When to use wood preservative

Q. Is it advisable to spray a wood preservative on a 20-year-old shingle roof? Jean Y. Gallaway Atwater, Calif.

A. We fired this question off to Marshall R. Ritchie, marketing manager for the Red Cedar Shingle and Handsplit Shake Bureau, Suite 275, 515 116th Avenue, NE, Bellevue, WA 98004. This is what he had to say:

"The majority of cedar roofs in basically dry, low-humidity climates, such as most of California, remain untreated and provide satisfactory service.

"In humid climates, where detrimental growth -- moss, fungus, and mildew -- are prevalent, fungicidal treatment is advisable. Also, under these climatic conditions one shuld clean the roof of debris at least once a year -- leaves, pine needles, and the like. Some fungicidal chemicals are toxic, so be sure to follow the manufacturer's directions explicitly.

"If an oil treatment, such as raw linseed oil or one of the commercial shingle-oil products, is considered in a dry climate, it should be done initially when the roof is relatively new and at periodic intervals thereafter. The frequency of treatment depends upon the amount of absorption obtained by each treatment. As a rule of thumb, at least one gallon per 100 square feet shoudl render the treatment reasonably durable for approximately five years.

Raw linseed oil may require a little thinning with paint thinner in order for the treatment to be readily absorbed into the shingles or shakes. We do not recommend roof-surface coatings.

"Any advantage in treating an older dry roof or the first time may be negated by damage to the shingles as they are walked upon during the course of treatment , possibly causing leaks.

"In some areas mechanical lift devices are available from which older roofs may be sprayed, thereby eliminating direct contact. Also, sheets of plywood or planks are sometimes used and moved about over the roof in order to distribute one's weight during the process of treatment.

"In short, anyone considering a roof treatment should be convinced of the integrity of the people doing the treatment, including its strenght -- that is, the percentage of thinner -- and the amount of treatment per square foot actually being used on the roof."