Ask Jon Eakes

Last Updated: Monday, June 25th, 2001, Created: Monday, June 25th, 2001

Getting Moss off of the roof shingles

Robert from Tamworth, Ontario writes:

Since insulating our attic we have developed a green moss on our roof. We seem to have a lot of condensation in the attic, so much so that we had to remove the vapour barrier during the summer. We need your advice before we start trying to correct the situation. Is this a ventilation problem? How do we correct this? How can we clean the mould or moss off the roof?

------ Response:

We need to sort out a series of potential problems here. From your letter I assume that the moss in on top of the shingles, and the condensation is inside the attic, and the vapour barrier you are talking about is between the ceiling and the insulation. At least that is where each of those things should be.

I don't understand removing the vapour barrier during the summer -- where was it located?

A lot of condensation inside the attic is a problem, but should not create moss growth on top of the roof shingles. In fact, unless we are talking about a roof deck rot problem that is growing through the shingles, there should be no real connection between the moss and the insulation -- other than it does create a change in temperature of the roof that might have had an effect on the moss.

Moss is removed (very carefully to not damage the shingles) with a bristle brush and Deck Restorer solutions. These are not bleach based deck cleaners, but wood restorers, which are strong enough to actually dissolve wood fibers. This will dissolve the moss without bothering the asphalt. Once it is off, then it is kept away by installing zinc strips just under the last row of shingles on the top of the roof, with about 1-1/2 inches of zinc exposed to the rain. This will continually wash an extremely diluted solution of zinc down the roof all the time, keeping moss at bay. The photo shows a stateside product called Shingle Shield, but more commonly in Canada you will find zinc in 3" wide rolls just for this purpose. For more details see Keeping Moss Off A Roof.

Ventilation (good through ventilation from the soffits up and out roof top vents) can help to reduce moisture in an attic, but the real solution is to find and block the sources of moisture, which are almost always the warm moist air from the house below which leaks into the attic. The vapour barrier (on the warm in winter side of the insulation) is part of this. However the most important part is air sealing all the holes from the house, including weather-stripping the attic hatch. Details of this work can be found in a free publication from Natural Resources Canada, called The Consumer's Guide To Keeping The Heat In.

Keywords: Cleaning, Environmental, Moss, Roof, Shingle, Zinc

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