Ask the Carpenter: What are those stains on my roof?

Ask the Expert Home Improvement

Q. Grayish stains recently appeared on the roof of my home, which was built in 1931. When the house was painted in June 2015, these stains were not there. I had a new roof installed in 1986. Before I call a roofing company, which would probably be glad to sell me a new one, I want to check on how long one should last. Also, I wonder what has caused the stains and whether they can be treated. My budget is limited; I am a 92-year-old widow, and while I am managing to maintain the house, I need to avoid unnecessary expenses.


A. I’m sorry to tell you this, but you are past needing a new roof. Thirty-two years for an asphalt roof is amazing. It’s also possible that you have two layers on your roof, which may have attributed to its longevity. If and when you reroof, please make sure that the installer strips it to the sheathing and replaces ALL of the flashing. Feel free to reach out to me if you have questions on this.

Roof stains

These are often attributed to an accumulation of dirt, defective shingles, mold, or mildew, but the most common culprit is actually blue-green algae known as Gloeocapsa magma ­­that are spread via airborne spores. I’ve been seeing more of this lately, and it does take some time to build up.

People often ask me whether these stains are a sign of roof wear. The algae will not damage the shingles and won’t make your roof leak. These stains usually show up on areas on the north side of a roof or in sections that do not get much sunlight.

The algae, which are found mostly on lighter roofs, look awful. You may also have noticed that algae stains are absent directly below the metal flashing around your chimneys and roof vents. This is because the copper and the zinc coating on galvanized sheet metal are toxic to algae. Every time it rains, trace amounts of these metals are washed down the roof, inhibiting algae growth.

Roof cleaning

Algae stains can be removed by cleaning, though they usually return. While an occasional cleaning might not harm your roof, repeated use of harsh chemicals, or the erosive effects of pressure washing, can shorten the life of asphalt shingles.

There are professional companies that will clean your roof. This is the safest route, because they have special lifts and equipment.

There are also consumer products (Ex. Wet & Forget) that you can buy to remove algae stains, but the best solution I’ve found is to use a mixture of two parts water to one part bleach applied with a garden sprayer. Be careful not to get it on your plants and be sure to protect your skin and eyes. too.

Preventing stains

Reroofing with darker shingles will make a difference, and so will using shingles laced with copper granules.

I’ve had great success installing strips of lead, zinc, or copper under the row of shingling closest to the peak, leaving an inch or two of the metal’s lower wedge exposed to the weather. It’s like an de-algaeing roof wash.

Good luck!

Dear Rob

From C.N. in Brookline: Just saw your answer to the question of a metal roof for a seasonal camp (“Ask the Carpenter: Up on the housetop, a debate on metal vs. asphalt shingle roofs,’’ Sept. 7). I wish you’d at least mentioned that sound is a factor only in uninsulated metal roofs. We installed a standing-seam metal roof and solar on our home in Brookline last year. In addition to the fact that the metal reflects rather then absorbs heat, making it cooler on the third floor, we do not hear the rain.

The use of non-fossil fuel products coupled with the extended life expectancy and temperature reduction makes it an excellent choice for homes. Although the initial outlay is higher, it is more cost-effective and environmentally sustainable in the long run.

From Nicholas DiBuono: Regarding your response to the homeowner who wanted to keep critters from sneaking behind his latticework (“Ask the Carpenter: Should you shut off the pilot light in your fireplace during the summer?’’ Aug. 20) Many people give up gardening because of the back-breaking work of burying fencing such as hardware cloth. This is unnecessary. You simply need to bring the fencing out at a right angle, lay about 6 inches of it on the ground, and cover it with a few shovelfuls of dirt. Critters will go up to the edge of the garden, dig down, hit the fencing, and not know enough to back up 6 inches to go under.

Rob: Great points, C.N. and Nicholas. Thank you.

Rob Robillard is a general contractor, carpenter, editor of, and principal of a carpentry and renovation business. Send your questions to [email protected] or tweet them to @robertrobillard. Subscribe to our free real estate newsletter at