Maintenance

Roof Moss Removal and Black Streaks on Madison Roofs

By Madison Roof Repairs · October 10, 2026 · 7 min read

Drive down any shaded street in the Vilas or Dudgeon-Monroe neighborhoods in October and you'll see it: black streaks running down the north-facing slopes, and on the houses under the biggest oaks, a green fuzz along the shingle edges. Lake humidity, tree cover and a long wet spring make Madison a good place to grow things on a roof. Roof moss removal is the part that matters most, and it is also the part people most often get wrong. Here's what each growth is, which one actually hurts the roof, and how to deal with it without wrecking the shingles.

The black streaks are algae

The dark streaks are a cyanobacteria called Gloeocapsa magma. It feeds on the limestone filler in asphalt shingles, spreads downhill with rain, and builds a dark protective coating that shows as streaks. It's on the north slope because that's the slope that stays damp longest.

It looks bad. It does not do much harm. Over many years it eats a little of the shingle surface and holds a little moisture, and on a dark roof it makes no real difference to attic temperature. If the roof is otherwise sound, algae is a cosmetic problem you can clean or ignore.

North-slope moss is the real problem

Moss is different, and in a freeze-thaw climate it's a genuine problem. North-slope moss grows on the shaded shingles, along the horizontal edges where a little debris collects, and in the valleys under the oaks. Once established, it holds water like a sponge, and in Madison that water freezes and thaws again and again through the winter. Each freeze pushes the shingle edge up a fraction; each thaw lets water a little further under it. After a few winters the shingles on a mossy slope are lifted, the seal strips are broken, and granules come off in sheets where the moss roots were. A mossy north slope ages noticeably faster than the clean south slope on the same house.

Moss also holds leaves and seeds in place, which holds more water, which grows more moss.

Lichen

Grey-green crusty patches, usually on older roofs. Lichen bonds to the shingle and pulls granules off when it's torn away. Treat it like moss, gently, and don't scrape.

What causes it

Shade and moisture. The silver maples and bur oaks on the older streets, a north slope, a roof pitch low enough that water lingers, and gutters that overflow and keep the eave wet. Humidity off Lake Mendota and Lake Monona keeps the near-east and near-west neighborhoods damper than the far west side. A roof in a Sun Prairie subdivision with no trees over it rarely grows anything.

What not to do

Don't pressure-wash. A pressure washer strips the granules that protect the asphalt from sun, drives water under the shingles, and can take years off the roof in an afternoon. Roofers here regularly see roofs that a "roof cleaning" crew pressure-washed the summer before, with the granules gone in stripes.

Don't scrape or wire-brush moss off. Same result, slower.

Don't use bleach at full strength. It kills the algae and also the shrubs below, corrodes gutter fasteners, and streaks aluminum.

Don't walk a mossy roof. It's slick when wet, and the shingles under it are already loose.

Roof moss removal that works

Kill it, then let it go. A moss-killing treatment — potassium salts of fatty acids, or a zinc- or copper-based product — applied in late spring or summer when the moss is actively growing. It dies over a few weeks, dries, and rain and wind carry most of it off over the season. A leaf blower from a ladder in the fall takes much of the rest. What's left deep in the seams can stay; pulling it out pulls granules with it.

For algae streaks: a soft wash. A solution of sodium hypochlorite diluted well below household bleach strength, with a surfactant so it clings, applied at low pressure, left to work, then rinsed gently. Wet the plants below first and rinse them after. The streaks lighten over a few weeks of rain. Cost from a company that does it properly: $300–$700 for a typical house.

Keeping it off: zinc strips and algae-resistant shingles

Zinc strips (or copper) installed just below the ridge are the standard prevention. Every rain washes a trace of metal down the slope, and growth below the strip slows or stops. Installing them on an existing roof runs $150–$400 and they work for ten years or so. They do less on a very wide slope, where the metal thins out before it reaches the eave.

Algae-resistant shingles, which have copper granules mixed into the surface, do the same job from day one. If you're replacing a roof on a shaded lot, they're worth the small premium; ask the roofer to name the product line on the quote.

Trim the trees. More sun and air on the north slope does more than any treatment.

Keep the gutters clear. An overflowing gutter keeps the eave wet all season, which is exactly where moss starts.

When moss means the roof is done

A light fuzz on a 10-year-old roof is maintenance. Heavy moss on a 20-year-old roof, with lifted edges and bare patches showing once it dies off, is a roof that's finished on that slope. Treating it will reveal the damage, not fix it. In that case, skip the treatment and get replacement pricing — the signs a roof is near the end post covers how to tell. Moss that's been holding water against the eave for years often means the decking there is soft, which a roof inspection will find from the attic.

Timing

Treatments need warm weather. May through September is the window; an application in October mostly washes off before it does anything. If you're reading this in the fall, put it on the spring list, clear the valleys and gutters now, and cut back any limb touching the roof. If moss is lifting shingles at the eave and you're worried about the winter, that's a roof repair question, and usually a cheap one if it's caught now.

Questions to ask a company that treats roofs

• What product will you use, and is it safe for the plants and gutters below?
• Will you use low pressure or a pressure washer? (The answer should be low pressure.)
• Will anyone walk on the roof, and how will they avoid loose shingles?
• What should I expect to see, and how long before the moss dies off?
• Do you install zinc or copper strips, and what does that add?

A simple yearly routine

Spring: look at the north slope and the valleys for new growth. Summer: treat any moss while it's growing. Fall: clear the gutters and valleys, and trim limbs that shade or touch the roof. Winter: leave the roof alone; just note where the snow sits longest, since those are the spots that stay damp.

Common questions

Does moss damage a roof?

Yes, over time. Moss holds water against the shingles, and in a Madison winter that water freezes and thaws under the shingle edges, lifting them and breaking the seal strips. A thin patch on a young roof is a maintenance job; thick moss on an older roof usually means that slope is near the end.

Do zinc strips work on a Madison roof?

They help, especially as prevention after the moss has been killed. They work best on shorter slopes, because the metal washing down from the ridge thins out on a long run. They won't remove established moss on their own, so treat first, then install the strips.

Will black streaks shorten my roof's life?

Not by much. The streaks are algae, mostly a cosmetic problem, and a soft wash will lighten them. If you're replacing the roof anyway, algae-resistant shingles keep them from coming back.