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Cedar Roof Cleaning

Cedar Roof Cleaning in Minneapolis

In recent years cedar roof maintenance has become a bit of a controversial issue in the construction world.  Some say you should never pressure wash a cedar roof while others insist you should.  Who is right?  They both are.  The truth is some cedar roofs benefit from pressure washing while others do not.  The key underlying question as to whether or not you should wash your roof depends on the condition of your roof and your motivation.  All else held equal, cedar roofs in Minnesota that are older than 18 years seldom benefit from cedar roof cleaning. Many times a week we conduct roof inspections where we deliver somewhat bad news to the homeowner; that message is simply that their roof is too far gone to benefit from washing. A frequent response from them is to go grab the estimate they just received from one of our competitors who suggested washing the roof. Usually these estimates come from a local franchise operation or a smaller company. These guys have never met a roof they don’t want to wash because it’s the only way they earn income. Our incentive is to tell you the truth. Period. Our incentive is not to wash cedar roofs purely because that is how we pay our bills. If we tell you not to wash your cedar roof you should listen.

Why Wash a Cedar Roof?

Depending on motivation, there are three reasons people choose to wash their cedar roof:

  1. To improve cedar appearance
  2. To remove organic material (moss, lichen, etc)
  3. To prepare the surface to accept a wood preservative.


As with anything you do to maintain the appearance of your home, the benefits of washing your roof are clear enough.  Dark, irregular stain patterns and accumulations of algae, lichen and moss can read as neglect from the curb.  Washing a cedar roof will almost always improve its appearance.  Some of our clients choose to wash their roof purely for the cosmetic benefits.  We often work on homes that are about to be sold for this very reason.

Cedar Roof Cleaning

Cedar Roof Cleaning Contractor Minneapolis



Removing Moss and Lichen from Cedar Roofs

Clean Your Cedar Roof

How to Maintain a Cedar Roof in Minneapolis

The second reason cedar roofs benefit from cleaning is also the most obvious; it’s smart to remove organic material to slow down the deterioration process. You see, nature wants to turn your wood roof back into dirt.  Don’t worry, it’s not personal.  Rot and decay will eventually affect all organic things, including your roof.  By far the most common form of deterioration on cedar roofs relates to the propagation of algae, lichen and moss, with the latter being the most destructive.  Moss lacks chlorophyll, the magic compound that enables most plants to feed themselves through the absorbtion of sun and water.  Instead, moss snacks on your roof to find the energy it needs to survive (Right). Adding insult to injury, both moss and lichen act as a natural sponge, retaining moisture on your roof long after each rain.  This creates a perfect habitat for other microscopic wood eating critters.

Fortunately, most homeowners intuitively understand the potential problem with moss and lichen on their roofs.  Calls to our office are often triggered by concern about the green stuff ‘up there’.  Less noticeable but equally troubling is the rot and decay that occurs in the keyways, which is often difficult or impossible to see from the ground.  A keyway is the space between cedar shakes or shingles.  When materials such as seeds, pine needles and leaves settle in the keyways they start to biodegrade quickly.  As with other forms of organic deterioration, rotting plant materials in the keyways will shorten the lifespan of a cedar roof, sometimes cutting it in half, by breaking down the wood until it has lost some or all of its structural integrity.

Cedar Roof Preservation

The third reason you may want to wash or clean your cedar roof is to prepare the surface to take wood preservatives. The advantages of washing a cedar roof to both make it look nice and remove organic rot seem simple enough.  Perhaps the most important reason why we wash roofs.

Most people understand the idea that applying a treatment to a dirty surface will be of marginal benefit. Prepping a roof for treatment is no different than prepping a home for painting or a deck for staining.  Put simply, we wash cedar roofs to prepare them for preservation.  On rare occasion we come across a roof that requires no washing before preservation.  In general, most roofs should at least be rinsed before the application of any wood preservative be considered.  Learn more about why treating your roof may make sense in our section on wood preservation.

Is Cleaning a Cedar Roof a Bad Idea?

