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
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.