What do dark stains on your roof mean?

Q. Many of the homes in our subdivision are 12 to 15 years old. In the last couple of years, I have noticed that many roofs, including our own, have dark stains flowing down one side or another. The stains are usually on the north side, which gets the least sunlight. I assumed this was because that side did not dry out as well and that something in residual dampness was causing this discoloration.

Today, I got a flier for a roof-cleaning contractor. It said such stain is from algae that “over time causes permanent damage by taking root on the shingles, dislodging the granules that protect and color the roof. This algae also holds water to the surface of the roof and causes decay and rot to the roof's underlayment.” I have not heard of roof-cleaning before. Does it solve the problem?

A. According to my resources, algae do not have a root system and cannot “hold” water against the shingles. The contractor must have been talking about moss.

The algae stains you see are most likely Gloeocapsa Magma, a species of cyanobacteria. Cyanobacteria have been reported to be responsible for the stains on asphalt-based shingles. Starting in the Southeastern states, the algae rapidly spread to the Midwest and are generally found on the north slopes of roofs where there is little sunlight. The bacteria accumulate over time as they feed on moisture already trapped on the shingles and the limestone that is used as filler in the manufacture of fiberglass shingles.

Algae stains were not common in the older “rag content” shingles used some 20 years ago because there was no limestone filler in those older shingles. As the bacteria feed on moisture or humidity and limestone (calcium carbonate), dark stains appear and then develop a hard UV protective outer coating.

Given the fact that a standard shingle has a life expectancy of 20 to 25 years and it takes at least five to eight years for algae stains to first appear, the shingles will most likely need to be replaced long before the algae could do any major harm.

It is still recommended that the shingles be cleaned by a reputable contractor and copper or zinc strips placed at the ridge of the roof to prevent future growth. The oxide from the metal covers the roof every time it rains and prevents the growth of algae. When it's time to replace the shingles, ask for algae-resistant shingles that have metal pellets embedded in the asphalt to prevent the stains from forming.

Q. I know you have talked about insulation before and you keep mentioning something about a value for the insulation and how much is needed. Where can I find information about my home area?

A. Insulation is measured by the R-value of the product. The higher the R factor — for instance, R-19 versus R-30 — the better the insulation is at preventing thermal loss or gain. The R-value depends on the type of insulating material used and its thickness or density of the product.

A common fiberglass batting material for a 2- by 4-inch wall stud cavity is rated at R-11, but a high-density fiberglass batt for that same 2- by 4-inch wall cavity has a rating of R-15 and a medium-density batt is rated at R-13.

If you are insulating an attic space, you want to install as much insulation as possible without letting the insulation come into contact with the underside of the roof's decking. Plastic or foam baffles are first installed in between each rafter space where the ceiling meets the roof at an outside wall. The baffle allows air to flow from the overhang to the roof vents for humidity control.

Then select the insulation materials you want to use. You can use fiberglass batts, blown-in, loose-fill fiberglass or cellulose. How much do you need? If you go to web.ornl.gov/~roofs/Zip/ZipHome.html and click on the “R-value Recommendation Calculator,” you can find the R-values for the attic, walls, floor and so on. A quick look at just one area in Washington state found that R-49 is recommended in your attic.

If you use loose-fill fiberglass, which has an average rating of R-2.7 per inch of thickness, or loose-fill cellulose, which has an average rating of R-3.7 per inch, you would need 13 inches of cellulose versus more than 18 inches of fiberglass.

Cellulose is basically 80 percent recycled newsprint that has been treated with a fire retardant and, when compacted by its own weight, provides a better air barrier than does the loose-fill fiberglass or the fiberglass batts.

Q. I have a whole-house fan at the landing of my second floor. It's great at cooling the house in the evenings. But, when I run the A/C during the day, the metal louvers radiate a lot of heat. Is there any way to put a second damper in the attic? Any other options?

A. A whole-house fan is great for cooling without using the air-conditioning system. But, as you have discovered, there is nothing between you and the heat of the attic except those small strips of metal in the fan's shutters.

In the past, homes built with the integrated fan also had a wood covering to shield the fan from the attic heat. The box was lowered and raised using a rope-and-pulley system often found inside a nearby closet. Because the wooden box was so heavy, it was difficult to open and the rope would often break and have to be replaced.

You can construct a box of ordinated strand board (OSB) to fit around the fan housing and then line the box with rigid foam insulation. The lid can be opened and closed using the old-fashioned rope-and-pulley system. When constructing the box to cover the fan, you can either line the box with rigid foam insulation or glue the insulation to the box's exterior. If the insulation is to be inside the box, make the box at least 6 inches wider and longer than the fan housing to allow for thickness of the insulation.

Secure the box to the rafters or fan housing using L-shaped metal garage-door track hangers and screws.

The lid will need a set of hinges and an eyebolt for attaching the rope.

If you want to install a whole-house fan, choose one that has an automatic insulated cover that opens and closes depending on the fan's operation.

Remember, the fan can only pull in as much air as it can push out through the attic vent system. Determine the cubic feet per minute (CFM) the fan can pull and divide that number by 750. This will give you the number of square feet of free venting the attic will need for the fan to work properly. To determine how much free venting you have, determine the number and size of the soffit vents or canned vents on your roof.

A 12-by-12-inch soffit vent has only 6 square inches of free venting because it is slotted and not fully open. Canned roof vents are the same. A ridge vent can have anywhere from 10 to 20 square inches of venting per lineal foot.

Add all the free vent openings together to determine the total amount of attic venting you have. Contact a professional roofing company if more ventilation is required. For more information, see www.cor-a-vent.com/pdf/balancedventilation.pdf.

Ÿ Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home improvement questions at d.Barnett@insightbb.com.

Scripps Howard News Service