Moss on your roof: leave the pressure washer in the garage

Moss on a roof can look almost harmless, a soft green velvet creeping up from the lower edge. If you live in Montclair, Livingston or Cedar Grove, where big oaks lean over a lot of rooflines, you've probably noticed it on the shadiest slope and told yourself you'd deal with it eventually. It's worth dealing with sooner than that, and it's worth dealing with carefully, because the obvious method does more harm than the moss does.

Why moss settles where it does

Moss wants the same things algae wants: shade, moisture and something to hold onto. A roof slope that faces north, or sits under a canopy of mature trees, may stay damp long after the rest of the neighborhood has dried out. Overhanging oaks add the rest. They drop leaves, acorn caps, twigs and pollen, and all of it collects in roof valleys and along the lower courses of shingles. That debris acts like a sponge. It holds water against the roof, breaks down into a thin layer of soil, and gives moss a seedbed.

New Jersey winters help it along. Water that sits in the gaps between shingle edges freezes, thaws and freezes again, and the moss that's already there tucks itself into every little ridge. The black streaks you see on sunnier roofs are a cousin of the same problem, algae rather than moss, and they spread for the same reason: a surface that stays slightly wet and slightly dirty.

Why blasting it off is the wrong call

Asphalt shingles are covered in tiny mineral granules. Those granules are the roof's protection against sun and weather. A high-pressure jet strips them off, and moss that gets torn away by force often takes granules along with it, leaving bare patches that age faster than the rest of the roof. Forceful water also travels upward under shingle tabs, since shingles are laid to shed water downhill and not to resist a nozzle pointed up the slope. You can end up with water in the attic and a roof that looks clean for one season and wears out early.

There's also the plain safety problem. A roof that's covered in moss is slick, and a wet one is worse. This is not a job for a homeowner with a rented machine and a borrowed ladder. Cleaning the roof is one of those tasks where the person doing it needs to know what the surface can take.

The low-pressure roof shampoo approach

Modern Powerwash's roof washing page lists mold and algae removal, stain treatment, black streak roof treatment and low-pressure roof cleaning, and it invites visitors to "ask us about our specialized roof shampoo method for thorough and safe roof cleaning." The company also lists shingle preservation as one of the aims. In general terms, a shampoo approach means applying a cleaner at low pressure, giving it time to work on the growth, and rinsing gently so nothing gets scraped or lifted. The roof gets washed, not sanded.

If you want to know exactly how the crew would handle your roof, that's a conversation to have when you get your quote. Tom outlines the job before the work starts, according to Greg H on the reviews page, and for something as particular as a roof you'll want that conversation.

What to do about the oaks

Cleaning a roof without addressing what grew the moss is a temporary fix. A few things help:

• Have branches trimmed back so they aren't resting on or hanging over the shingles.
• Clear leaf litter out of valleys and gutters. Debris that holds water is what gives moss its start.
• Let the gutters drain freely so water isn't backing up along the roof edge.
• Letting a few more hours of sun reach the slope helps more than any product you can buy.

Gutters matter here too. Modern Powerwash lists gutter cleaning and gutter brightening alongside roof work, and it makes sense to treat the roof and the gutters as one job, since debris in one feeds the other.

If you'd like to see what the shaded slope of your roof looks like to someone who does this for a living, the roof washing page has the details, and quotes are free. Call (973) 777-6336, or use the form on the site and you'll hear back by the next business day.