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Roof Moss

Over the years I have often heard repeated the statement that moss on a tar and gravel roof is harmful to the roof, and should be removed. The theory being that the moss has “roots” which burrow into the tar and damage it.  I have never seen a shred of empirical evidence or research to support this contention.

My own view, based on observation, inspection, and working on dozens and dozens of roofs in Hollin Hills is that the moss, rather than being harmful to the roof, is beneficial. Here’s my reasoning. The primary agent of roof senescence is the sun. The ultraviolet light and heat cause the tar to dry out, shrink, and crack. As the cracks develop and deepen, at some point the roof membrane is breached, allowing water to seep through.

The tar roof is covered with white gravel for two reasons. One reason is to provide ballast; the weight of the gravel helps prevent the roof membrane from lifting in the wind. The second reason is to protect the tar from the sun. A roof typically starts deteriorating in spots where the gravel has washed away, leaving a bare area exposed to the sun’s damaging rays. Think of the gravel as sunscreen for the roof.

As anyone who has removed moss from their roof knows, the moss has no roots. It forms a mat of growth in the gravel, but is never attached to the tar. The beneficial aspects of the moss are three fold. First, the moss helps anchor the gravel and prevent it from washing away, maintaining the integrity of the gravel solar barrier.  Secondly, the moss acts as an additional barrier to the sun. And thirdly, the moss acts as an insulator, keeping the tar, and the house, cooler on sunny days.

Finally, walking around on the roof to remove the moss, and the removal of the gravel that is imbedded in the moss, probably reduces the life of the roof.

So my view is, leave the moss alone.

If you have a shingle roof however, the moss should be carefully removed. The moss can hold water on the roof and allow water to back up under the shingles, causing leakage.

Robert Fina
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