Moss on the roofs of the Northwest are almost synonymous.
One of the recommendations to reduce the amount of moss growth is to install zinc strips to help control the moss. This sometimes seems to work while other times it seems to have almost no effect. I have no clue as to why this is other than perhaps different species of moss respond differently or perhaps zinc looses its effectiveness over time.
In this picture of a very moss covered roof it is very clear that the various galvanized plumbing vent-pipe penetrations are in fact reducing the moss growth in the areas beneath them.
In this next picture, the black algae discoloration—another type of vegetative growth that occurs on roofs—-can also be affected by zinc given off by nearby galvanized components on the roof. In this case the chimney restraint brackets and chimney flashings are helping to keep the roof clear of algae in a distinctive pattern that corresponds to the bracket and chimney installation. It almost looks as if someone got up there and cleaned that area.
Left and right of the clean area you can see the distinctive marks of where someone has pressure washed the roof—but not really consistent with the area cleaned by the zinc from the galvanized components.
So why don’t we see more zinc strips on roofs?
I think the main reason is because they are considered “unsightly”—-people don’t like the way they look and if they aren’t installed about every 3 feet on the roof they won’t keep the entire roof free of moss & algae growth. Note how in the second picture the roof is only “totally” clean about three feet below the bracket? A small amount starts to appear again down near the bottom of the clear area.
In this next picture we can see that effect even more clearly—and one could argue that the zinc strip is really not helping very much as all.
I know you are waiting for my recommendation for an easy solution—-but alas I have none. Some roofing materials supposedly have zinc granules included in the roofing material itself, and the effectiveness of this approach has yet to prove itself to me. It would seem to me that these roofs would stick out like a sore thumb in the NW but I have yet to see this.
As near as I can tell the algae doesn’t represent much of a problem for the roofing materials itself—primarily a cosmetic concern. This is unlike the moss, which should be controlled. Sweeping the roof during the dry season seems to be effective. Pressure washing should always be avoided as it will take far more years off the life of the roof than sweeping will—or apparently even letting the moss grow. Moss will kill your roof—but so will a lot of the attempts to eliminate it.
Moss is more of a problem with wood shingle type roofs and composition type roof coverings that contain wood fibers.
I think a combination of manual maintenance, zinc strips and eco-friendly chemical treatments can do the job.
The most important thing I can say about moss on your roof?—-be careful up there. Only people experienced to work on roofs should do maintenance work on roofs.
Charles Buell, Seattle Home Inspector
If you enjoyed this post, and would like to get notices of new posts to my blog, please subscribe via email in the little box to the right. I promise NO spamming of your email