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Cleaning Red Streaks on Rendered Walls

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  • Post last modified:April 14, 2021

Render is a standard exterior for buildings. It’s a practical method of sealing a structure’s surface against the elements while also changing its aesthetic appearance. Instead of paint, render involves applying a mixture of wet cement and sand “type material” to the outer walls.

While this is a great way to fortify your home’s exterior against severe weather and water damage, some common problems can arise. One of these is the appearance of red stains, marks, or streaks on rendered walls. These streaks can be unsightly and can drive down the value of your property.

If you’ve noticed red stains on your home or business’s rendered walls, you’ll want to know not only what causes these marks and how to get rid of them without damaging the structure. Don’t worry— we’ve got you covered! Read on to learn all about cleaning red streaks on rendered walls.

What Causes Red Stains on Rendered Walls?

Cleaning Red Streaks on Rendered Walls

It is easy to mistake red stains on rendered walls as something else, such as rust or silicone runoff. However, these streaks are actually manifestations of algae growing on the outside of the walls. Humid environments or rainwater trickling down the sides of your home provide sources for algae with the moisture it needs to flourish.

There are certain types of algae that can be hazardous. However, the algae typically found on render are not harmful to humans. They won’t cause excessive damage to your home, either. That said, the stains are not pleasant to look at, and it can become an obstacle if you’re looking to sell your home.

How to Clean Red Streaks on Rendered Walls

Luckily, the solution to the problem is very simple. You just need to clean the algae off the rendered walls.

You’ll need a specialist biocide to kill and remove the algae. This is applied at relatively low pressure so as not to damage the render.

If you have concerns about damaging the exterior, we recommend consulting a professional. You don’t want to apply too much pressure, and selecting the right biocides for the specific algae strain can be a tricky task best left to the pros.

What About Painting Over the Stains?

Painting over the red streaks might work as a temporary solution, but it won’t keep the algae from coming back. It will continue to grow under the paint, and eventually, the red marks will return to the outside of the rendered walls. At this point, it will be more challenging to clean off because there will be a layer of paint in the way.

For these reasons, we strongly advise against painting over the algae. Cleaning the walls and killing the algae is the only way to make sure it doesn’t return.


Cleaning red streaks on rendered walls will restore your home and maintain its curb appeal. If you’ve noticed red stains on your rendered walls, get in touch to find out how we can help.