Our shake roof, like many others, is dark from aging and moss. Every year I'm visited by guys with power washers selling their service. Some also want to treat the roof. Lots of neighbors take them up on it, and from a distance their roofs look like new. Is this a good way to remove moss from a shake roof? R.H., Kirkland
DEAR R.H.: The best removal method depends on the age and condition of your roof. In addition to power washing, there are chemical treatments. Each method has its pros and cons.
Moss growing on shingles and shakes can do double damage so you must take steps to remove it. During a rain, moss retards the shedding of water, which promotes leaks. After the rain stops, it prevents the surface from drying, which promotes the growth of wood-rotting organisms. Another major reason to keep your roof in good shape is to protect insulation. A leaky roof means your insulation is getting wet and losing its insulation value.
-- Power washing is an effective method, but must be done carefully since cedar is a soft, low-density wood. If you hold the wand too close to the shake, apply too much pressure, or hold it in one place too long, you can detach shakes or erode many years of wear in a matter of seconds.
If you're going to do the job yourself, make sure you know how to determine and regulate the pressure and flow of the power washer (available through rental firms). Jack Eddy of the Cedar Shake and Shingle Bureau, recommends no more than 1,000 pounds per square inch (psi), at a flow of about four gallons a minute. For more information on cedar shakes, contact Eddy at (206) 453-1323, or write to him at 515 16th Ave. N.E., #275, Bellevue, WA 98004.
If you don't choose to climb on the roof yourself, professional companies providing this service are listed either under "Moss Control" or "Roofing Contractors" in the Yellow Pages of your phone book. As you would with any contract for service, choose a reputable firm. Ask the contractor to provide proof of insurance, a written description of the work to be done, a cost estimate and references (if you ask for them).
-- Chemical treatments (available through hardware stores, building supply centers, or lumber yards) will perform to varying degrees of effectiveness depending on a variety of environmental conditions. The frequency of application required for long-term protection depends on the chemical you use, the amount of rainfall, average temperatures, and the condition of the roof. The type of chemical used depends on the type of gutter, downspout and flashing on your roof.
Remember, chemical treatments are toxic and pose health and environmental risks. They must be applied safely and with extreme care, otherwise you can seriously harm surrounding vegetation and people or pets. Wear protective clothing, read and follow the instructions closely (for application and disposal), and be cautious.
And use care when handling and disposing of moss and litter treated with chemicals to avoid accidental exposure. Know your legal responsibility as a pesticide applicator. You may be liable for injury or damage resulting from pesticide use.
If you have any questions about safe handling, call the Seattle - King County Department of Public Health Hazards Line at 296-4692, the WSU/King County Cooperative Extension at 296-3900, or phone the closest poison control center.
A non-toxic alternative
Applying zinc and copper strips across your roof is a non-toxic alternative. Galvanized metals have a surface coating of zinc; and zinc can effectively kill or retard the growth of moss and fungi. According to the Oregon State University Extension Service, although it could find no scientific study verifying how long this treatment is effective, the effect of zinc is prominent below chimney flashing and vents.
Fasten a two-inch-wide strip to either side of the ridges and run it the entire length of the roof. Rain will leach zinc from the metal's surface and wash it down the roof. After a year, you should be able to judge how well the strips are controlling the moss; add more if needed. A word of caution. Since the leachate contains zinc, don't collect the water in rain barrels for use on your lawn or garden.
Care and Maintenance
Now that you know the hazards of removing the moss, let's look at ways to increase the life of your wood roof. Eddy says "Keeping the roof clean is one of the most effective ways to prevent damage.
You can add years to the life of your roof through minimum maintenance. Once or twice a year, remove all leaf litter, pine needles, and debris that accumulate over time between the shingles and shakes. Wash off most of the debris with a garden hose. A stiff broom will take care of the leaves and pine needles.
When climbing on a roof use good judgment and safe equipment. As much as possible, walk across the roof surface, not up and down. On slopes with a 5-12 pitch (about 23 degrees), or greater, use safety lines and belts. On roofs greater than 16 feet from eaves to ground, use safety lines and belts when you work within six feet of the edge. If you're leaning a ladder against the gutter, be sure to put a wood block in the gutter to prevent it from being crushed or damaged. Also, secure the ladder to the block to prevent the ladder from sliding.
Finally, remove overhanging branches that excessively shade sections of the roof and retard its drying. Never let the tree branches touch and rub against the surface of the roof.
Written by Lee Benner from the Education and Information Network of the Washington State Energy Office. This information can be made available to people with disabilities; please call 1-800-962-9731 (Voice and TDD).