Skip to Content

Building Stains

Improving Curb Appeal

Article written by Jack Carr, P.E., R.S., LEED AP, Criterium Engineers
Published in Condo Media, February 2014

With real estate brokers touting the importance of curb appeal to maintain a condo's market value, it is no wonder condo boards rake building stains so seriously. The problem is many stains are not only difficult to remove, but the reason for the staining and how to prevent its return is not always well-understood.
We have all seen light-colored asphalt shingle roofs with ugly black splotches or V-shaped stains ruining the building's front elevation appearance. Though roof stains can be caused by chimney soot and rusting metal components, most roof stains originate from either algae or extractive bleeding. It is important to know which type of stain it is to properly remove it or not touch it at all. These stains are often difficult to diagnosis, but there are signs to assist in making the determination.

Diagnosing and Treating Stains

Even though the stain is black, it may be a form of black, brown, or green algae. It is not mildew as this only grows on living plants. If it is algae, the signs to spot include broad staining patterns; staining only on the non-sunny side of the building; and less staining below dormers or other building obstructions to the flow of roof surface water. If it is algae, that is good news as it can be removed by professional roof cleaning companies.
Unless you have algae resistant (AR)-rated shingles, the manufacturer will not provide warranty assistance. If you do have AR-rated shingles, most manufacturers will warrant them for five years after installation. Whether the shingles are warranted or not, make sure you check with the manufacturer for its recommended cleaning procedures as power washing, bleach, or other chemicals can reduce the life of the shingles.
It should be noted if you do clean the algae from your roof, do it for cosmetic reasons only. Do not do it because of reported unsubstantiated myths of it causing shingle granule loss, the release of harmful toxic spores near the home, or shingles tab lifting making them subject to wind damage. If the stains are caused by extraction bleeding, that is a different story.
If your roof has extractive bleeding (i.e., bleed through), you have product defect. This kind of staining can also occur on rolled roofing materials. The roof's chemistry is allowing black asphalt pigment to leach out of the shingle surface and appear as streaks down the roof starting at a pinpoint. These types of long triangular stain streaks are a typical sign of extractive bleeding. These stain characteristics will also appear on all sides and areas of the roof. Also, whereas mineral salts washing down the roof from metal flashings will often kill algae, the salts will have no effect on extractive bleeding stains.
The roofing industry refers to extractive bleeding as a cosmetic problem, saying it will not affect the life of the roof and is not covered by warranty. We do not agree, though, as the bleeding is a sign of loss of bitumen. This indicates the shingle batch lacked adequate bonding, which correlates to premature aging and wear, and the industry recognizes the loss of shingle weight correlates to aging and shingle failure. Similarly, the loss of volatile material causes loss of flexibility contributing to shingle tears and cracking. We also do not recommend attempting to clean this type of staining as that action could cause more damage.
Extractive bleeding also is a term used to describe the black stains seen on wood siding, such as cedar, redwood, and Douglas fir. This type of staining is caused by excess moisture dissolving the natural tannins causing them to bleed to the surface resulting in reddish-brown stains. These kinds of stains can be removed with mild detergent and water, or in severe cases, the use of oxalic acid solutions is advised.
Algae stains can also occur on both wood and vinyl siding. Typically, the non-sunny side ofthe building is most affected. Often, the stain forms in repeating vertical patterns telegraphing the location ofthe exterior wall studs due to a phenomenon called "thermal bypass temperature variation" where the temperature differential at the stud locations allows the moisture conditions necessary for organic growth. Fortunately, this type of cosmetic staining call be cleaned with sodium hypochlorite solutions or other professional products. Another type of typical staining on vinyl siding takes the form of small black dots. This form of algae call also be similarly
With a little research or a few questions posed to your building consultant, most stains can be diagnosed and your curb appeal will be restored.