have properly dried. Rain and dew (whether morning dew or dew from humidity) are the most likely culprits for causing wash-offs.

Wash-offs have happened even when the water-based coating was applied by a contractor two full days before any rain occurred. This situation was the “perfect storm”—humidity was high, temperatures were moderate, and dew set in. After dew sets into water-based coatings, temperatures need to rise high enough in order to “bake” off the dew, otherwise, the dew will continue to set into the coatings each day and ultimately ruin the application. Though this was a rare situation, it is still possible with any water-based coating application.

While there are exceptions to this rule of water-based coatings, it is better to instead use solvent-based coatings (when possible) in the fall and winter to avoid potential wash-off—especially if the area in which the coating is being applied typically has warm temperatures and/or high humidity.

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Using Water-Based Coatings During the Fall & Winter

Based on this information, roofing contractors working in the southern United States may think they cannot use water-based acrylic systems at all—such as our Met-A-Gard or Met-A-Gard+ systems—but this is simply not true. Roofing contractors in areas of high humidity should be cautious and not take any chances, but can otherwise proceed with the installation of a water-based system as long as there are high temperatures to match the high humidity.

The rule of thumb: low temperatures and high humidity do not mix when applying water-based coatings.

It is essential to be mindful of both the climate and weather of the area when installing water-based coatings and choose appropriate days on which to perform their application. Weather-permitting, stop applying all coatings at least three hours before sunset to minimize the risk of wash-off occurring in the nighttime temperatures.

Optimal Conditions for Water-Based (Acrylic) Coatings

Obviously, contractors should ensure that acrylic coatings are not being applied on days when rain is expected. Dew point and temperatures should also be taken into consideration. For more information on dew point versus temperature, click here. Again, the lower the humidity and higher the temperature, the better chance of a successful application and successful day.

Contractors should also take care to cautiously apply water-based coatings in thin passes. If the application requires 2 gallons for every 100 sq. ft., applying with three thin passes of the roof rather than two will help to protect the coatings.

Finally, coatings that are a darker color—such as a gray base coat—will dry faster. Keep in mind that a single day’s forecast will not suffice; the weather for the 2-3 days surrounding the initial application need to be monitored in order to make an informed decision. Contact our Technical Services department for help with best practices on this matter.

Conclusion: Alternative Systems

If it’s possible to use a system other than an acrylic system on a project, some alternative options include our Ure-A-Sil or Met-A-Sil roof restoration systems for both flat and metal roofs, respectively.

These systems are comprised of moisture-cured urethanes and silicones rather than water-based acrylics, so there is no risk of wash-off. Moisture-cured roof coatings are immune to failure from freezing or wash-offs, so long as they are applied on a clean, dry surface. We can help with any questions or concerns about applying a urethane or silicone system.