Roof Cleaning in NJ: Soft Wash Removal of Black Streaks, Moss and Algae

Black or dark streaks are algae, green fuzzy clumps are moss, and flat gray-green or white crusty circles are lichen. Each one does different harm and needs a slightly different approach.

Algae. The streaks are mostly Gloeocapsa magma, which GAF and Owens Corning describe as a cyanobacteria often called blue-green algae. It travels on the air and settles on damp shingles. Algae is mainly an appearance problem. It does not lift shingles. The streaks make a roof look old, can make a home harder to sell, and spread to neighboring slopes. It responds quickly to a soft wash and usually looks clean the same day.

Moss. Moss is a small plant that grows in the shaded, damp gaps between shingles. It is the most damaging of the three. As This Old House explains, moss can lift and separate shingles as it grows, opening a path for water, and it holds moisture against the roof. In winter, wet moss can freeze and thaw repeatedly, which is hard on shingle edges. Moss treated with a soft wash dies and loosens; the remaining dead material either rinses off or releases over the following weeks of rain.

Lichen. Lichen is a partnership between a fungus and an alga. It grows slowly and grips the surface hard. Ripping it off pulls granules with it, so it should be killed in place and allowed to weather away. Lichen can leave faint ghost marks, especially on slate and older shingles.

Many New Jersey roofs have all three: algae streaks on the open slopes, moss in the shaded valley, and lichen on the oldest section near a tree. The inspection at the start of our cleaning tells you what is there and how heavy it is, which also drives the price.

Algae is mostly cosmetic, but moss and lichen cause real physical damage: lifted shingles, trapped moisture and granule loss that shortens roof life.

Manufacturers refer to algae as discoloration, and their bulletins focus on appearance. That matches what we see. Algae streaks on an otherwise sound roof rarely cause leaks by themselves. The practical costs are curb appeal, resale value and the fact that algae on a roof usually means conditions are right for moss too.

Moss is different. It roots into the keyways and under shingle edges. As it thickens, it lifts the tabs and can break the adhesive seal strip that holds them down in wind. Once tabs are unsealed, a strong nor’easter can fold them back or tear them off. Moss also acts like a sponge. It keeps the shingle surface wet for days, which speeds up the breakdown of the asphalt and can let water wick under the laps. In freeze and thaw cycles, common in Sussex, Warren, Morris, Passaic and Hunterdon counties, that trapped water freezes and expands at the shingle edges.

Lichen bonds to the granules. Over years it loosens them, and when it finally falls off it takes granules with it. Bare spots age quickly under the sun.

The National Park Service’s guidance on wood shingle roofs puts it plainly: moss and lichen trap moisture, rot the shingles and shorten the life of the roof. On cedar, the damage is faster than on asphalt.

Debris adds to all of it. Leaves in valleys hold water against the most water-loaded part of the roof, and clogged gutters keep the bottom courses damp. That is why we clear valleys and gutters on every cleaning, and why a roof that has been mossy for years should be inspected, not just washed.

Most New Jersey roofs need cleaning every three to five years, while heavily shaded roofs on wooded lots may need it every two to three years.

There is no single schedule because growth depends on shade, humidity, trees and the shingle itself. ARMA’s bulletin notes that cleaning is temporary and discoloration will likely return, and Owens Corning says the same. The goal is to clean before moss gets a foothold.

Here is a practical way to plan:

• Sunny lot, algae-resistant shingles. You may go many years without needing a cleaning. Watch the north slope.
• Typical suburban lot with some trees. Plan on every three to five years, more often if streaks reappear quickly.
• Wooded lot in North or Central NJ. Homes under heavy canopy in places like West Milford, Ringwood, Vernon, Sparta, Kinnelon and the Hunterdon hills often need attention every two to three years, plus yearly debris clearing.
• Shore homes. Humidity and fog keep algae active even in sun. Check every couple of years.

Between cleanings, a few habits stretch the interval. Keep gutters and valleys clear, ideally after the fall leaf drop and again in spring. Trim branches back so the roof gets sun and the canopy does not touch the shingles. Consider zinc strips near the ridge on shaded slopes.

Pair each cleaning with a roof inspection. The cleaning visit is the best time to see the roof clearly, and the photos give you a record of its condition over time. That record is useful when you sell, when you talk with your insurer and when you plan a replacement.

Late spring through early fall is the best time to clean a roof in New Jersey, on a dry, mild day with no rain expected for several hours.

Soft wash solutions work best when the roof is dry before application and temperatures are moderate. Very hot afternoons in July and August can dry the solution too fast on a dark roof, so crews often start early in the day. Cold weather slows the treatment, and freezing temperatures make a wet roof dangerous to work on.

Rain right after a treatment can dilute it before it finishes working, so we schedule around the forecast. A day or two of dry weather afterward is ideal, then normal rain helps rinse loosened moss in the following weeks.