The Best Reason Not to do a Cesar Roof Cleaning:

We frequently inspect roofs that are determined to be ‘beyond’ washing, while some roofs may only have certain sections that are worth washing.  We use the Embrittlement Scale (right) to determine whether or not washing makes sense.  Put simply, Category

Should you wash your cedar roof

Should you wash your cedar roof?

One roofs will always be worth washing.  Category Two roofs are marginal, with some being worth washing and others not.  Roofs in embrittlement Category Three should never be washed.  A few of the cedar roofs we inspect every week fall into Category Three.  Even so, there are roof cleaning companies that frequently wash these roofs, costing the client good money while shortening the lifespan of the roof.  Although Category Three roofs might appear cleaner and brighter from the ground after being washed, in reality are damaged through the process.

Scale for determining whether or not to wash or clean a cedar roof

Here is something that may come as a surprise to you.  The cedar roofing on your home does nothing to keep water from getting inside.  The tar paper under the cedar serves that purpose.  That’s right.  Your cedar roofing is in place for one functional reason and that is to protect the tar paper from the sun and wind.  As long as your tar paper remains in tact your roof will continue to keep you dry.  Once the tar paper is exposed to the elements, however, it quickly begins to deteriorate, resulting in leaks soon thereafter.  See photos below of ‘open keyways’.

What does this have to do with using the proper equipment to wash a cedar roof?  Everything.  You see, the question isn’t whether or not pressure washing will result in a net loss of wood material from your roof.  It will.  The question is, how do you minimize that loss?

Our goal as cedar roof restoration professionals is therefore to keep as much wood on your roof as possible; to keep your tar paper from seeing the elements.  This is where the use of professional equipment makes all the difference in the world.

Some companies use wood ‘cleaners’ or other chemicals during the washing process under the guise that it enhances the outcome.  The real reason chemicals are used is to artifically brighten the wood that is otherwise left inadequately cleaned by their cheap, low-volume pressure washing equipment.  Chemicals are entirely unnecessary when commercial equipment is used.  Period.

Damage from Cedar Roof Cleaning

Beware: Some Minneapolis Roof Cleaning Companies us Crappy Equipment

If your roof is in good enough condition to wash your cedar, congratulations.  You have the opportunity to add years to its remaining life through occassional maintenance.  But it is important to understand that a cedar roof should only be washed using specialized equipment.  Standard pressure washers, like the one pictured to the right, will do a good deal of damage to a cedar roof regardless of the user.  That damage will most likely not be visible from the ground because it is a matter of hundreds of an inch.  In fact, consumer-grade pressure washers will remove between five and eight times the amount of wood from your cedar roof than professional equipment.  If you have ever seen a cedar deck that looks furry and splintery after washing you know what we are talking about.

Kuhl only uses high volume, low-pressure equipment to wash cedar roofs. Our trailer mounted washing unit is custom built for washing cedar roofs and cost us $25,000, not including Steve’s free labor to weld it all together.  There are only a couple of wood restoration firms in the country that use this specialty equipment.  Where the standard consumer-grade machine will push 3-4 gallons per minute through the wand, ours pushes 12!  This is why we often refer to our process as power rinsing, not power washing.

Why should you care?  Because regular pressure washers–the kind used by painters, handymen and most of our competitors–are an entirely different machine.  Such units should never be used on a cedar roof as they operate at inappropriate water volume to pressure ratios, resulting in significant wood loss.

Companies that don’t invest in industrial equipment do so for one reason; it is much cheaper to use a standard machine than an industrial unit.  From the ground the results look about the same to you.  The discoloration on your roof is gone, the roof looks fresh and clean again.  But upon close inspection you will see that those machines have removed a lot of wood.  Not good.  Regardless of the appearance from the ground, using a typical pressure washer on a cedar roof will take years off its existing lifespan regardless of whether or not wood cleaners and/or brighteners are used.  Ultimately, wood loss in the keyways of your roof is one of the primary reasons your roof will fail in the future.


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