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

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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.

[Click to enlarge any image]

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. [Photo below]

Other larger spots, 1/8" or so but with the same properties may be remaining footprints from vines that were growing on the wall.

Damaged vinyl siding (C) Daniel FriedmanAlgae 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

In our photo at above 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.

Leak stains on exterior vinyl sided wall (C) Daniel Friedman

But extensive water streaks down exterior siding [photo above] may indeed indicate a wall or roof leak that needs to be tracked down and repaired.

Our photo 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:

Reader Question: Chalking surface of older vinyl siding - is this normal

I am wondering why when I rub my fingers across our siding I come away with my fingers being white.

Is this a normal vinyl siding thing or does it mean it has been previously painted or what? I want to clean the siding but would like to know what the white stuff is first. - Anon 8/18/2011



Yes indeed, some formulations of vinyl siding, especially some older products, will oxidize, producing a dull or powdery film of oxidized material similar to paint chalking.

This appears especially on building areas exposed to the most sunlight. If the siding is not cracking and breaking then it's a cosmetic defect not a functional one, but it is also an indication of siding age, formulation, and sunlight exposure. If your white stuff correlates with weathering or sun exposure then the white stuff is most likely oxidized vinyl.

Watch out: any older oxidized plastic may have increased in brittleness and be more vulnerable to mechanical damage.

On occasion I've seen powdery white stuff on vinyl siding that was not from the vinyl itself but was instead a wash-down of oxidized paint or coating on other building components such as windows or trim. If your white staining shows up specifically just below such objects then they are probably the source.

You can remove some of this oxidized coating by scrubbing with any household cleaner and a soft bristle brush; watch out not to blow water into the siding - so be careful about trying power washing.

Reader Question: brown water leaking from behind vinyl siding on older home

Brown leak stains coming out of clapboard siding (C) Daniel Friedman

(Mar 17, 2014) Anonymous said:

I live in wi. I have brown water leaking from behind my vinyl siding. I think it is from frost melting on the back side of the vinyl.

I have tyvek stapled to the osb. It started on the south side of the house. as the outside temp went up, it started on the east side and now the west side.

The west side has a 8 foot roof over our deck on the gable end. so I dont think it has any thing to do with the gutters. the house is seven years old. This is the 1st year this has happened. It was also the coldest windyest winter in 30 years. any Ideas


Yes anon: I have often seen red or brown stains from water leaking down behind building siding on older homes. Most often the root cause is wind-blown rain, condensation or frost accumulation behind siding that was not installed over a modern housewrap water barraier.

Older siding jobs were installed over red rosin paper (not waterproof) or often roofing felt (not very water proof).

Water leaking into the wall behind the siding absorbs tannings from wood sheathing or even chemicals from the roofing felt in some cases; that stained water drips out between siding courses or drips out at the wall bottom leaving unattractive stains.

I suspect that at your home the unusually cold weather caused frost accumulation behind the siding at a quantity sufficient to produce stains when the weather warmed.

Above I've included a photo of brown leak stains on a wood clapboard home in Fishkill New York: you can see that this problem can occur on many types of sided buildings, not just with vinyl siding. If you were to re-side the home you'd want to

See VAPOR BARRIERS, VINYL SIDING where we discuss use of housewrap behind vinyl wall coverings. But if your home only suffers these stains once in 40 years I'm doubtful such dramatic repair is justified.


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