Removing moss, algae, and lichens from the roof (asphalt shingles) and sidewalks

Like many in the Pacific Northwest, we had some moss on our roof and algae stains on our gutters. Also, the moss had embedded itself under a lot of the shingles and was pushing them up.

embedded moss

embedded moss 2

The algae and moss seep into the gutters, and if the gutters get clogged, the moss makes its way onto the deck and around the house perimeter.

The driveway and sidewalk around the house were very dark. This was a combination of years of dirt and dark algae growth due to continual wetness. The cracks between the driveway sections were filled with moss and dirt.

I read about various ways to handle this, and started first with a solution called “Wet & Forget” (benzalkonium chloride, aka BAC) that claims to remove algae and “crud” over a period of weeks or months following application. It did indeed work in some areas of the driveway, but other areas seemed to resist clearing.

So I began cleaning the driveway last year with a pressure washer. I’d read enough to know not to do this to our roof, since it would drastically reduce the life of the asphalt shingles. I cleaned the driveway (and completely clogged the driveway inset drain with the dirt and algae), and part of the sidewalk before stopping. That was last year.

This year, I began seeing the darkness of algae returning to the driveway, and I still had plenty of sidewalk and curb that were stained dark, not to mention the roof and rain gutters.

The benzalkonium chloride did clear some of the areas, but it starts to get a bit expensive given the amount of sidewalk and driveway I have. Pressure washing is faster, and I have the option to seal the concrete afterward. I bought 5 gallons of sealant for the driveway, but need to figure out how to do the washing without further clogging my driveway drain or flooding my garage.

I found a site dedicated to asphalt roofing and they had these two approaches:

1. A 50/50 water and bleach solution sprayed on with a hand sprayer. Let it set for 30 minutes and then wash off with a hose. This kills the algae and begins loosening the moss and algae. Bleach has the added benefit of removing stains.

2. A solution of benzalkonium chloride sprayed on with a hand sprayer. This one is just left on the roof, no washing should be done. It kills green algae and moss, and to some extent lichens, and then over the next 3-6 months of weather washes off the remains, leaving a nice clean and unstained roof.

Either method should be done when the temps are cool and there is little wind. Early morning is usually recommended.

There are pre-fab products containing benzalkonium chloride such as “Wet & Forget”, and some much higher concentrates in chemicals intended for clearing algae from pools. The latter is a far cheaper approach when mixing in a hand sprayer, though as always I’m not going to recommend you do this since it technically violates the labeling, even though it is the same chemical.

I started spraying the sidewalk and driveway in mid-August 2018. I sprayed a solution of benzalkonium chloride on my driveway, some rain gutters, and quite a bit of sidewalk that had dark algae staining. The weather has been hot and dry for quite some time, which is unusual but becoming more usual in the past few years.

The roof cleaner recommended a 25:1 ratio of water to 50% concentrate BAC. That is right around 10oz per 2 gallons water. I had been using 1oz or 1/10 the strength I should have been using.

9/8/18 I got up on the roof and sprayed the roof with the full strength mix and and ounce of liquid Moss Out.

10/14/18 No real sign of change in the roof or the sidewalk. Starting to think that bleach water is a better approach.

11/17/18 The moss actually died, but wasn’t obvious. Fresh green moss came out where I hadn’t sprayed. I sprayed some of it two days ago and it is clearly dying, so going to spray the rest with the mix.

05/02/19 It was clear that the spray and some later application of granules of Moss Out (zinc) had killed all the moss on the roof. The spray had also worked to clear the darkness from the sidewalk.

However, the embedded dead moss in the shingles of the roof still remained embedded. I went up on the roof with a knife and began the tedious task of cleaning out the moss and dirt with a knife. Since it was this way over nearly all of the roof (10+ years of accumulation), it took about 12 hours of careful balance work to clear it most of it out. Some of it I then swept off with a big broom, other I left for the wind and rain to clear. The roof looks a lot better now. Note: use an old screwdriver rather than a nice knife. The abrasive asphalt shingles turned my nice blade into a butter knife.
knife damage

To keep from getting near the edge on the 2nd story, I am using a 20ft extension pole with a piece of sturdy bent metal that hooks back toward me. I can sit near the peak and run the edge of the metal along the edge of the shingles to clear what I can see. I added a piece of scrap sheet metal to the pole to get at other edges that are more lateral to my location, but next to the edge of the roof. Falling from the 2nd story would be death or permanent damage, just not worth it.
moss tool

I still need to clean the outside of the gutters with some 409 and a bunch of towels.

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