The short answer: Moss colonizes asphalt and cedar roofs around Puget Sound because granules and wood fibers hold moisture and give spores something to root into, and because the wet season keeps a shaded north slope from ever fully drying. Interlocked aluminum ends the cycle by offering neither — no granules, no fibers, no seams that trap debris.
The mechanism, not the folklore
Moss is not a sign of a dirty roof. It is a plant doing exactly what its substrate invites.
An asphalt shingle is mineral granules bonded to a mat. Those granules are rough, porous, and slow to dry — a seed bed with the moisture retention built in. Cedar is organic matter outright, and once it weathers it wicks. West of the Cascades, rain arrives in autumn and does not really let go until late spring, and a north-facing or tree-shaded slope simply never dries out between systems. Spores land, root, and hold water directly against the roof surface. From there it compounds: moss lifts shingle edges, water sits where it should have shed, and the roof ages from two directions at once.
This is why roofs in Seattle, Tacoma, Everett and Bellevue rarely die of sun. They die of standing moisture.
What the treatment cycle actually costs you
Not in dollars — in effort and in roof life.
The standard prescription is wash, treat, install zinc strips, repeat, more or less annually on shaded slopes. Each round means someone on the roof: foot traffic on a brittle surface, granule loss down the gutters, and a chemical schedule to remember. Skip a season and the regrowth is worse than what you cleaned. Keep it up faithfully and you are maintaining a material that is aging out on its own timetable regardless.
Homeowners tell us the same thing at estimates across Puget Sound: they are not tired of their roof, they are tired of managing it.
Why moss cannot colonize interlocked aluminum
An Interlock® roof is smooth-surfaced aluminum in panels that lock to their neighbors on all four sides. There are no granules to root into, no organic fiber to wick, no exposed fasteners collecting grit, and no open seams holding debris where spores start. Rain sheds and the surface dries. No zinc strips, no treatment schedule, no washing.
That is a property of the material and the geometry, not a coating claim that wears off. The panels are 3105-H24 / 3003-H24 aluminum to ASTM B209, roughly 95% recycled content, finished in an AAMA 2605 70% PVDF Alunar® coating warranted 30 years. The system is evaluated under ICC-ES ESR-1790 for wind uplift resistance to 177.5 psf and carries a UL 2218 Class 4 impact rating; a Class A fire rating is available as an assembly under ASTM E108, dependent on the underlayment and deck build-up.
The honest caveat
Moss is not the only thing that ends a roof, and a metal roof does not exempt a house from physics. Attic ventilation, gutter drainage and overhanging limbs still matter, and we assess all three at every estimate — a roof that never dries underneath has a problem no surface material solves. What aluminum removes is the part of the cycle you have been personally responsible for.
What the Washington record shows
We have been installing this system for Pacific Northwest homeowners since 1997: 3,223 completed Washington installations and 215 documented project photographs you can look through yourself. Start with the gallery, read what homeowners wrote, or check the proof page for how the record is compiled.
The Guardian Lifetime Limited Material Warranty runs for life to the original owner, transfers once, and covers the second owner to 50 years from substantial completion.
If your roof is on the treatment cycle right now, that is the signal that the material — not the maintenance — is the problem. The aluminum roofing page makes the material case in full, the FAQ answers the questions that come next, and a written estimate gives you the once-and-done number for your own roof.
