Preventing external render wash-offs

// the building envelope

Q: In the past, I’ve had a few external render wash-offs following rainfall and / or frost, is there any way to prevent this?

A: Joe Ragdale, Technical Manager for Wetherby Building Systems says:

"This is a common problem with external render applications in the winter months. If the conditions are too wet, humid or cold when the render is applied, it can significantly extend drying times, leaving the render open to damage from rain and frost.

The only way to prevent wash-offs and damage to thin coat external renders in these conditions is to ensure they are installed in the right atmospheric conditions. They also need to be allowed to cure sufficiently and protection should be provided to keep the render dry and temperatures above freezing point.

Successful application of polymer modified renders, adhesives, primers and silicone products can only occur when the temperature is 3°C and rising or 5°C and falling, with no frost forecast.  These products will only cure in temperatures above 3°C and applying such finishes in lower temperatures will result in the render remaining wet on the wall, increasing the chances of damage due to frost or wash offs in heavy rain.

Humidity levels should also be taken into consideration when applying external render, with the majority of products only able to cure in relative humidity levels of less than 90%. When humidity levels exceed 90%, the amount of relative humidity in the air becomes higher than the moisture content of the material itself, preventing the render from curing. Applying render in high humidity is another cause of render remaining wet on the wall for extended periods, making it susceptible to frost damage and wash offs.


Product selection is also an important consideration when working in winter conditions, as certain systems can cope better with cold, damp weather. Mineral render, such as Wetherby’s EpsiCoat Mineral Render Plus, is the most suitable as it relies on a chemical reaction during the curing process and cures independently of humidity, unlike traditional renders, such as silicone, which set by air drying. Rain resistant in one hour and touch dry in two hours, EpsiCoat Mineral Render Plus is less susceptible to delamination and wash-offs in adverse weather conditions."

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