Common Siding Stains and How to Remove Them

Siding covers a large portion of your home’s exterior. So when black streaks or yellow blotches appear, they can make the siding look grubby and your home uncared for. Most often the colour of the stain will be a good indication of what is causing it. To help you keep your siding looking as good as new, here are some common siding stains and tips on how to remove them.

Green Stains on Siding

Green stains appearing on exterior walls – cedar siding, aluminum siding, vinyl siding, or composite wood siding – are typically caused by mold, algae, or lichens. Mold, lichens, and algae growth can take hold on areas of siding that don’t get as much sun as the other exterior walls and/or where there is moisture buildup.

Black Stains on Siding

Black marks or stains on siding are generally the result of some type of fungus, including algae and mold. Again as with the cause of green stains, the right combination of shade, warmer temperatures in the sheltered space than the surrounding area, and the presence of moisture can encourage fungi growth.

Brown Stains on Siding

Rusting nails are often the cause of brown stains on the siding. The nails hold the siding in place or they hold the sheathing in place and somehow water has penetrated behind the siding panels. If it is cedar siding, and rusting nails aren’t the issue, it could be the natural oils of the wood coming to the surface.

Yellow Stains on Siding

When water infiltrates behind the siding and soaks insulation or sheathing that is deteriorating, the result can be yellow blotches on exterior walls.

Clear Stains on Siding

Water streaks or stains will be “clear” or have no colour. When water marks are the same color as the siding they are usually caused by common gutter problems such as overflowing or inadequate gutter pitch.

Black or Grey Streaks on Siding

Unlike other types of stains that might indicate siding or gutter system problems, cosmetic stains or discolorations aren’t particularly harmful, unless it’s to your curb appeal. Black or grey in colour, they are typically the result of environmental pollution such as dirt, pollen, grime, and soot.

Removing Stains from Siding

Once you’ve identified what might be causing the streaks, discolorations, or marks that you see on your siding, it will be easier to determine the appropriate cleaning solution you will need to use to clean the siding panels.

To get rid of green, black, brown, yellow or water stains, use a one-to-eight-part solution of oxygen bleach and water or follow the manufacturer’s instructions. Apply with a spray bottle; scrub as needed with a soft bristle brush; let sit for a while (from 4 hours to overnight when removing mold, lichens, etc.); then thoroughly rinse using a garden hose with a spray nozzle.

Cosmetic stains due to everyday exposure to the elements can usually be removed with a water/detergent/vinegar solution.

Note 1: Oxygen bleach has no negative impact on the environment. However, if you are using chlorine bleach in a cleaning solution to remove discolorations and stains from siding, cover nearby landscaping to protect from harmful chemicals.

Note 2: It is better to apply an oxygen bleach solution to cedar siding since chlorine bleach can break down wood fibers.