Some roofs have mossother roofs have MOSS!

Some roofs have mossother roofs have MOSS!

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.

Heavy moss covering of a roof

In this next picture, the black algae discolorationanother 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.

The effects of zinc on moss/lichen growth on roofs

Left and right of the clean area you can see the distinctive marks of where someone has pressure washed the roofbut not really consistent with the area cleaned by the zinc from the galvanized components.

So why dont we see more zinc strips on roofs?

I think the main reason is because they are considered unsightly-people dont like the way they look and if they arent installed about every 3 feet on the roof they wont 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 clearlyand one could argue that the zinc strip is really not helping very much as all.

Moss covered roof barely affected by zinc strip

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.

Some roofs have mossother roofs have MOSS!

As near as I can tell the algae doesnt represent much of a problem for the roofing materials itselfprimarily 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 willor apparently even letting the moss grow. Moss will kill your roofbut 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.

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Charles Buell, Seattle Home Inspector

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