Knowledge Base · Moss & Rain
Do You Have to Clean Moss Off a Metal Roof?
No. An Interlock aluminum roof has no granules or wood fiber for moss spores to root into, and its smooth coated surface sheds water rather than holding it, so there is no recurring moss-treatment cycle. Leaves and needles from overhanging trees should still be cleared, and gutters still need attention.
Technically reviewed by Scott Plumptree, Director of Marketing, Interlock Metal Roofing · Updated 2026-08-15
At a glance
- There is no recurring moss-treatment cycle on a smooth coated aluminum roof, because there is no porous surface for spores to anchor into.
- Water sheds off the panel rather than sitting in granule texture, so the surface does not stay wet the way asphalt does.
- Debris from overhanging trees still lands on any roof in western Washington and should still be cleared off.
- Gutters and downspouts still need seasonal attention; that is property maintenance, not treatment of the roof covering.
- Interlock has completed 3,223 Washington installations since 1997, including 394 in Seattle since 1998 and 80 in Everett.
Why does moss not establish on a metal roof?
Moss needs two things from a surface: something to anchor its rhizoids into, and moisture that persists. An asphalt shingle supplies both, because the mineral granules create a textured, water-holding face. Cedar supplies both, because wood fiber is porous and absorbs water directly. An aluminum panel supplies neither. The Interlock panel 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. That coating is a continuous, hard, non-porous film. There are no pockets holding a film of water between rain events and nothing fibrous for an anchor to grip. Water behaves differently as a result. On a granulated shingle, rain has to work its way across a rough surface and some of it stays behind in the texture. On a coated panel it runs off, and the surface dries as soon as the rain stops rather than hours or days later.
So what maintenance does a metal roof in Washington actually need?
The honest answer is that the maintenance is of the property around the roof, not treatment of the roof surface itself. Under a Douglas fir or a western red cedar, needles will land on any roof regardless of material. They collect in valleys, behind chimneys, in the gutter line and along any low edge. They should be cleared. The difference is what that debris is doing while it sits there. On asphalt, wet needles hold moisture directly against a porous surface and give moss a starting bed, so the debris is actively feeding a problem. On a coated aluminum panel the debris is simply debris. Removing it restores the roof to its original condition rather than revealing damage underneath. Gutters and downspouts still need seasonal attention, particularly on wooded lots along the I-5 corridor where needle fall is continuous rather than seasonal. Blocked gutters back water up at the eave on any roof, and that is worth staying ahead of on any house in this climate.
Can algae, lichen or staining still appear on metal?
Surface deposits are not the same thing as rooted growth, and it is worth being precise about the difference. Airborne dust, pollen, pitch from overhanging conifers and general grime settle on every exterior surface in this region, and a north-facing panel in deep shade under constant needle fall can develop a dull film over time. What that film does not do is anchor into the panel or hold a saturated mat of vegetation against the surface. It rinses off with plain water, and in most cases the rain does it unaided. That is a cosmetic matter on a schedule the homeowner chooses, not a structural one on a schedule the roof dictates. No roof material is entirely immune from the environment around it, and we do not describe our roofs as immune. What changes is that the surface no longer supplies the two things moss needs, so the recurring treat-and-retreat cycle that porous coverings require in this climate does not apply.
What about the cleaning methods used on asphalt roofs?
Most moss remediation on asphalt is a compromise between removing growth and damaging the covering. Scraping strips granules. Pressure washing strips them faster and can force water up under the shingle courses. Zinc or copper sulphate treatments kill growth chemically but have to be repeated, and the runoff has to go somewhere. None of that applies to a coated aluminum panel, because there is nothing to remediate on the field of the roof. Clearing needles by hand or with a leaf blower from the ground, and keeping the gutters open, covers ordinary upkeep. Anything more aggressive is unnecessary and should be avoided on any finished metal surface. If a panel does need washing for appearance, plain water and a soft brush are sufficient. Abrasives, wire brushes and harsh solvents are not appropriate on an AAMA 2605 PVDF finish, which carries a 30-year coating warranty and is meant to be left intact.
How does this change what the roof costs you in time?
The relevant comparison is not one cleaning against another, it is a repeating obligation against an occasional one. A moss-prone asphalt or cedar roof in western Washington needs periodic treatment, and each treatment either removes protective material or has to be redone, because the surface conditions that produced the moss are unchanged afterward. With a metal roof the recurring item disappears and the occasional item remains: clear debris after heavy needle drop, keep the gutters flowing, and look at the flashings and penetrations after a significant windstorm as you would on any roof. That is a seasonal habit rather than a service contract. Interlock's system is listed under ICC-ES ESR-1790 for wind uplift to 177.5 psf and tested to UL 2218 Class 4 for impact. It is warranted lifetime to the original owner, transferable once, with the second owner covered to 50 years from substantial completion. Washington licence #INTERMR75406.
Common questions
Do I need to treat a metal roof for moss every year?
No. A smooth coated aluminum roof gives moss spores nothing to root into and sheds water instead of holding it, so there is no annual moss-treatment cycle. What remains is ordinary upkeep of the property: clearing needles and leaves that fall from overhanging trees, and keeping gutters and downspouts flowing.
Should I still clear debris off a metal roof?
Yes. Under conifers in western Washington, needles land on any roof and collect in valleys, behind chimneys and along low edges. Clearing them is straightforward on a smooth panel and is worth doing seasonally. On a metal roof that debris is not feeding growth, but it can still hold water at the eave.
Can you pressure wash a metal roof?
It is unnecessary and not advised. An AAMA 2605 PVDF coating carries a 30-year finish warranty and is designed to be left intact, so abrasives, wire brushes and harsh solvents should be avoided. If a panel needs washing for appearance, plain water and a soft brush are sufficient for the job.
Will a metal roof stay clean under trees?
It will not stay clear of debris, because needles and leaves land on every roof under a canopy. It does stay free of rooted moss mats, because there are no granules or wood fiber for spores to anchor into. The difference is that debris on a coated panel simply lifts off.
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