Most homeowners who call about roof moss start the same way: “it looks bad, but does it actually matter?” It’s a fair question, because a green roof is the kind of thing a neighbour notices before it’s actually a problem — so it’s worth separating what’s cosmetic from what isn’t.
What moss actually does to the surface underneath
Moss holds water. That’s the mechanism, and everything else follows from it.
On asphalt shingle, the granules on the surface are what shield the asphalt layer underneath from UV and weather. Moss growing under and around them holds moisture against that layer continuously rather than letting it dry between rainfalls, and the roots work into the granule surface as the moss establishes itself. Over time that accelerates granule loss — exactly the layer the shingle depends on to last its rated lifespan.
On cedar shake, the effect is more direct: constant moisture against wood is what causes rot, and moss is a moisture-holding mat sitting directly on the wood. A cedar roof with established moss growth is aging faster than an identical one without it, not just looking different.
Tile is the most forgiving of the common materials, but moss and the organic debris that collects with it can still work into the gaps between tiles and hold water there, which affects the underlayment moss itself was never going to touch directly.
Where “cosmetic” is actually a fair description
Light surface algae — the grey-green staining that shows up on a shaded slope without the raised, cushion-like texture of established moss — is mostly a cosmetic issue. It isn’t holding water the way thick moss does, and it’s reasonable to leave it if it isn’t bothering you.
The distinction is texture, not just colour. If you can see a raised, carpet-like growth rather than a flat stain, water is being held against the surface and the “it’s just cosmetic” argument doesn’t hold up anymore.
The part that isn’t about the roof at all
Established moss also does a second kind of damage that has nothing to do with granule loss: it lifts shingle edges as it grows, which creates a path for water to get underneath during a heavy rain rather than running off the surface. That’s a leak risk forming gradually, not an appearance problem.
So when does it actually need treating
As a rough guide: thin, flat discolouration on a shaded slope can usually wait. Raised, cushion-textured growth — especially anywhere it’s thick enough to hold water after a dry day — is doing one of the two things above and is worth treating rather than watching.
If you’re not sure which one you’re looking at, send us a photo of the worst slope. Roof cleaning and moss removal covers the assessment as part of the visit — we tell you what the roof needs and what it doesn’t, rather than quoting a treatment it doesn’t need. Our guide to roof washing covers how the method changes with the material, and our note on winter treatment timing if the moss you’re looking at is at its most visible right now.
Every roof treatment carries our moss-free guarantee, with the term confirmed in your written quote — if moss returns to the treated area within that period, we treat it again at no charge.