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Moss Removal, Treatment, Air-blowing, and More

Due to a long overcast and rainy season here in the northwest, combined with all our beautiful trees, our roofs can take a beating over time. Roof cleaning and care are essential. Moss, lichen, leaves, needles, and other debris not only serve to make a home look run-down, but they also trap moisture onto the shingles. This speeds the aging process, causing water damage and decay. Regular roof maintenance can delay the cost of an expensive re-shingling by years.

Moss has it’s way with roofs in the Northwest.

Depending on the situation, we use leaf-blowers, brushes, or pressure-washers to clean most types of roofs. This is often done in combination with a cleaning agent and/or moss prevention treatment.

We clean most roof types: asphalt composition, torch down, PVC membrane, standing-seam metal roofs, cedar shake, and certain tile and slate roofs. Most of the sturdier roof type–such as asphalt comp, torch down, PVC, metal, and thick cedar shake–can be walked on in the cleaning process (though steep pitch or bad condition might preclude our efforts). Fragile roof types like slate, certain light-weight tiles, faux-tile aluminum shingles, and thin cedar shake should not be walked on and cleaning will need to be performed from the roof’s edge.

Asphalt composition roofs are the most common and we have four methods for cleaning them. These apply to torch down roofs as well. Here they are:

Option A:  Routine Care

  • Air-blow loose debris from roof
  • Apply moss treatment
  • Clean out gutters and downspouts
  • Clean up debris from ground

While this option will not get rid of old dead moss, it is the easiest on your roof and budget. It is also the norm for routine roof cleaning. We air-blow the roof of any loose debris and apply the treatment (usually zinc sulfate monohydrate, powdered or sprayed, depending on roof type and weather) before cleaning out the gutters and our mess from the ground. If your roof has very thin moss or is just beginning to green, this is the way to go. Here in the northwest, we generally recommend doing this annually, or every other year, depending on the amount of sun your roof gets. Please note again: If you want us to remove any moss, this is not the option for you.

A yearly Option A, keeps the moss away.

Option B: Roof Brush

  • Brush remove moss from roof
  • Air-blow loose debris
  • Apply moss treatment
  • Clean out gutters and downspouts
  • Clean up debris from ground

Roof cleaning by brush can be a good way to go in certain circumstances. We brush off the moss, air-blow the roof, lay down the zinc treatment (see description in Option A) to kill off any moss left behind, and clean up our mess. Though this won’t leave the roof quite as clean as Option C, it is often a very close second, requires no water, and can be less expensive.

Option B in progress

Option C: Roof Wash

  • Air-blow loose debris
  • Clean out gutters and downspouts
  • Apply cleaning agent
  • Wash moss from roof
  • Re-clean gutters and downspouts
  • Pick up debris from ground

For roof cleaning that gets the roof looking as new as possible, a roof wash may be the best way to go. We begin by blowing off the roof and cleaning out the gutters if needed. Then we spray the whole roof with an outdoor cleaning agent to loosen the moss and wash the whole roof with a low-pressure, high-volume pressure-washer. After clearing out the gutters and downspouts (again), we rinse any debris that might have splashed onto your siding, clean up the ground, and treat the roof to prevent moss from growing right back. The Option C is a favorite method for those who simply must have their roof looking as sharp as possible.

Before and after an Option C

Option D: Soft Wash

  • Air-blow loose debris from roof
  • Clean out gutters and downspouts
  • Clean up debris from ground
  • Apply cleaning agent
  • Rinse roof

Many roofers prefer the soft wash for roof cleaning. This involves a pre-cleaning: air blowing off the loose debris, cleaning out the gutters, and cleaning up any mess we’ve made from the ground. Then we return to the roof to completely soak it with an outdoor cleaning agent (e.g. 30-Seconds Outdoor Cleaner). While this method will not remove the moss, it does kill it completely and immediately. This method is best used on roofs with thick shingles or very steep pitches where a zinc treatment will be less effective.

Cedar Shake and Shingle Roof Cleaning

  • Clean out gutters and downspouts
  • Thoroughly apply cleaning agent
  • Soft-wash cedar shake shingles
  • Rinse roof and gutters and flush downspouts
  • Clean up any mess made on siding and ground

Perhaps more than any other roof type, cedar must be kept clean to protect their lifespan. In their general technical report, “Installation, Care, and Maintenance of Wood Shake and Shingle Roofs,” the U.S. Department of Agriculture makes this clear: “Keep a clean roof. Eliminate the causes of debris accumulation and mold and moss growth. Remove all overhanging branches and if necessary, trees that encroach on the roof. If moss is growing on the roof or if leaves and other debris collect on the roof, they trap moisture and encourage decay, thereby decreasing the life expectancy of the roof.”

We do this by first cleaning out the gutters and downspouts and blowing off any loose debris. Then we will apply a cleaning agent (a mixture of either sodium hypochlorite or sodium percarbonate) and wash the shingles, line by line, with low to moderate pressure to return the shingles to their natural color.

After we have cleaned the roof and it has time to dry (typically 4 days of solid dry weather), it’s a good idea to have a painter or cedar shake restoration company apply a fungicide/preservative. This is not something we do as it is outside our scope of work, but please feel free to call the office at 206-232-1266 for a referral if you are interested.

