When Is the Best Time to Have Your Roof Washed in NH?

What those black streaks actually are

Before getting into timing, it's worth understanding what you're dealing with. The dark streaks you see running down asphalt shingle roofs aren't dirt, pollution, or water staining — they're a cyanobacteria called Gloeocapsa magma. It spreads via airborne spores, feeds on the limestone filler in asphalt shingles, and gradually works its way down the roof as it grows.

Left untreated, the bacteria doesn't just look bad. It eats away at the shingles over time, reducing their ability to reflect UV rays and accelerating the breakdown of the granule layer that protects your roof from weathering. An infected roof can lose years off its lifespan compared to one that's been maintained.

Spring vs. fall: which is better?

Both spring and fall are good times for a roof wash in NH — but for different reasons, and if you can only choose one, late spring (May–June) is generally the better option for most NH homeowners.

Here's why spring has the edge:

• The roof has just come through a winter of freeze-thaw cycles, which can cause existing algae to expand cracks in shingles. Treating early in the season stops further damage.
• Spring and early summer are when algae growth accelerates — catching it before it spreads prevents the streaks from darkening and spreading through summer.
• After treatment, the roof dries quickly in the longer spring days, and natural rainfall helps rinse any residual solution over the following weeks.
• You get the visual benefit for the entire outdoor season, not just a few months before winter covers everything in snow.

Fall is a solid second choice if spring doesn't work for you — particularly late September through October, before heavy leaf fall. A clean roof heading into winter means no algae going dormant and re-emerging aggressively in spring.

When NOT to have your roof washed

There are a few timing windows to avoid:

• During freezing temperatures. Cleaning solutions don't work effectively below 40°F, and water on a roof in freezing weather creates ice hazards. We don't wash roofs when frost is in the forecast.
• Immediately before heavy rain. Rain within 24–48 hours of treatment washes away the cleaning solution before it has adequate dwell time to kill the algae completely. A light rain a week later actually helps rinse away dead material.
• In peak summer heat. Temperatures above 85–90°F accelerate solution evaporation and reduce effectiveness. Early morning appointments or scheduling in shoulder seasons is better.

How long do results last?

A professional soft wash roof treatment typically keeps algae from returning for 2–4 years in NH conditions. The exact duration depends on:

• How much shade the roof receives (shaded roofs re-infect faster)
• The age of the shingles (older shingles with more limestone filler are more susceptible)
• Proximity to trees, which drop spores and debris that accelerate growth
• Whether any algae-resistant shingles or zinc strips were installed

Compare this to pressure washing, which physically removes the visible growth but doesn't kill the root organism — results often last only a season before the dark streaks return. Soft washing is more effective because it kills the bacteria, not just the surface appearance.

What about moss?

Moss is different from algae and more common in heavily shaded NH yards. It grows thicker, holds moisture against the shingles, and can physically lift shingle tabs over time. Moss treatment follows the same soft wash approach — cleaning solution applied, dwell time allowed, followed by a rinse. We recommend treating moss aggressively since it causes more direct structural damage than algae alone.

After treatment, zinc or copper strip installation along the ridge can inhibit moss regrowth — the metal slowly leaches down the roof in rain and creates an inhospitable environment for moss and algae. It's not a magic fix, but it can meaningfully extend the interval between cleanings on shaded roofs.

A note on pressure washing your roof

If you've gotten quotes from contractors who want to pressure wash your roof, be cautious. High-pressure water on asphalt shingles strips the protective granule layer — the same granules that block UV rays and protect the shingles from weathering. Granule loss accelerates aging and can void manufacturer warranties. Most roofing manufacturers explicitly recommend against pressure washing. Soft washing at low pressure (under 500 PSI) is the safe, effective method for asphalt shingles.

Ready to schedule?

If your roof has visible black streaks, dark patches, or green moss growth, it's worth addressing before summer. We serve the NH Seacoast, Southern Maine, and the Lakes Region. Request a free estimate or call us directly at (603) 416-1498.