Seasonal considerations for New Jersey:

• Spring (April to June). Good for cleaning and checking what winter did. Ice dams, freeze and thaw, and nor’easters can loosen shingles and flashing, and spring is the time to find it. Plants are budding, so careful protection matters.
• Summer (June to August). Reliable weather, but heat and humidity mean early starts. Summer is also when algae grows fastest.
• Early fall (September to October). A strong choice. Many plants are going dormant, temperatures are mild, and the roof will be clean and inspected before nor’easter season. Plan a gutter clearing after the leaves finish falling.
• Late fall and winter. Usually not a good time for chemical cleaning. Debris clearing and gutter cleaning can still be done on dry days.

If you are selling in spring, book the cleaning a few weeks before listing photos so moss has time to release and the roof looks its best.

You can clear light debris from a low, single-story roof with care, but chemical cleaning on a steep or two-story roof is a job for a crew with fall protection.

The manufacturers do publish homeowner recipes, which tells you the chemistry is not complicated. The risk is everything around it. A treated roof is slippery; ARMA, GAF and Owens Corning all warn about it, and Owens Corning advises fall protection. Falls from ladders and roofs are a common cause of serious home injuries, and a wet, steep New Jersey colonial roof is one of the worst places to work.

If you do it yourself on a low roof, follow the safety basics:

• Use the shingle maker’s recipe and nothing stronger. Never mix bleach with ammonia or acid cleaners.
• Work from a ladder at the eave where possible, and use a proper harness and anchor if you must go on the roof.
• Wear eye protection, gloves and soft-soled shoes.
• Pre-wet and cover plants, and rinse them afterward.
• Never use a pressure washer on shingles and never scrub.
• Brush moss gently from the top down so you never push debris under a shingle edge.

Do not do it yourself if the roof is steep, two stories or higher, slate, or near power lines, or if you are not comfortable on a ladder. Slate cracks under careless footsteps, and the National Park Service says slate should not be walked on if at all possible.

The other thing a DIY wash misses is the inspection. A roofer cleaning the roof sees the lifted tab, the cracked pipe boot and the worn valley. A homeowner with a sprayer usually does not. That is a large part of the value of having a roofing contractor do the cleaning.

Residential cleaning is mostly about algae and moss on steep shingle roofs, while commercial cleaning is mostly about debris, drains and membrane condition on flat roofs.

On homes, the work is steep-slope: soft washing asphalt shingles, slate or cedar, protecting landscaping, and clearing valleys and gutters. Appearance matters to the owner, and the cleaning visit doubles as a roof check.

On commercial and multi-family buildings across New Jersey, from office buildings in Parsippany to warehouses in Middlesex County and retail strips along Route 22, most roofs are low-slope EPDM, TPO, PVC or modified bitumen. The priorities shift:

• Drains and scuppers. Blocked drains cause ponding water, which stresses the membrane and adds weight to the structure. Clearing them is the most important part of the visit.
• Debris. Leaves, windblown trash and gravel collect around rooftop units and against parapets.
• Membrane-safe cleaning. Reflective white TPO and PVC roofs lose reflectivity when dirty. They are cleaned with a membrane-safe product and low pressure. Follow the membrane manufacturer’s care guide, because the wrong chemical can void a commercial warranty.
• Seams and penetrations. With the roof clean, seams, flashings at rooftop units, pitch pans and coping are easier to inspect.
• Documentation. Many commercial roof warranties require periodic maintenance. Dated photos and reports help keep coverage in place.

Commercial work also involves access planning, tenant notice and sometimes after-hours scheduling. For maintenance programs and repairs on larger buildings, see commercial roofing.

Choose a contractor who uses low pressure on shingles, can name the cleaning product, carries liability insurance, is registered with the NJ Division of Consumer Affairs, and puts the price in a written contract.

Ask these questions before anyone climbs a ladder:

• What pressure and method will you use on my shingles? The answer should be a soft wash at low pressure. Any mention of pressure washing or surface cleaners on asphalt shingles is a red flag, since GAF, Owens Corning and ARMA all advise against it.
• What product are you using, and how do you protect plants? A good contractor names the active ingredient and explains pre-wetting, covering and rinsing.
• Are you registered and insured? New Jersey home improvement contractors must register with the Division of Consumer Affairs and receive a registration number beginning with 13VH, which requires at least $500,000 in commercial general liability insurance. Look up the number at newjersey.mylicense.com.
• Will I get a written contract? Under the Contractors’ Registration Act, N.J.S.A. 56:8-151, home improvement contracts must be written and signed, show the contractor’s legal name, address and registration number, list the total price, include a copy of the insurance certificate, and give you a Notice to Consumer of your right to cancel before midnight of the third business day.
• Who checks the roof itself? A washing company can make a roof look clean. A roofing contractor can also tell you whether the shingles, flashing and deck are sound and fix small problems on the spot.
• What does any guarantee actually cover? If a company offers a no-streaks guarantee, ask for the written terms, the length and what voids it.

Be wary of door-knockers after storms offering cheap roof washes, prices that seem far below the national ranges above, and anyone who wants full payment up front. Ask for before photos taken from the roof, not just the street.