Black Streaks, Algae and Moss on Overland Park Roofs
Stand on the north side of almost any street in Overland Park and look at the roofs. Half of them have dark streaks running down from the ridge, heaviest under the trees and on the slope that never gets afternoon sun. Homeowners ask about black streaks on roof shingles constantly, usually in the fall when the leaves come off and the roof becomes visible again. Here's what that roof streaking is, whether it matters, and what to do about it.
What the black streaks are
Dark streaks on roof shingles here are not dirt, not mold, not a defect in the shingle. They're a cyanobacteria called Gloeocapsa magma — a blue-green algae that feeds on the limestone filler in asphalt shingles and grows a dark protective coating against UV. It arrives as airborne spores, takes hold wherever the roof stays damp longest, and spreads downhill with the runoff, which is why it streaks rather than blotches.
Overland Park has the humidity for it — summer dew points in the 70s, a wet spring — and plenty of shade from the mature silver maples, pin oaks and sycamores over the older neighborhoods. The classic picture is a 1980s two-story in Nall Hills or Cherokee Hills with a clean south slope and a streaked north one.
Does it hurt the roof?
Mostly algae discoloration hurts the look of the roof, and it lowers what an appraiser or a buyer thinks of it. The algae itself eats limestone filler very slowly, and the dark coating raises the shingle's surface temperature a little, which in theory shortens its life. Algae rarely kills a roof here. What kills roofs is hail, sun, freeze-thaw and age. A streaked 12-year-old roof is a 12-year-old roof.
Where it matters more is when the streaking is a symptom: a north slope that stays wet enough to grow algae heavily is also a slope that stays wet enough to rot fascia, soften decking at the eave, and hold ice longer in winter. If the streaks are thick and the gutter under them is always full, look at the trees and the gutter, not just the algae.
Moss is the different case
Moss is rare on Overland Park roofs — too much sun, too dry in summer — but it turns up on roofs under heavy tree cover, on the north side of steep-lot houses backing to the creek corridors, and on roofs that haven't been cleared of leaves in years. Moss is a problem. It holds water against the shingle like a sponge, its roots work under the shingle edge and lift it, and in winter that trapped water freezes and expands. A mossy patch on a 15-year-old roof is where the first leak will be.
If you have moss:
- Don't scrape it off dry. You'll take the granules with it.
- Kill it first, then let the rain and wind clear it over a season.
- Trim the tree that's shading it, or it will come back.
What not to do
Don't pressure-wash a shingle roof. Ever. A pressure washer strips the granules that are the shingle's UV protection and drives water under the shingle edges. Some roofs lose years of life to one afternoon with a rented washer. The same goes for the "roof cleaning" trucks that show up after a storm alongside the roofing trucks.
Don't use straight bleach at full strength. It works, but it also kills the boxwoods and hostas under the eave, corrodes the aluminum gutters and can discolor the shingle.
Don't walk a wet or streaked roof. Algae is slick.
What works
A soft wash. A solution of sodium hypochlorite (pool chlorine or household bleach) diluted with water, applied at low pressure with a garden sprayer, left to sit, and rinsed gently. Most roofers and roof-cleaning companies use roughly a one-to-one mix. Wet the plants below first and rinse them after. The streaks turn light within a day and wash off over a few weeks of rain. Expect $250–$600 for a professional soft wash on a typical Overland Park two-story, and expect it to last three to five years.
Zinc or copper strips. A strip of metal along the ridge releases ions in the rain that keep algae from establishing below it. Works well on a freshly cleaned roof, less well on one that's already colonized. $150–$400 installed. You'll see the effect as a clean band below any existing metal — look at the roof below a galvanized chimney cap or a copper flashing on a neighbor's house.
Algae-resistant shingles. Most architectural shingles sold now, and Class 4 shingles typically, come with copper-coated granules mixed in that keep the roof clean for the life of the warranty period on staining — usually 10 to 25 years. If you're replacing the roof anyway, this is the fix, and it's already in the price. The replacement page covers the rest of what's in a new roof.
Trees and gutters. Getting the canopy off the roof and the gutters flowing does more for the north slope than any chemical.
Keeping the north slope drier
The streaks follow moisture, so anything that helps the north slope dry faster helps. Trim branches that shade the roof, especially the ones that drip on it after rain. Keep the gutters clear so water leaves the eave instead of sitting there. Make sure the attic is ventilated, since a warm, damp attic keeps the deck moist from below. None of this removes streaks that are already there, but it slows the return after a cleaning.
Streaks and selling a house
Buyers notice a streaked roof from the curb, and some read it as an old roof even when it isn't. If you're listing soon, a soft wash a few weeks ahead is often worth it. If the roof is near the end anyway, cleaning it won't change what an inspector writes, so put the money toward the replacement instead.
Questions to ask a roof-cleaning company
- Do you use low pressure only, never a pressure washer?
- What solution do you use, and how strong is it?
- How do you protect the plants and rinse the gutters?
- Do you walk the roof, and how do you keep from scuffing the granules?
- Can I see a certificate of insurance?
Streaks on a new roof
If you're replacing the roof, ask for shingles with algae-resistant granules; most architectural lines include them. It also helps to fix the shade and drainage at the same time, since the new roof will face the same damp north slope as the old one. Keep the product paperwork, because the staining coverage is often a separate term from the shingle warranty.
How often to treat
A professional soft wash usually holds for three to five years here, longer on a slope with less shade. Zinc or copper strips stretch that interval. A good rhythm is to clean once, add a strip if you want, and then simply watch the north slope each fall. When the streaks start to show again from the street, it's time. There's no need to treat on a schedule if the roof still looks clean.
A quick guide
| What you see | What it is | What to do |
|---|---|---|
| Dark streaks from ridge down, north slope | Algae | Soft wash, zinc strip, or ignore it |
| Green, fuzzy, thick, lifts shingle edges | Moss | Kill it, trim the tree, check for damage |
| Uniform gray-black darkening, whole roof | Age and granule loss | Get it inspected |
| Streaks plus a soft fascia and full gutter | Drainage problem | Fix the gutter and tree first |
When to call a roofer
Not for the streaks by themselves. Call if the streaked slope has a soft spot, a sagging gutter or moss lifting the shingles, or if you're deciding whether a 15-year-old streaked roof has life left before you pay to clean it. A roof inspection answers that. Repair and replacement estimates from local roofers are usually free; a formal written inspection report typically runs $150–$300. Ask whether the fee is credited if you hire that roofer. Our post on signs a roof is near the end lists what to look for — streaking isn't on it.
Need roof work in Overland Park?
Call for a free estimate. Mon–Sat 7am–7pm.