Seattle Home Inspector's Blog

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Roofs can't roll---so they gather plenty of moss.

       

To state that we see a lot of moss on the roofs of houses in the Northwest would be an understatement.  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.  Perhaps there are different strains of moss and some are controlled by zinc and some are not.

     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.

Moss on a Roof 

     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.

Algae on a Roof 

     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 "unsightly"----people don't like the way they look and if they aren't installed about every 3 feet on the roof they will not 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?

     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 materials 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----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.

     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  

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Comment balloon 25 commentsCharles Buell • November 29 2008 08:42PM
Roofs can't roll---so they gather plenty of moss.
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