Moss on the north slope
On a great many Connecticut houses one slope of the roof looks a decade older than the other. It is almost always the north face, and the culprit is not the sun that the south side gets. It is the moisture the north side never loses.
- 1 Ridge caps. Thin, exposed, and worked loose by a winter of freezing and thawing.
- 2 The field. One layer or two, and the answer changes what a tear-off costs.
- 3 Valleys. Leaves mat where two slopes drain as one, and snow sits on top of them.
- 4 Flashing. Old masonry chimneys, and the saddle behind them that should shed snow.
- 5 The vent boot. A collar that goes brittle in the cold and splits without warning.
- 6 Eaves and ice dams. Where escaping attic heat melts snow that refreezes at the edge.
Why one slope and not the other
A north-facing slope in this latitude gets little direct sun through the winter half of the year. Under the mature canopy that covers most older Connecticut neighborhoods, it may get almost none. It stays damp after rain, holds snow longer, and dries slowly, which is precisely what moss needs. Once established, moss holds water against the covering permanently rather than letting it run off.
From there the damage is mechanical. Moss grows under the edges of shingles and lifts them slightly, which breaks the seal and lets water travel sideways. It traps debris. And in a climate with a real freeze, the water it holds expands in place, night after night, which is the same freeze-thaw mechanism that opens every other seam on the roof, concentrated in the one area that never dries.
Removing it without wrecking the roof
Never with pressure. A pressure washer strips granules and takes years off an asphalt roof in one afternoon, and anyone proposing it is proposing damage. The correct approach is a low-pressure application that kills the growth and lets weather carry the residue off over the following weeks. It looks slower because it is, and it leaves the covering intact.
Prevention is a materials decision made at replacement: algae- and moss-resistant shingle lines carry copper-bearing granules that suppress growth for years, and on a shaded lot they are worth their small premium every time. Zinc or copper strips near the ridge help more modestly. And trimming back the branches that keep the slope in permanent shade does more than any product: though on the streets this region is known for, that is a conversation about the tree as much as about the roof.