Why Do Roofs Grow Moss in Western Washington?

What makes an asphalt or cedar roof surface hold moss?

Moss does not feed through roots the way grass does. It anchors with fine rhizoids, absorbs water directly through its surface, and needs only a damp, textured place to sit. An asphalt shingle provides exactly that. The mineral granules bonded to the shingle face create thousands of tiny shaded pockets, and each one holds a little water long after the rain has stopped. Cedar behaves the same way for a different reason. Wood fiber is porous and absorbent by nature, and as a shake weathers the grain opens further, so the shingle itself becomes the reservoir. Both surfaces give a spore somewhere to land, something to grip, and a water supply that outlasts the weather. Colonisation starts small. A green haze appears in the keyways between shingles, or a fuzzy line forms along the butt edges where water sits longest. Once a mat establishes, it holds still more water against the roof surface, and the process accelerates from there.

Why does a north-facing slope in Seattle never dry out?

Western Washington's wet season runs roughly from October into June. The important part of that sentence is not how hard it rains, it is how long the season lasts. Something close to nine months of frequent wetting means a roof surface rarely gets a long, warm, uninterrupted spell in which to dry all the way through. A north-facing slope makes that worse. Through the winter the sun sits low in the sky and the days are short, and a north pitch may receive no direct sun at all for weeks at a time. Water that lands there in the morning is often still there in the evening, and still there the following morning when it rains again. This is why moss on a Seattle, Tacoma or Everett home is so frequently lopsided. The south slope looks weathered but clean; the north slope is carpeted. Nothing different was done to either side of the roof. One side simply never gets the chance to dry.

How much do trees and shade change the picture?

Tree canopy works against a roof twice over. First it blocks the low winter sun that would otherwise dry the surface between storms, extending the damp period on the slopes beneath it. Second it delivers organic debris directly onto the roof, and in this region that debris is largely conifer needles, which knit together into a mat that holds water like a sponge. Douglas fir, western red cedar and hemlock all shed year-round rather than in one autumn drop, so the supply is continuous. Needles collect in valleys, behind chimneys, along the low edge of a slope and in the gutter line. Those are the same places moss is usually thickest, and that is not a coincidence. A homeowner on a cleared lot in Olympia and a homeowner on a wooded lot two streets away can have identical roofs installed the same year and see completely different results at year twelve.

Where along the I-5 corridor does moss appear fastest?

The pattern follows shade and drainage far more than it follows city limits. Along the I-5 corridor from Everett through Seattle and Tacoma down to Olympia, the homes that show moss earliest are the ones on wooded lots, on the shaded side of a hill, or with a low-pitch north slope under a canopy. Elevation and exposure matter too. A house on a bluff over Puget Sound with wind across the roof dries faster than a house in a sheltered valley pocket a mile inland, even though both get the same weather on paper. Wind is a drying mechanism, and blocking it changes the outcome. Interlock has recorded 3,223 completed Washington installations since 1997, with 3,092 pinned on the live project map, including 394 in Seattle since 1998, 180 in Tacoma since 1997, 80 in Everett and 67 in Olympia. Across those jobs the roofs being removed on shaded north slopes are consistently in worse condition than the sunlit slopes of the same house.

Is moss actually damaging the roof, or just unsightly?

Moss causes damage through water retention rather than through the plant itself. A saturated mat keeps the shingle surface wet continuously, and on a cedar roof that sustained moisture is what drives rot. On asphalt, growth in the keyways can lift and separate shingle edges, which lets wind get underneath and lets water travel sideways instead of down the slope. Removal carries its own cost. Scraping strips granules from asphalt, and granules are the shingle's protective layer against ultraviolet light. Pressure washing removes granules faster still and can drive water up under the courses. Chemical treatments work, but they treat the visible growth and not the condition that produced it, so the cycle restarts. That is the underlying issue with moss on a porous roof in this climate. Every method of dealing with it either shortens the life of the covering or has to be repeated indefinitely, because the surface itself keeps offering moss what it needs.

Does a metal roof change the mechanism?

It changes the two conditions moss depends on. An Interlock aluminum panel has no mineral granules and no wood fiber, so there is nothing porous for a spore to root into, and its coated surface sheds water rather than holding it in surface pockets. The substrate is 3105-H24 or 3003-H24 aluminum to ASTM B209 with roughly 95 percent recycled content, finished with an AAMA 2605 70 percent PVDF Alunar coating carrying a 30-year coating warranty. That is a change to the roof surface, not to the weather. Needles and leaves will still land on any roof under a fir tree, and they should still be cleared, along with the gutters. The difference is that debris sitting on a smooth coated panel is debris that can be removed, whereas debris sitting on a granulated shingle is feeding an organism already anchored into the surface. Interlock's aluminum system is also listed under ICC-ES ESR-1790 for wind uplift to 177.5 psf and tested to UL 2218 Class 4 for impact resistance. A Class A fire rating is available depending on the assembly used. Washington licence #INTERMR75406.

Common questions