Algae stains a shingle. Moss lifts it. One is a cosmetic problem with a deadline measured in years, and the other is a leak forming right now.
Both are green or dark, both show up on the shaded slope, and both get called roof algae by most people. Mechanically they have very little in common.
| Algae | Moss | |
|---|---|---|
| What it is | A single-celled organism, Gloeocapsa magma, feeding on limestone filler in the shingle | A plant, with a root-like structure that grips into the granule layer |
| What it looks like | Flat dark streaks running down from the ridge | Raised green or gray-green clumps, thickest at the bottom edge of a shingle course |
| What it does | Stains, holds moisture, slowly loosens granules | Physically lifts the shingle edge and holds standing water under the course |
| How urgent | Years. Cosmetic first, granule loss slowly | Months to a couple of seasons. This is a leak forming |
| Where it starts | The whole shaded slope, from the ridge down | The bottom edge of courses, in valleys, around penetrations, and above a poor gutter line |
The important line is the third one. Algae sits on the shingle. Moss gets under it. A roof sheds water because every course overlaps the one below, and moss growing at that overlap props it open and parks wet organic matter in the gap. Water then sits against the mat and wicks sideways, which is the one direction a shingle roof has no defense against.
This is the instinct and it is the single most expensive thing you can do to a shingle roof short of pressure washing it.
Moss anchors into the granule layer. Those granules are the roof: they take the UV and the rain, and the asphalt mat underneath them ages fast once they are gone. Pulling moss off mechanically, with a scraper, a stiff brush, or a pressure washer, removes the moss and a patch of granules with it. You have traded a green clump for a bald spot, and the bald spot is the one that shortens the life of the roof.
You also cannot tell how much you took. The granules wash into the gutter, which is why people scrape a roof, look at it, decide it worked, and find out four years later.
Brushing downward is marginally less damaging than brushing upward, because upward lifts the shingle edges you are trying to protect. Neither is a good idea, and the difference is not large enough to make scraping a method.
Beds, shrubs, and lawn under the roofline soaked with clean water first, and runoff controlled at the downspouts. Roof mixes are stronger than wall mixes and a downspout is a hose pointed at your landscaping.
A treatment goes on at roughly garden hose pressure. Nothing is scrubbed and nobody drags equipment across the shingles.
The treatment works where it sits. The color change on algae starts while we are standing there; moss takes longer because there is more of it and it holds water.
This is the part people are not told. Dead moss does not vanish on the day. It loses its grip and comes off over the following weeks of rain, and that gradual release is exactly why it does not take granules with it.
Trim the branches, clear the gutters, and if there are repeated wet spots look at why that slope is not drying.
So the honest expectation after a moss treatment is a roof that looks somewhat better immediately and substantially better over a month or two. A company promising a clear roof the same afternoon is either only treating algae or planning to scrape.
Moss needs more moisture than algae does, and for longer. Something on that roof is keeping it wet.
Sun is the cheapest treatment there is, so trimming back what shades the roof does more than any product. After that, keep the water moving: clear gutters, clear valleys, and nothing left sitting on the shingles.
Zinc or copper strips at the ridge are worth asking about. Runoff carries a small amount of metal down the slope below, which suppresses regrowth. It works, it works only below where it is fitted, and it is a slow effect rather than a treatment.
Expect three to five years between roof washes in the Triangle and Johnston County, sooner under heavy tree cover. If you have moss rather than streaking, treat it on the shorter end of that, because the damage is mechanical and it does not wait.
Most people have never actually seen their own roof. Our free drone inspection flies the roofline and shows you the footage on a screen, which is the fastest way to find out whether you have streaking, moss, or both, and whether the gutter line is part of it.
We wash roofs across Clayton, Apex, Cary, Wake Forest, and everywhere else on our list. If a roof is far enough gone that washing is not the answer, we will say so. Call (919) 698-3669.
“Did the roof and the gutters in one visit and showed me photos of everything I could not see from the ground.”
“The black streaks on our roof were embarrassing. Gone in one afternoon and the shingles were not touched.”
“Windows inside and out, screens included, and not one streak. They even wiped down all the sills.”
Everything on the outside of a building, what method each surface actually wants, how often it needs doing, and the handful nobody thinks about until something fails.
Read the guide →WindowsMost of what people call dirty glass is three different problems with three different fixes, and one of them permanently etches the pane if you leave it long enough.
Read the guide →GuttersA pressure washer empties a gutter fast, which is exactly why people try it. It also dents the trough, drives water behind the fascia, and packs the downspout tighter than it was.
Read the guide →We fly the roofline and show you the footage on the spot. You see your own gutters, joints, and downspouts on the screen instead of taking our word for it. It costs nothing and you are not committing to the cleaning.
Free for homes and businesses across the Triangle and Johnston County. No obligation to book any work afterwards.
Call (919) 698-3669 for a free quote. No obligation.
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