Seasonal

Moss Season Starts Now: What October Rain Does to Gutters

By Portland Gutter Works · October 7, 2026 · 7 min read

The rain came back this week, the way it does every October — not a storm, just the switch flipping from dry to wet for the next seven months. Within a month there will be a green fuzz along the north edge of every roof in the city, and by February the gutters under it will have a mat of moss thick enough to hold a coffee cup. This is the Portland winter. It does not freeze much and it does not snow much. It grows moss.

Why gutters are where it starts

Moss needs three things: shade, constant moisture and something to root into. A Portland gutter in winter provides all three. The trough is wet from October to May. The north and east eaves get no direct sun for four months. And the sediment that collects in the bottom of a gutter — roof grit, needle dust, maple leaf sludge — is a perfect growing medium. Spores land in September, take hold by November, and the mat is established by the time the daffodils come up.

From the gutter, moss spreads up onto the bottom course of shingles, where it lifts the edges and holds water underneath, and down onto the fascia, where it keeps the paint film wet until it fails.

What it does to a gutter

It holds water. A moss mat is a sponge. A gutter that would drain in ten minutes stays wet for days, and the fascia behind it stays wet with it. This is how old-growth fir fascia on a 1920s house finally rots: not from a leak, but from 90 straight days of damp.

It blocks the outlet. Moss grows toward the light, and the outlet is a hole. A ring of moss around a 2-inch outlet closes it to nothing in a season.

It adds weight. Wet moss and the sediment under it can weigh 5 to 8 pounds per foot on top of the water. On 40-year-old spike hangers that is what starts the sag.

It traps needles. Fir needles that would have washed to the outlet catch in the mat. By spring the gutter is a strip of peat.

What it does to the roof edge

On composition shingles, moss along the bottom course wicks water up under the shingle and lifts the tabs. In a freeze it pries granules loose. On cedar shake it is worse — the shake stays wet, cups, and rots from the butt end. On Portland houses, a lot of roof-edge damage sits in the 18 inches above the gutter, and it usually started in the gutter.

Removing it safely

From the gutter: by hand, into a bucket. There is no shortcut. A leaf blower leaves the roots and the sediment. A pressure washer forces water behind the gutter and into the fascia, and on a roof it strips the granules or the shake surface and can void a roofing warranty. Moss removal from gutters in Portland is a hand job, which is why a proper cleaning takes as long as it does.

From the roof edge: a stiff brush on a dry day, working downward so you do not lift the shingles, followed by a treatment. Never brush wet shakes.

Treatments that work: zinc sulfate or potassium-salt moss killers applied to the bottom courses in fall or early spring, when the moss is active. They kill what is there and slow regrowth for a year or two. Zinc strips nailed along the ridge release a little zinc with every rain and keep the roof below them cleaner; they do nothing for the gutter itself. Skip bleach on shake and anything near a rain barrel or a garden bed.

What it costs

Slowing it down

You cannot stop moss in Portland. You can make the gutter a worse place for it:

• Fix the pitch so the trough drains instead of holding water. A dry gutter grows nothing.
• Enlarge the outlets so sediment washes through instead of settling.
• Trim limbs that shade the north eave. An hour more sun a day makes a visible difference.
• Clean in late fall, after the maples, so the mat never gets its winter start.
• Consider micro-mesh guards under fir. They do not stop moss on their own, but they keep the sediment out, and moss without sediment is a film that rinses off.

The timing

If your gutters have not been cleaned since spring, the next four weeks are the window. Get the summer's needles and grit out before the spores have something to root in, and the gutters go into the wet season draining. Wait until January and you are cleaning out established moss with a scraper. The cleaning page has what a visit includes, and the fall leaf calendar tells you whether to wait for the maples first.

A monthly look through the wet season

Once the rain returns, a quick look each month tells you how the season is going:

• November: Check that the downspouts run freely after the leaf drop. Any that stay quiet in a steady rain need attention.
• December: Look along the north eaves from the ground. A green fuzz on the bottom course of shingles is the first sign.
• January: After any east-wind cold snap, look for split downspout seams and ice damage.
• February: This is when growth is most visible. Note which eaves look worst so the spring cleaning starts there.
• March to April: Book the spring visit for after the blossoms and the spring needle drop.

Binoculars from the yard are enough for most of this.

Doing the roof edge yourself, safely

Roof-edge work is only for single-story eaves you can reach from a ladder without stepping onto the roof.

1. Wait for a dry, mild day. Wet growth on shingles is slick.
2. Brush downward with a soft or medium brush, with the shingle overlap, never against it.
3. Work in short sections and stop if granules start coming off in quantity.
4. Apply any treatment exactly as the label says, and keep it off plants, ponds and rain barrels.
5. Leave anything above the first story to someone with the right equipment.

Mistakes that make it worse

Scraping shingles. A metal scraper takes the granules with the growth.

Pressure-washing. It is fast and it damages roofs.

Bleach on cedar. It can discolor and weaken the wood.

Treating in a dry spell. Many treatments work best when the growth is active, in fall or early spring.

Ignoring the shade. If nothing changes about the shade and the standing water, the green comes back on schedule.

Questions to ask a cleaner

• Do you remove growth from the trough and outlets by hand?
• Do you brush the roof edge, and how?
• What treatment product do you use, if any, and where do you apply it?
• How do you protect plants and rain barrels below?
• Will you photograph any soft fascia or lifted shingles you find?

Clear answers to those tell you the cleaner understands a wet-climate roof. Vague ones are a reason to keep calling.

The long view

Shade, damp and standing water are what Portland roofs fight every winter. Fixing the pitch, opening up the outlets and trimming back limbs makes each year easier than the last. After two or three seasons of that, the spring visit often turns from a scraping job into a quick clean-out.

Keep a note of what was found at each visit and which eaves were worst. Over a few years the notes show whether the changes are working, and where to spend next.