Renovation & Design

ASK THE INSPECTOR: Proper removal of moss on cedar shingles

Postmedia/Inspecting a moss-covered cedar roof.

QUESTION: I read your articles in the Winnipeg Free Press and I have a question about moss growing on our cedar shingles on our roof. Is it OK to leave it, or what can I spray to kill the moss? I would appreciate your comment.

-- John Friesen

ANSWER: Issues related to cedar or wood-shingle roofs are becoming fewer and fewer as many of these high-quality roofs are being replaced with different materials. The high cost of cedar combined with increasing types of other esthetically pleasing shingles are making them a thing of the past.

But maintaining your current cedar roof is still a priority, and removal of moss is one of the key elements in that effort.

Moss will grow on many different types of roofing, but especially wood shingles because of their porous nature. Natural wood roofing, particularly cedar shakes or shingles, is quite permeable to moisture. If roofing dries quickly after it becomes wet from precipitation, there will be little chance of moss growth. Because cedar shingles absorb moisture easily, they are also able to shed water and dry quickly, as long as they are readily exposed to sun and wind. Moss will normally grow more easily on wood roofs on the north side or other areas that are shaded by large trees.

Some of the same properties that makes cedar roofs subject to absorbing water also make them susceptible to easy collection of dirt and other debris. When roofs of any kind have a lot of material sitting on the surface, especially organic material such as leaves and other tree litter, there is a much higher chance of moss or fungal growth. Again, debris holds moisture and moss can easily take root any time there are wet areas. This is particularly important with homes that have lower-pitch roofs or multiple valleys where junk can easily collect.

So the first order of business in preventing moss growth is to regularly clear your roof of loose material. This can be tricky, as wood roofs can be quite slippery and you really should clear debris from top to bottom. Extreme care should be taken any time you are walking on a wood roof, especially if it's wet or covered with damp debris or moss.

If you are cleaning your roof with a hose or pressure washer, you always want the spray to be aimed downward to prevent forcing water underneath the shingles. Once you have cleared the roof of leaves and twigs, small amounts of moss should come off fairly easily with a very stiff broom, wire brush or water from a hose.

The most difficult areas to clean are the bottom of the shingles, which are particularly susceptible to moss growth because of the exposed end grain. They're also the thickest portion of the shake or shingle and normally the last part to dry. These are the areas that may need a wire brush or putty knife to physically pry off embedded moss.

Using chemicals or detergent to remove the moss is not normally necessary or wise, with one exception. The one element that is frequently used to clean or prevent moss growth on wood roofs is zinc. Zinc can eliminate moss from troublesome areas and is often used in a very passive manner to accomplish this task.

Installation of small zinc strips near the top of wood roofs can be quite effective in maintaining moss-free sections below. When rain falls on these strips, some zinc will wash down onto the shingles below, which will not allow the moss to grow. This can often be accomplished by using common galvanized metal flashings in strategic areas, because they contain zinc to prevent corrosion.

The only drawback with this method is that the areas directly below the metal will change colour, relative to other areas, as the zinc is washed down. This may leave streaks or lighter spots in an otherwise uniform grey roof. Conversely, if your roof is almost black because of fungus and moss growth, it may dramatically improve the look at the same time as killing the moss.

But your main question is whether it's necessary to remove the moss at all. Yes, it's very important to remove the moss to prevent premature deterioration of the cedar shingles. While cedar has natural phenols that help prevent rot, they do not eliminate it. Even cedar will deteriorate from wood rot over time if there is water present. Anything that holds moisture, such as spongy moss, will prevent the wood from drying out and will allow wood rot to get a foothold. The thicker the moss is on your cedar roof, the more quickly the shingles will rot.

While I discourage the use of harsh chemicals due to safety and environmental reasons, there may be specific cleaners containing zinc that can be used to help remove the moss from your roof. But use them wisely. If you have a small or moderate amount of moss in a few locations on your roof, you should be able to get rid of it with a little elbow grease and water.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors -- Manitoba ( Questions can be emailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at


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