How to remove moss from a roof?

Is this moss or algae?

They look different and they matter differently.

Algae shows as dark streaks running down the slope, usually on the north or shaded side. It is essentially cosmetic. It does not shorten the roof’s life meaningfully, and the main reason to know that is so nobody sells you a replacement for it.

Moss is green, three-dimensional, and grows in clumps you could get a fingernail under. It holds water like a sponge, and that is the whole problem. A shingle kept permanently damp deteriorates faster, and as the clump thickens it physically lifts the shingle above it, opening the course to wind and wind-driven rain.

Lichen is a third thing — flat, crusty, grey-green, and bonded hard to the granules. Removing it takes granules with it, which is why it is usually left alone.

Why can’t I just pressure wash it?

Because pressure washing an asphalt roof destroys it.

The granules are the shingle’s ultraviolet protection and they are held in a thin asphalt bond. A pressure washer strips them off in sheets. You get a roof that looks clean for a season and has lost years of service life, and the granules in your gutters are the evidence.

Directing water upslope is the second problem. Shingles are lapped to shed water downhill; a pressure stream aimed under the courses drives water into the assembly, past the underlayment laps and onto the deck.

If a contractor proposes pressure washing your asphalt roof, that alone is enough to decline. On tile and slate a low-pressure rinse can be appropriate — but the pressure and the angle are the whole question, and tile also breaks underfoot.

What does a professional actually do?

A soft wash: a low-pressure application of a cleaning solution, dwell time, then a gentle rinse — or in some cases no rinse at all, letting weather carry the dead moss off over following months.

Solutions are typically sodium hypochlorite or a proprietary roof cleaner, sometimes with a surfactant. The important details are dilution, dwell time, and protecting what is below: the run-off will kill plantings, and it goes somewhere. A competent crew pre-wets and rinses landscaping and controls where the water lands. Follow the product label; roof biocides are regulated and the label is the legal instruction.

Scrubbing, scraping and stiff brushes all take granules with the moss. Gentle chemistry and patience do not.

Why is this pro-only when it looks like cleaning?

Because of the surface, not the task.

A moss-covered roof is one of the most slippery things a person can stand on, and the work is done while wetting it further. Add the ordinary factors — slope, height, and an old covering that is by definition in poor condition if it has grown moss — and it is a bad combination.

There is also an assessment element. A roof with established moss is often near the end of its life anyway, and someone who can read the covering will tell you whether cleaning it is worth doing or whether you are spending money to postpone a replacement by a year.

What does it cost?

Soft washing a typical single-family roof commonly runs a few hundred to around a thousand dollars, driven mostly by roof area, pitch and access rather than by the moss itself.

Compare that against the roof’s remaining life before you commit. On a covering with five years left, cleaning is usually not worth it. On a ten-year-old roof in a shaded, damp location, it is straightforward maintenance.

How do I stop it coming back?

Address the conditions, not just the growth.

Light and airflow. Moss grows where the roof stays damp. Overhanging branches keep a slope shaded and drop debris that holds moisture; cutting them back changes the microclimate more than any treatment.

Zinc or copper strips at the ridge. Rain carries trace metal ions down the slope, which suppresses regrowth. It works reasonably well on the area directly below the strip and less so further down.

Keep gutters and valleys clear. Trapped debris holds water and seeds the next colony.

Check the ventilation. A roof deck kept cool and dry from beneath dries faster on top.

Common mistakes

• Pressure washing. It strips granules, drives water under the courses, and takes years off the roof. The single most damaging thing done to asphalt roofs in the name of maintenance.
• Treating algae streaks as urgent. They are cosmetic. Spending on them is optional; spending on moss is not.
• Scrubbing or scraping. Granules come off with the moss.
• Ignoring the run-off. It kills plantings and it goes into the storm drain. Read the product label.
• Cleaning a roof that needs replacing. Established moss frequently means an old, damp, shaded covering. Get the remaining life assessed first.