Four Seasons Roofing and Construction · Homeowner guide
Black streaks on your roof: what they are and what to do
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Black or dark brown streaks running down an asphalt shingle roof are usually algae. The Asphalt Roofing Manufacturers Association says there is no scientific evidence that algae damages asphalt shingles, although it does change how the roof looks. Moss is a different matter because it can lift shingle edges. A pressure washer should not be used on either.
ARMA's technical bulletin on roof discoloration lists the Eastern Seaboard among the areas where algae staining is most widespread, which takes in North Jersey. Four Seasons Roofing and Construction repairs and replaces roofs across that part of the state, and what follows sticks to what the shingle industry itself publishes about the streaks.
What causes black streaks on a roof?
Airborne algae, in most cases. ARMA's bulletin names Gloeocapsa magma as probably the most prevalent of several species involved. The algae travel through the air, settle on roofs and other outdoor surfaces, and the natural pigments they produce can gradually turn a white or light colored roof dark brown or black. The stain can look uniform or come in streaks.
According to the same bulletin, algae grows to some degree in every region of the U.S., especially where conditions are warm and humid, and almost every type of roofing is susceptible. It shows most on light colored roofs and far less on darker shades.
Is it algae, moss or something else?
ARMA lists what algae gets mistaken for: fungus growth, soot, dirt, moss and tree droppings. The difference it points to is how far the marks spread. Moss and tree droppings usually leave localized spots, while algae covers a slope evenly or runs down it in streaks.
Moss deserves more attention. In ARMA's guide to algae and moss on asphalt roofs, moss can make the leading edges of shingles lift or curl, and that increases the risk of shingles blowing off in wind. The same guide says moss loosens over time and may be removed with a leaf blower, and a severe case can take more than one bleach treatment to kill.
Dark areas that are patchy rather than streaked can also be spots where shingles have worn and shed their granules. That's wear, and it usually comes with other signs like granules collecting at the bottom of the downspouts, which our fall roof checklist for New Jersey homeowners covers.
Should you clean algae off the roof?
For algae alone it's mostly a looks decision, given ARMA's finding that it does not damage the shingles. The bulletin also says cleaning only works for a while, and the discoloration may come back.
If you do want it cleaned, ARMA's method is a 50:50 mix of laundry strength liquid chlorine bleach and water, applied with a sprayer, left on the roof for 15 to 20 minutes, then rinsed off thoroughly with low pressure water. It calls for eye protection and rubber gloves and care not to damage the rest of the building or the landscaping around it, and it warns the roof will be slippery and potentially hazardous. ARMA recommends that only trained professionals do this kind of work.
No pressure washer. ARMA says it causes granule loss and very likely premature failure of the roof system, and the bulletin rules out brushes, brooms and scrubbing as well. Mixing directions can vary between shingle manufacturers, and ARMA says other cleaning chemicals or methods shouldn't be used without the shingle manufacturer's approval.
Can you keep the streaks from coming back?
Not much can be added to a roof that's already on. ARMA does not recommend adding zinc or copper strips to an existing roof, because the strips need either exposed nails, which can cause leaks over time, or a broken sealant bond to slide them under the shingles, which may lead to wind damage.
A reroof is when you get a real choice. ARMA says several types of algae resistant shingles are commercially available, designed to inhibit algae growth for extended periods. If streaking has been a problem on your house, ask whether the shingle in your roof replacement estimate is an algae resistant line, and read what that manufacturer's documents say about it.
Color matters too. Because the staining is most visible on light roofs, a darker shingle hides it better. Our roof replacement page puts the trade off in one line: "Dark roofs hide algae streaking longer but show patch repairs more." A lighter roof can still be the right choice for other reasons, so weigh both when you pick the color.
When should a roofer look at it?
Streaks by themselves are not urgent. Moss along the shingle edges, lifted or missing shingles, granules piling up at the downspouts, any water inside the house: those are different. Lifted or torn shingles are usually a roof repair, and a roof that is old and heavily stained may be worth pricing against a replacement.
If you cannot tell streaks from wear, book a roof inspection and ask for photos of what the inspector finds.