Algae growth on vinyl siding (C) Daniel Friedman Causes of Vinyl Siding Stains, Algae, Lichens, Soot, Other Markings

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Vinyl siding stain diagnosis:

This article discusses common causes of stains or discoloration found on vinyl building siding. Distinguishing the type of stain found on siding is useful in deciding on the cause and thus the cure or prevention of staining. Dirt from rain splash-up, algae, lichens, mold, moss, or even smoke or chemical stains have different causes and different preventative measures.

Shadows, moisture variations that collect or don't collect dust on siding, even roof leaks or air leaks through building walls can often be diagnosed by a close look at wall stain patterns discussed here.

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Stains on Vinyl Siding: Algae, Fungus ("Mildew"), Artillery Fungus, Smoke, Dirt, Leaks

Algae growth on vinyl siding (C) Daniel Friedman

As we complain throughout, mildew, a proper subset of the larger family of molds or fungi, does not grow on buildings - mildew grows on living plants.

But people often refer to mold growth on surfaces as "mildew" and so product manufacturers selling cleaners intended to remove mold will understandably call them "mildew removers". It's mold - if you care.

The photo at above left shows green algae on vinyl siding. Algae may also look black or brown, as may some molds.

Small gray or darker round spots on siding that are about 1/16" in individual diameter, particularly if the spots are slightly raised may be artillery fungus - a mold spore.

Damaged vinyl siding (C) Daniel FriedmanOther larger spots, 1/8" or so but with the same properties may be remaining footprints from vines that were growing on the wall.

Algae on building walls: Algae is often confused with mold. In our photo at above left shows a common condition that is found on many types of building siding: green (or brown or black) algae growth. Algae is found on siding that is shaded and stays damp.

While algae stains on exterior walls are principally a cosmetic concern and can be cleaned using scrubbing, mild soaps, or more aggressive TSP-substitute cleaners or detergents, there may remain a hidden problem: any building wall that stays damp and shaded may be at higher risk for hidden insect or rot damage.

If there is an accessible crawl space or basement below a wall that looks like the one in our photo, that's a good place to look for signs of building leaks, rot, or insect damage.

In our photo just above we think these black specs may be a fungal growth. A pen cap is included in the photo for scale. Artillery fungus can produce black specs on a building wall, trim, or other outdoor surfaces. Similar black specs may be found on exterior siding where the attachment points of vines have adhered to the surface.

Dirt and water stain on vinyl siding (C) Daniel Friedman Leak stains on exterior vinyl sided wall (C) Daniel Friedman

In our photo at above left you are looking at dirt and dust on building siding; that vertical light colored streak shows where water has washed away some of the surface debris. "Is that a problem?" is a common question. Probably not. Look above the water streak for a drain opening that is permitting condensate to drain off from the back side of the building siding ouit onto the siding exterior surface. Such markings may be normal.

But extensive water streaks down exterior siding may indeed indicate a wall or roof leak that needs to be tracked down and repaired. Our photo at above right shows that water has been leaking out of this vinyl-clad wall (in winter in a northern climate); further investigation and repair are needed, and there is a risk of hidden rot damage, insect damage, or mold contamination in the wall cavity; I'd also worry that water leaking into the wall cavity may have wet insulation.

Advice for Dealing with Algae or Mold on Outside Walls

Black stains on vinyl siding (C) Daniel FriedmanThe black stains on the vinyl siding shown at left may be a fungus or a black agae: by the growth pattern we think this is almost certainly a dark-colored algae.

A microscopy lab examination of a sample from this surface would be needed to know for sure. But testing is probably not justified.

Where there are apparent "growth" substances on a building exterior wall we offer the following added advice:

  • The same cleaning methods - soap and water, or stronger cleaners - work pretty well on either mold or algae on a building wall
  • Skip testing: Testing to identify the material by collecting a lab sample (TEST KITS for DUST, MOLD, PARTICLE TESTS) is easy and not very costly ($25 to $50. typically) but in general we do not recommend this step.

    It's not necessary before cleaning off the mold or algae, and except for the presence of mold or algae on a wall as an indicator of damp conditions, exterior building molds or algae are not a threat to building occupants except in very rare circumstances.
  • Fix the cause of wall mold or algae: if you don't want recurrent mold or algae growth on a building wall it needs to be more dry and to receive more sunlight. We hate cutting down mature trees, but don't hesitate to trim back dense shrubs close to a building exterior wall.
  • Increase mold/algae resistance of your exterior wall - if it's a wood wall that's being stained or painted - by using a fungicidal additive to your paint or stain. Obviously this isn't going to be of use siding materials that are not going to be painted.


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