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Exterior siding stains are like a mystery that needs to be solved before you can rest easy. They seem to appear out of nowhere for no reason at all and ruin the appearance of your home’s exterior. But if there was a bright side to stains on exterior siding, it’s that it doesn’t take much detective work to determine their cause. The color of the stain holds the clue to its origin.

Exterior Siding Stains Identification Guide

Brown Stains – Long streaks of brown stains on exterior siding are often the result of rusty nails. However, if the home is old and has cedar wood siding, brown staining also might be oil residue from the cedar wood caused by water getting under the shingles. If it’s a reddish brown stain, it could be the end result of an ice dam over the winter season.

Black Stains – Black stains are usually caused by a form of algae, mold, or other type of fungus. It’s often found on the side of the house that doesn’t get much sun to dry moisture from wet weather. However, black staining also could be a result of soot, particularly for homes near industrial areas where wood-fired or oil-fired chimneys are in use.

Green Stains – Green stains on exterior siding stem from algae or moss growth. It tends to grow on siding where the right mix of sun and moisture exists and there is inadequate ventilation.

Yellow Stains – Yellow stains on siding are commonly caused by deteriorating insulation or sheathing behind the siding.

Water Stains – Water streaks or stains in the same color as your siding are caused by gutter problems. If the gutters on your home haven’t been inspected lately or are old, they could be out of alignment and causing rainwater to flow behind them or the siding.

How to Remove Exterior Siding Stains

In most cases, exterior siding stains can be removed with an applicable cleaning solution. Here are some cleaning tips:

  • To remove mold, use a sodium hypochlorite solution. Spray the solution on the siding, scrub with a soft brush and rinse with water. You may need to repeat the process a few times to completely remove the mold or mildew.
  • For cedar wood staining due to excess moisture, it’s important to identify where water is getting under the shingles. As part of our roof repair services, the Roofing Annex troubleshoots roofing problems and gets to the root of the cause. Then, the stains can be removed from the wood with mild detergent mixed with water. In some cases, a stronger cleaning agent, like an oxalic acid solution, is needed. To preserve the affected cedar wood siding, a stain-blocking primer or sealer can be applied.
  • Yellow staining can be removed using a mixture of trisodium phosphate, water and bleach. Scrub the stained area with a brush and rinse well with water.
  • Water stains can be removed with mild detergent and water. But to stop water stains from occurring, use the Roofing Annex’s free gutters and downspouts inspection to ensure they are correctly aligned to your home and functioning properly.

When using any type of chemical or bleach to clean exterior siding stains, remember to protect shrubs or plants around your house with a tarp or a plastic sheet. If you’re still not sure what’s staining your exterior siding, rely on our expertise. As always, the Roofing Annex is available to answer any questions you may have about your exterior siding, roof, windows and doors. If it pertains to the exterior of your home, we’ve got you covered!


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