Moss Treatment

Maintaining Asphalt Composition Roofs

Asphalt composition roofs seem to be the most common type of roof covering here in the Pacific Northwest.  These roof systems come in a variety of styles and estimated expected life spans, ranging from 3-tab “strip” shingles (20-year roof system) to dimensional architectural shingles (25-40 year roof system) to premium dimensional architectural roof shingles which can carry a 50-year or lifetime warranty.  The basic difference between these types of shingles is their thickness (and cost), but the maintenance is still basically the same.

I usually recommend clients purchase the longest lifespan roof system they can afford (e.g., the thicker the roof coverings, the better).  Sometimes, with competitive bidding, you can negotiate with a roofing contractor to install a longer lifespan roof for the same price as lower lifespan roof coverings.  On my home, for instance and to get the job, the roofing contractor “upgraded” me to a 40-year dimensional architectural roof system, charging me the same quoted price as a 25-year architectural dimensional roof system.  Whenever contemplating hiring contractors, it often pays to get three (3) competing bids if you can.  And, please don’t make your contractor selection solely based on price; prices should be reasonably competitive, but ultimately reputation and experience are the most important factors in contractor selection.  Ask around and check out references!

For asphalt composition roof system maintenance of homes in the Pacific Northwest, I recommend treating roof systems at least twice annually with zinc powder, zinc granules or a zinc solution, all readily available at hardware stores.  Basically, you sprinkle zinc product along the roof peak, known as the ridge line.  Rain will then dissolve the zinc, washing it downslope to kill moss/algae and inhibit future growth.  If you live in the woods- in an area with little sunlight- or have a low slope roof, you may need to apply the zinc treatment 2-4 times annually.  With a zinc solution, you simply spray it on with a garden hose or low pressure sprayer.  It’s simple and economical.

If you see moss growth, and your roof looks “dirty” and unmaintained, please don’t be tempted to pressure wash your roof system!  Composition roofs have granules embedded in asphalt which, in addition to giving your roof color and style, serve an important purpose- they are the roof’s protection from ultraviolet radiation and Mother Nature!  Pressure washing will, at a minimum, displace granules and reduce life expectancy.  At a maximum- and I’ve witnessed this thousands of times- pressure washing will ruin a perfectly good roof system.  Keep the pressure washer on the ground, don’t allow it onto your roof.

Once treated, the green moss growth will eventually turn brown and then black, but it takes time.  Moss will then disintegrate and- again with time and patience- the moss will begin to fall off your roof system and go away.  Try to not be tempted to use any mechanical means to remove moss growth from your roof system.  Moss has roots, called rhizomes, which can embed into the roof coverings.  We want these roots to die and “release” their hold on the roof.  If you brush the roof, you risk pulling chunks of roof coverings away because the moss rhizomes tightly hold onto the roof coverings.  Treat your roof and the moss will eventually release.

Of course, if you let a roof go to the extreme and become completely covered with a thick mat of moss, the moss mat may very well be the only thing keeping your roof from leaking.  I’m often asked to inspect a home, clients worried in particular about the roof covered completely with moss.  Politely, I tell them that I can’t see a thing and can’t properly assess until the poorly maintained roof is cleaned.

Unfortunately, in the real estate industry, we usually don’t have the luxury of time.  The ideal solution is to be patient, don’t “clean” the roof, treat the moss and let it gradually die away.  But this isn’t always practical.  Sellers often stage their home wanting it to look perfect and have the roof cleaned, which puts the roof at risk of mechanical damage.  Buyers want to know the status of the home’s roof, so they insist the moss-covered roof be cleaned so it can be properly assessed which, again, puts the roof at risk of damage.  Insurance companies, too, will not want to see moss all over the roof and may require it to be cleaned.  All of these interests combine to, unfortunately, damage a lot of roofs in the Pacific Northwest and, consequently, keep roofers quite busy.

Here’s my “take home” point.  Treat your roof at least twice annually.  Personally, I treat my roof four times annually with zinc powder, simply sprinkling it at the ridge line.  My roof is now ten years old and still looks brand new.  Even if you don’t see moss, keep treating it at least twice annually.  With proactive (and reactive) treatment, you’ll eventually get ahead of moss growth and enjoy a beautiful roof for years to come.