Cedar Shake Moss Removal

Cedar is the covering that most rewards patience, and the one where a fast method does the most damage. A shake roof on this part of the Island is often twenty or thirty years old, has been wet for eight months of every one of those years, and is considerably softer than it looks from the driveway.

The keyways are the job

Growth on a cedar roof is not really on the shakes. It is between them.

The keyway — the gap between two shakes in a course — is narrow, shaded from every angle and slow to dry. Moss establishes there first and spreads out from it, and by the time it is visible across the face of the roof the keyways have been full for years. A roof cleaned across the surface and not through the keyways looks finished from the ground and is essentially untouched where it matters.

Clearing them is slow. It is done by hand, along the run, working down the slope so nothing is driven up under the course above. There is no version of this that a machine does faster without doing it worse.

What age does to a shake

A weathered cedar shake has a grey, slightly fibrous surface that is doing real work — it sheds water and takes the UV. Under it is wood that has been damp for most of its life.

That combination sets the limits. Pressure lifts the weathered layer and exposes soft wood underneath, which then holds more water than it did before anyone touched it. A stiff blade dragged across the grain does something similar on a smaller scale. So the tools stay soft, the direction stays with the grain and down the slope, and the pace stays slow.

The butt line — the bottom edge of each course — is where cedar gives out first, and it is the part that has been sitting wettest. That is where we start, and it is the part we look at hardest on the way through.

What gets flagged

Loose shakes. Cupped shakes that have curled away from the course below. Splits running up from the butt. Any patch where the roof has already been repaired with something that is not cedar.

None of that is ours to fix, and none of it gets quietly worked around either. A shake that lifts under a hand is one that was coming off in the next southeaster anyway, and you are better off knowing which slope it was on.

Why cedar rots from the bottom up

Water leaves a shake roof by running down the face of each shake and dripping clear at the butt. That butt edge is end grain, which takes up water far faster than the face does, and it is the part sitting closest to whatever is growing in the keyway below it.

So cedar tends to fail along the butt lines first, in a fairly even band across a slope, rather than in patches the way asphalt does. A shake that has gone soft there will feel spongy under a thumb while the top two-thirds of the same shake is still sound.

That is worth knowing when you are deciding whether a roof is worth clearing. A cedar roof with sound butts has years in it. One that is soft along the butt line across most of a slope is telling you something the moss is not responsible for.

What you get afterwards

The keyways clear, the butt lines clear, the valleys and hips done properly, and the growth off the roof and away rather than raked into the gutter. Then a plain account of what the roof is doing: which slopes are worst, where the shakes are going soft, and roughly how long before it wants looking at again.

Shaded cedar under fir tends to want attention every two or three years here. Open cedar goes longer.

Different covering? See asphalt shingle or concrete tile.