What Causes Black Streaks, Algae and Mold on North Carolina Roofs?

The black streaks on most North Carolina roofs are algae. The Asphalt Roofing Manufacturers Association explains that algae spores are carried through the air, settle on roofs and grow, and that natural pigments produced by the algae turn light-colored roofs dark brown or black over time. It names Gloeocapsa magma as probably the most prevalent of the species involved.

The streaks usually start on the north-facing and shaded parts of a roof, where the surface stays damp longest after dew or rain, and run downward as water carries the growth. They are most noticeable on lighter gray, tan and white shingles, and on roof planes that sit in the shade of nearby trees.

ARMA says discoloration is most widespread in the Gulf States and along the Northwest and Eastern Seaboards, and it occurs to some degree across the country, especially in warm, humid conditions. That describes North Carolina well: humid summers, frequent dew and long stretches of warm, damp nights. If your roof is streaked, you are in large company across Raleigh, Charlotte, Greensboro and the coast.

Algae on its own is mainly a cosmetic problem. ARMA's guidance focuses on the discoloration and how hard it is to remove; it does not describe the algae itself as damaging the shingles. Streaks can still matter if you are selling the house or your HOA expects roofs to look maintained.

Moss and lichen are a different story. Moss grows in thick, spongy mats on shaded, debris-covered roofs, holds moisture against the roof surface and can lift shingle edges as it grows. That trapped moisture speeds up wear and can let water work under the shingles. Lichen, the flat, crusty gray-green growth, clings tightly and can be hard to remove without damaging the surface.

If you see moss, deal with the conditions that feed it: shade, debris and poor drainage. Trim overhanging limbs, keep valleys and gutters clear and let sun and air reach the roof. See pine needles and debris on metal roofs for safe ways to keep a roof clear, and signs you need a new roof if growth has been there for years.

Clean algae off a roof gently, or not at all. ARMA recommends lightening the stains with a diluted household bleach solution, leaving it on for about 15 to 20 minutes and then rinsing gently with water. It warns not to scrub, because friction loosens the granules that protect asphalt shingles, and it states plainly: do not use a power washer or any type of brush or broom to clean algae from the roof surface.

Some cautions apply:

• Follow the shingle maker's directions. ARMA notes that manufacturers' recommended mixtures vary.
• Protect plants and siding. Bleach runoff can harm landscaping; wet plants first and rinse them afterward.
• Expect it to return. ARMA says the cleaning is temporary and the discoloration may come back.
• Hire a professional. ARMA recommends professionals, because the treatment makes the roof slippery.

Pressure washing is the most common mistake. It can strip granules from asphalt shingles, force water under shingles and trim, and damage painted metal finishes. If a roof cleaning company offers to pressure wash your roof, look for someone else.

You prevent most algae growth by choosing the right roofing and keeping the roof dry, clear and sunlit. On asphalt, algae-resistant shingles are designed to inhibit growth for an extended period; ARMA describes them as designed to inhibit algae for extended periods of time, not forever. Ask what algae resistance a shingle has and what its warranty says about discoloration.

On metal, a smooth painted surface gives algae far less to hold onto than a granule-covered shingle, and it dries faster after dew and rain. Painted aluminum roofs do not grow the streaks common on asphalt, though a metal roof under heavy shade can still collect pollen, sap and some organic film that a gentle rinse removes.

Color plays a part in what you see. The U.S. Department of Energy notes that light colors can make mold or algae stains more visible in warm, humid areas. If your lot is shaded, a mid-tone color shows less staining than the lightest shades; see metal roof colors for NC heat.

Whatever the roof, keep trees trimmed back and gutters and valleys clear so the surface dries quickly.

Mold in the attic means moisture is getting into the attic and staying there. The U.S. Environmental Protection Agency puts it simply: the key to mold control is moisture control. EPA notes that roof leaks or insufficient insulation can cause hidden mold, and that water-damaged areas should be dried within 24 to 48 hours to prevent mold growth.

In North Carolina homes, attic moisture usually comes from one of these:

• Roof leaks at flashing, vents, skylights or damaged roofing.
• Warm, humid indoor air leaking into the attic through gaps around lights, ducts, hatches and plumbing.
• Bathroom or kitchen fans venting into the attic instead of outdoors.
• Leaky ductwork in the attic, which can create condensation on cold duct surfaces in summer.
• Poor ventilation, which lets moist air sit against the roof deck.

Dark staining on the underside of the roof deck, damp or rusty nail tips, and a musty smell are the usual signs. Find and fix the moisture source first; cleaning the mold without fixing the cause only buys time. See attic ventilation in NC heat and humidity and roof leaks after a thunderstorm.

Algae streaks alone do not mean a roof needs replacing. If the shingles are otherwise flat, intact and holding their granules, cleaning or living with the streaks is reasonable.

If you are selling, expect buyers and their home inspector to notice heavy streaking and moss, and to ask about the roof's age and condition. A gentle professional cleaning before listing, along with records of the roof's age and any repairs, usually answers those questions better than a rushed replacement.

Replacement becomes worth discussing when growth comes with other problems: shingles curling, cracking or losing granules; moss lifting shingle edges; soft or spongy decking when walked on; repeated leaks; or attic staining and rot that point to long-term moisture. At that point the growth is a symptom of an aging roof, not the main issue.

If you do replace the roof, treat it as a chance to fix the conditions that fed the growth. Replace damaged decking, correct ventilation, make sure bathroom fans vent outdoors and trim back the trees. Our guides to when to replace a roof and how long a roof lasts in NC cover the decision in more detail.

A metal roof is a good choice where algae has been a problem on asphalt, because painted metal does not develop the dark streaks that asphalt does and sheds water and debris readily. It does not change attic moisture, though, which depends on ventilation, air sealing and leaks.

Interlock aluminum roofing uses the Alunar 70% PVDF finish, built to the AAMA 2605 standard and warranted for 30 years, on panels that interlock on four sides over concealed fasteners. Material and finish are covered by the Guardian Lifetime Limited Material Warranty from Interlock Roofing for the original purchaser while they own the home, transferable once, with the second owner covered to 50 years from substantial completion.

Metal Roof NC, an authorized Interlock aluminum roofing dealer, can check attic ventilation and decking as part of a project-specific estimate. See aluminum roofing, the warranties page and our service areas.