Q: I read the Q&A on using treated cedar shingles in the March 2016 issue of JLC. Do pressure-washing and sealing a cedar roof also help to promote longevity?

A: Chris Yerkes, a cedar-shingle installer certified by the Cedar Shake and Shingle Bureau (CSSB), and owner of Cedarworks, in Brewster, Mass., responds: Pressure-washing a cedar roof is generally a bad idea, and my company does not offer that service. I have two main problems with it: Pressure-washing forces water into the shingles, and it can cause surface damage and actually promote rot.

Pressure-washing effectively drives water into the wood while removing the top layer of material at the same time. The resulting dramatic increase in the moisture content of the shingles can lead to rot and mold, and the force behind pressure-washing can erode or “scallop” away the soft material between grain rings of the shingles, weakening them and potentially causing the shingles to crack and curl. Additionally, if the wrong spray angle or nozzle tip is used, the process can create leaks by forcing water underneath the shingles or shakes, or it can give the roof a streaked or uneven appearance.

One of the biggest problems with pressure-washing is the easy accessibility of the machines. In trained and experienced hands, a pressure-washer might be effective in the most extreme cases. But there’s always the danger that someone will go down to the local rental center and grab a pressure-washer without knowing exactly how to use it. An inexperienced pressure-washer operator will probably do more harm than good to a roof.

Instead, we’ve found that the best way to keep a roof clean is to passively maintain the roof from day one. This can be accomplished by using treated shingles (as was mentioned in the Q&A referred to above); by installing copper, zinc, or lead strips along all the hips and ridges, which helps to prevent mold and moss from growing; and by keeping valleys and low-slope roofs free of debris. Don’t forget to keep the gutters clean as well.

As far as the use of sealers is concerned, I do not advocate using any type of sealer on a wood roof. Sealers can trap moisture and prevent drying, which can lead to premature roof failure.