Moss and Algae on Your Roof: Causes, Removal, and Prevention

Moss and algae are common problems on Massachusetts and Rhode Island roofs, particularly on north-facing slopes and homes with heavy tree cover. Beyond being unsightly, these biological growths can actually damage your roofing material if left untreated.

The Difference Between Moss and Algae

**Algae (Gloeocapsa magma):** The black or dark gray streaking you see on many asphalt shingle roofs is most commonly a cyanobacteria (blue-green algae). Despite the name, it appears almost black because the bacteria produce a dark pigment as a UV shield. Algae doesn't physically damage shingles directly, but it's unsightly and can hold moisture against the surface.

**Moss:** Green, fuzzy plant growth - usually more prominent and thicker than algae. Moss is more damaging than algae because its root structures (rhizoids) physically penetrate the shingle surface and lift granules, undermining the shingle's waterproofing layer. Moss also holds significant moisture against the shingle surface.

**Lichen:** A combination of algae and fungus that appears as gray-green crusty patches. Lichen is the hardest to remove and most damaging - it physically bonds to the shingle and pulling it off removes granules.

Why New England Roofs Are Susceptible

Massachusetts and Rhode Island's climate creates ideal conditions for biological roof growth:

• •**High humidity:** Summer humidity levels in the 70-85% range support algae and moss development
• •**Moderate temperatures:** Neither too hot nor too cold for extended periods - the sweet spot for biological growth
• •**Shade:** Heavily treed properties with north-facing slopes stay moist longer after rain, giving moss a foothold
• •**Organic debris accumulation:** Leaves, pine needles, and other organic debris in valleys and at intersections hold moisture and provide nutrients

How Much Damage Can Moss and Algae Cause?

Algae

Primarily a cosmetic problem in early stages. However, algae-stained shingles hold more moisture than clean shingles, which can:

• •Slightly accelerate granule loss over time
• •Create conditions for moss to establish
• •Reduce curb appeal and potential resale value

Moss

More serious than algae:

• •Rhizoids lift and separate granules from the shingle surface
• •Creates persistent moisture retention that accelerates shingle deterioration
• •Can work under shingles at edges, lifting them and allowing water infiltration
• •Adds weight when saturated with water

Lichen

Most serious:

• •Chemical adhesion to the shingle surface causes granule removal when disturbed
• •Cannot be simply rinsed off - requires treatment and time to die before removal
• •Leaves pitted, damaged surface even after removal

Safe Removal Methods

What to Avoid

• •**Pressure washing:** High-pressure water strips granules from asphalt shingles - never use a pressure washer on an asphalt shingle roof
• •**Wire brushing or abrasive scraping:** Damages shingle surface
• •**Bleach applied undiluted:** Can damage shingles and surrounding vegetation

Low-Pressure Soft Washing

The industry-accepted method for roof cleaning is "soft washing" - low-pressure application of a cleaning solution, followed by a period of dwell time, then a gentle rinse.

**Typical solution:** Diluted sodium hypochlorite (bleach) at 1-3% concentration, often combined with a surfactant. This kills the biological growth without the mechanical damage of pressure washing.

**Process:**

1. Protect surrounding landscaping with a water rinse and tarps before treatment

2. Apply cleaning solution at low pressure from the ridge down

3. Allow dwell time (15-30 minutes) for the solution to kill biological growth

4. Rinse thoroughly with low pressure

5. Moss may not fall off immediately - dead moss loosens over several rainfalls

For Active Moss Growth

Apply zinc sulfate or potassium bicarbonate-based products specifically formulated for roof moss. These are less harsh than bleach and are effective for active moss removal.

Dead moss that doesn't fall off with rain can be gently removed with a soft brush - always brushing downward (toward the eave) never upward.

Prevention: Keeping Moss and Algae Off Your Roof

Algae-Resistant Shingles

Most major shingle manufacturers (GAF, CertainTeed, Owens Corning) offer "algae-resistant" or "AR" versions of their popular shingle lines. These shingles have copper granules mixed into the surface - copper is naturally toxic to algae. AR shingles typically carry a 10-15 year algae-resistance warranty.

When your roof is due for replacement, specifying algae-resistant shingles is the single most effective prevention measure.

Zinc or Copper Strips

Install a strip of zinc or copper flashing near the ridge. Rain carries trace amounts of these metals down the roof surface, inhibiting algae and moss growth. This can extend the time between cleaning treatments on existing roofs.

Trimming Overhanging Trees

Reducing shade on the roof surface helps it dry faster after rain, reducing the moisture window that supports biological growth. Trimming branches that directly overhang the roof also reduces debris accumulation and physical damage from branch contact.

Annual Cleaning

For properties in shaded, humid locations, plan for a professional roof cleaning every 3-5 years to remove growth before it causes significant damage.

Roofing Doctors provides professional roof cleaning and moss/algae treatment services across Massachusetts and Rhode Island.