How to Identify and Remove Black Stains on Your Building’s Exterior

What do old buildings, new homes and farm structures all have in common? These and other types of buildings can all fall victim to discoloration on their exterior walls over time and throughout the seasons. It’s often called “black staining,” as the thing causing the discoloration is naturally dark. In most cases, black staining can be found on the north side elevation of properties.

So, what exactly are those black stains on your building’s exterior, and how do you get rid of them? Here’s some information from siding contractors in the Twin Cities about how to handle this problem.

Black staining on the exterior of buildings

That dark discoloration on the side of your house can be one of three things: algae, mold or moss. These three types of growths can survive in similar environments, making it difficult to tell exactly what’s growing on your exterior walls. Don’t guess the growth—here are some tips to help you correctly identify the issue:

• Mold: The color of mold depends on the type of mold, but can be a shade of green, red, yellow, white or black.
• Moss: Moss kind of looks like a houseplant or ground-cover, in the sense that it sprouts stems and often appears to be structured with tiny, light, feathery leaves.
• Algae: Algae that grows on buildings is usually a deep green. Spray it with water and it will look slimy and be slippery under your fingers. It doesn’t have stems, leaves or roots, and algae is still difficult to remove from surfaces like concrete and stucco. It is sticky and clings mightily to hard surfaces, especially in spots where water is always moving.

Depending on how long the growth has been on your wall, removing it can be a difficult task, if attempted manually. Luckily, you can hire a local company that offers exterior building cleaning services.

Removing stains from stucco

It’s relatively easy to remove black stains from your stucco walls using oxygen bleach (NOT chlorine bleach). It’s a powder that you mix with water until dissolved, then pour into a hand-pump sprayer and saturate the stucco. Do this on a cloudy day and keep the stucco wet for about 30 minutes for the best results. Then, using a scrub brush dipped in the solution, go after the black stains. Rinse immediately with water from your garden hose.

Removing stains from concrete

Concrete is a resilient material and can take a lot from a pressure washer. The powerful stream of water will loosen the growth from a large surface area over the concrete rather quickly, but you should adjust the water pressure by testing it on an out-of-sight spot first.

A safer alternative to non-oxidizing bleach is oxidizing bleach, which does not contain chlorine. When this bleach is sprayed on algae, the cells immediately start dying off. This makes using a brush to remove the growth much easier.

When it’s time to clean your stucco or concrete surfaces, call Diamond Roofing, Remodeling & Roof Shampoo. We’re your go-to exterior cleaning service and siding contractors in the Twin Cities!