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Why pressure washing your asphalt shingle roof is a bad idea.

why you should never power wash your asphalt shingle roof

Unless you want to intentionally remove a sizeable chunk of the remaining life of your asphalt composition roof and dump in in your gutters, then you should NEVER power clean your roof. 

Power washing an asphalt roof is a bad ideaSpring is in the air and homeowners and home sellers finally emerge from winter hibernation, lean backward and get an eye full of their roof for the first time in a while.

"Where did all the moss and other unidentifiable gunk come from?"

There's the instant urge to go into full manly-mode, extract that manly pressure washer from the garage, climb up your manly ladder, and unleash the full power setting on that pesky moss.  The urge can be overwhelming. 

"I didn't buy this pressure washer for nuthin'!"

It's OK, we understand. It just feels good!

Alternatively, you ask Bob your handyman, who does wonders with his power washing machine removing gunk from your patio, driveway and decks if he could also do your roof.

Real estate agents often advise their sellers to have their roof power cleaned to improve the home's curb appeal.  A freshly power-washed composition roof can indeed look fabulous from the ground, almost as good a new roof. But in this case, looks are really deceiving.  

Here is what I have personally learned in my Realtor job and from following home inspectors up ladders onto power-washed roofs. So, before you go destroy your home's roof, you might want to read this first...

First of all, what is an asphalt roof tile?

The most commonly used roofing material throughout the US is called asphalt shingles (tiles), aka, composition tiles. In some parts of the country clay or concrete tiles, wood shingle or metal roofs will be more common, each of which have their own pros and cons.

In simple terms, asphalt shingles have a base layer(s) and a top, outermost layer. The top layer is exposed to the elements and is coated with granules of slate, schist, quartz, ceramic granules, brick, or stone. New shingles feel like really rough sandpaper.

The purpose of the outer layer is to prevent UV deterioration of the roof, protect the asphalt core, and to add color to the tile. That granules in the rough outer layer are there for a reason and it's important that they stay right there... and for as long as possible.

The culprits: moss and algae.

The black discoloration often seen on roofs in coastal or humid areas of the country is not due to mold or mildew. This staining is caused by algae which starts as small spots but ten develops into large unsightly streaks on the roof.  Algae are more of an aesthetic annoyance and probably do not actually damage asphalt roofs. 

Moss on the other hand can damage your roof. Our little friend doesn't have any roots and absorbs moisture through its leaves and so concentrates in the more shaded areas of the roof. That's why you will see the most moss growth on the north sides of the home and where the roof is shaded by nearby trees.

Moss damages roofs by causing the leading edge of the tile to lift up or curl and makes them more susceptible to being blow off in strong winds. Excessive moss growth can also cause moisture to get access to the plywood sheeting underneath the shingles and lead to the roof leaking.

So why is power cleaning bad for your asphalt roof?

Power washing a composition roof will remove the protective granular from the outer layer of the shingles leaving the shingles less protected from the sun and other harsh weather conditions. It will also leave gouge marks in the lower layers leaving them more vulnerable to the elements.

Power cleaning damages asphalt composition roofs

Take a good look in the gutters. That's where 50% of the remaining life of your roof is now sitting. It will have a nice thick layer of all that gritty material from the outer layer of the roof.

Now run your hand hands over he shingles. Nice and smooth eh? 

So now, a home seller that had a slightly grubby looking roof with 10 years left on it now has a clean asphalt roof with less than 5 years left on it...and buyers asking for a new roof after doing their home inspection. 

What about pressure washing "specialists" who say they can safely clean your asphalt roof?

Sorry, such a company does NOT exist!

Even if that discount mailer or friendly salesperson confidently states that they are an experienced/expert roof cleaning company and have "special low-pressure washer nozzles" don't believe the sales pitch and don't let them anywhere near your roof.

Feel free to have clean your driveway and patio, but keep them firmly on the terra firma!

So how do you clean the composition roof?

First, consider ways of reducing roof moss, lichen, and algae growth in the first place.

  • Cut back trees that are growing close to the home which keep the roof in moss-loving, perpetual shade. It will also stop trees dumping debris on the roof and filling your gutters.  Added bonus: it also removes a bridge that helps rats and other critters get access to your attic space.
  • Remove debris from the roof using a leaf blower...facing down the slope of course so as not to get under the leading edge of the shingles. Also, keep your gutter clean so they drain properly.
  • Avoid having gutters from an upper roof lazily dump water onto a roof below. Run a downspout all the way to the gutter instead.

Moss removal: you're probably not going to like this but... you, or someone you pay, needs to get up on the rood with a brush and start scrubbing it off with some good old elbow grease.

"Elbow Grease? Is that some fancy new moss killing chemical I can buy at the hardware store?"

Umm, no. It's just you and everything from a stiff toothbrush to a yard broom having a bonding experience with your roof (while wearing a safety harness of course!).  It's quite therapeutic! 

For those black algae stains, spray a 50:50 bleach: water solution on the roof, wait about 15 minutes and then rinse off the roof with a garden hose. The roof may need more than one treatment but the algae will be killed off and then washed away by rain. This treatment will also kill off some moss growth as well. 

If like me,  you have a rain garden where all the rain that hits the roof is diverted to a fast-draining planted area in your yard, then do not use this bleach cleaning method. 

So, in conclusion: do yourself and your roof a favor and leave the power washer on the ground. A power-cleaned roof looks great but you also now years closer to needing a new one. 

Here are some additional helpful resources on the subject:

Signs That You Need a New Roof: Bill Gassett lists some of the things to look out for that might indicate that your roof needs replacing, including both interior and exterior telltale signs. 

Buying a Home With and Old Roof: Eric Jeanette advises buyers considering a house with an old roof and your options when it comes to negotiating with the sellers.

A Homeowner's Guide to Roof Maintenance: John Cunningham's article list all the things you can do to get the maximum lifespan out of your roof and how to maintain different types of roofs. 

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