We do not recommend having your cedar roof oiled here in the Northwest. According to the Cedar Shake and Shingle Bureau, “In humid areas oiling shakes does not allow them to breathe and they cannot dry out properly which could shorten their life span.” Any application

PVC Membrane Roofs

  • Air-blow loose debris from roof
  • Clean out gutters and downspouts
  • Clean up debris from ground
  • Soft-brush PVC with mild soap/water solution
  • Rinse roof and flush out gutters

Gently, is the only way to clean a PVC roof. We clean them with a soft brush and a mild soap-and-water solution followed by a thorough rinse. We don’t pressure wash PVC.* Nor do we use bleach or apply moss treatment. An ugly, algae-slimed PVC roof almost always turns out looking brand-new.

* NOTE: We often use pressure washing machines on these job, but only with a rinse tip that produces pressure equivalent to that of a garden hose nozzle. Our pressure washers with a rinse tip are only superior to most garden hoses because they produce a more volume. Higher volume–not pressure–helps us get the job done faster (and that makes the job more cost-effective).

Standing-Seam Metal Roofs

We begin by blowing off the loose debris and cleaning out the gutters. Then we apply a solution of water, detergent, tri-sodium phosphate, and hypochlorite to the metal. Then we clean the roof by brush or pressure washer, depending on the level of dirt, mildew, and algae. After a final rinse and flushing of the gutters and downspouts, we clean up any mess we’ve made on the siding, decks, and ground.

Frequently Asked Questions about Roof Cleaning and Care

What types of roofs do you clean?

We clean most roof types: composition, aluminum, cedar shake, torch-down, pvc, and many types of tile. There are a few exceptions, however. We don’t clean soft aluminum tiles for example, due to damage liability. We also don’t clean roofs we aren’t equipped to clean safely or roofs in need of replacement or serious repair. If you have any questions about our roof care options for your own roof, please contact us.

I’ve heard you should never use a pressure-washer washing for roof cleaning. Why does your company pressure-wash roofs?

It is a well-known fact that pressure washers can do a lot of damage. A high-volume, high-psi pressure washer can actually bore a hole right through concrete without much problem. Knowing this has rightfully made most homeowners wary about having anyone pressure-wash their roofs. And a lot of contractors, who know less about pressure washers than they do about the potential damage they can do, insist that a pressure washer should never be used on a roof. But to that we say, “abusus usum non tollit,” the abuse of something doesn’t negate its proper use. Just because knives pose a threat to small children, doesn’t mean we don’t keep a few in the house and use them for preparing meals. We all know about knives because we all use them. Not everyone, however, knows about pressure washers.

The truth is that a commercial pressure washer in the hands of a well-trained tech is often the softest and best approach to moss removal, especially where the only other alternative is to use a wire brush. Because there is so much misinformation about pressure washing roofs and too many “cowboys” out there with pressure washers, it might be useful here to share one piece of knowledge our techs have about washing roofs that most people and many contractors* don’t.
For example: You might know that the type of tip used at the end of a pressure washing wand is one of the essential components in determining the outcome of a project. And you might also know that pressure washers come with a set of tips of varying degrees, from 0° to 40° plus a rinse tip. But did you know that those tips are sized to match your specific machine? If you own a 4-gpm 4000psi machine, you will have a set of four tips labeled 0004, 1504, 2504, 4004. The first two digits in each of those numbers tell you the degree of the flow of water from those tips: 0°, 15°, 25°, and 40°, respectively. The last two digits tell you the size of the orifice. In this case, 04, for the 4 gallons per minute (gpm) of water your machine is designed to draw. Now, if you were to replace these tips with size 08 tips on this particular machine, your pressure would drop substantially while you would maintain the same flow rate. You would now have a 4gpm 1000psi machine. In our case, we ordinarily use 8 or 10gpm machines at 3000 psi for concrete, aggregate, and other hard surfaces. The tips we most often use are size 09 or 10 tips. When cleaning roofs, however, we use size 12, 15, or more, depending on the roof type and condition to drop the psi significantly and eliminate the potential for damage.

This may be more than the average homeowner cares to know. Still, there are many other things a well-trained technician knows about pressure washing such as where to hold the wand in relation to the surface, pre-soaking with cleaning agents, what types of other tools can be used, when not to pressure wash, to name a few. All our techs are well-trained, smart, and experienced in cleaning roofs. If we offer the Roof Wash as an option for your roof cleaning, it will be done carefully with minimal abrasion and the end result will be beautiful.

*In reality, many roofing contractors know that psi can be adjusted downward. They also know that a pressure washer in the hands of a well-trained tech is often preferable for roof cleaning to rubbing it over with a wire brush. But, since they have no idea who’s doing the roof cleaning, or what kind of training they might have, they often simply recommend against pressure washing roofs for anyone except themselves.

How often should I have roof washed?

The best option for the life of your roof, whatever type, is to never wash it (or brush it, for that matter), but to maintain it regularly by blowing off the debris and treating the moss. However, this might not cut it for aesthetic preferences, especially if maintenance has been long-neglected. Aluminum, rubber tile (faux slate), and thick concrete tile may be washed every few years as needed without causing leaking or reducing the life of the roof. And those who simply must have their composition roof looking completely new in a very short period of time (e.g., the house is going on the market), washing may be the best option. If you have questions about your own situation, please don’t hesitate to ask one of our techs.

Do you clean with air-compression?

We use air-blowers (leaf-blowers) to remove loose debris from roofs, and we use high-volume, low-pressure pressure washers for more thorough roof cleaning, but we don’t use compressed air. We want to provide excellent service to our customers and haven’t been convinced that air-compression cleaning does a great job. Unlike water, compressed-air leaves a lot of dirt behind, yet it is still hard on the roof. If a roof must be cleaned and made to look like new, pressurized water in the hands of an experienced tech is the way to go. If routine leaf/debris removal is required for roof cleaning, a top-of-the-line commercial backpack blower is all that’s needed to get the job done.