Moss and algae roof care
Annual · typically April
Why it matters
In a wet climate, moss isn't a cosmetic issue — it's a slow roof killer. It colonizes the shaded north slope first, roots under shingle edges, and holds moisture against the roof around the clock. Black streaks (algae) are uglier than they are harmful; green moss carpet is the one that costs money.
The spring timing matters: kill the moss as the growing season starts, and let summer dry out what's left.
What skipping it costs
$800–$12,000
Moss doesn't just sit on shingles — it roots under the edges and holds water against them like a wet sponge that never dries. The shingle edges lift and rot, and in freeze weather the ice pries them further. A $20 treatment each spring versus shingle replacement on the shaded slope — or a full reroof years early on a house that just needed the moss dealt with.
Two ways to get it done
Do it yourself
You'll need: Soft-bristle brush on a pole, Garden sprayer, Binoculars, Moss killer labeled for roofs (potassium-salt based, ~$20), Zinc or copper strip kit for the ridge (~$30–$60, optional).
- From the ground with binoculars, map where the growth is — usually the north slope and under tree overhangs. (Trimming those branches back is half the cure; sun is the best moss killer there is.)
- Apply a roof-labeled moss killer with a garden sprayer on a dry, calm day. The potassium-salt products are gentler on plants and metal than bleach mixes. Wet the foundation plantings first and rinse them after regardless.
- Wait. The moss browns and releases over weeks — let rain and gravity do the removal. If you must brush, use a soft bristle brush, stroking downslope only, from a ladder at the eave, never standing on the roof.
- For the long game, install zinc or copper strips just below the ridge. Every rain washes a trace of metal down the slope, and moss hates it — that's why you see clean stripes below chimney flashing.
- Never, under any circumstance, pressure wash a shingle roof. Yours or anyone else's.
Hire it out
$300–$800National fair range — local labor moves this number.
A professional soft-wash moss treatment runs $300–$800 depending on roof size and pitch — legitimate crews use low-pressure application, roof-safe chemistry, and protect the landscaping. Ask the pressure question first; it's the fastest honesty test in the business.
Pay once for the treatment, then make them quote the zinc or copper strip install while they're up there — it's the cheapest part of the visit and it's what keeps you from becoming an annual customer. A moss company that never mentions prevention is planning its return trip.
Ask the tech
- Soft wash or pressure wash? If the answer is pressure, we're done here.
- What product are you applying, and is it safe for the plants and gutters below?
- Will you install zinc or copper at the ridge so I'm not paying for this again in three years?
Red flags you're being upsold
- A pressure washer anywhere near a shingle roof. It strips the granules that ARE the roof — you're paying someone to sand years off it.
- 'Your roof is ruined, it needs replacement' from a cleaning crew. Moss grows on healthy roofs in wet climates; green does not mean gone.
- Bleach-heavy mixes applied with no plant protection or gutter rinse — dead foundation shrubs are the calling card.
Second opinion time: Any cleaning visit that turns into a replacement quote — get a roofer who does repairs to look, not the crew holding the wand.
The call
DIY the treatment — it's a sprayer and patience. Hire the cleaning if there's an established carpet, and fire anyone who unrolls a pressure washer.