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Asphalt Roof Cleaning Guide by Ryan Cash (Rooftec Systems)

Ryan Cash

UAMCC Associate Member
Why should you have your roof cleaned?
What is the cleaning pie?
How do you clean an Asphalt Shingle Roof?
Is it ok to pressure wash a roof?
How do you soft wash a roof?
Can you downstream roof?

Whether there is an excessive build up of moss, lichen or other staining, there are a lot of reasons why a homeowner should have their roof cleaned. While the damaging effect of mosses and lichens may be more noticeable, many homeowners may not know why staining should also be removed.

Let's discuss the black staining you'll often see.

The most likely culprit for this staining is a type of Cyanobacteria called Gloeocapsa Magma.

Here are a few of the reasons a homeowner should have this removed from their roof:
  1. The bacteria feeds on the calcium carbonate in the limestone that is used for filler in the shingles
    1. This causes pre-mature breakdown of the shingles and release of protective grit
  2. A darker, stained roof will hold in excess heat leading to higher utility bills
  3. The bacteria will hold water on the roof longer, causing it to break down faster
  4. It is unsightly and harms the appearance of the home
After understanding why is should be cleaned, we need to look at HOW it should be cleaned.

The first and foremost thing to remember when roof cleaning is this:


“High pressure washing systems are likely to damage asphalt roofing and should NOT be used on asphalt roofing for removing algae or for any other purpose” (asphaltroofing.org)

A single use of a high pressure on an asphalt roof can not only void it’s warranty but can also easily cut the life of the roof in half by removing large amounts of the protective shingle grit.

So what is the proper way to clean an asphalt roof?

Low-Pressure Chemical Treatment (commonly referred to as soft washing) is the recommended process for asphalt roofs.

In the cleaning industry, there is something known as the cleaning pie

cleaning pie.png

The idea is simple. There are four components to cleaning:
  1. Chemical
  2. Agitation
  3. Heat
  4. Time
All cleaning applications rely on some combination of these. Some pressure washing applications actually use all four:

You may pre-treat a driveway with a degreaser (Chemical), let it dwell (Time) and use a hot water machine (heat) to deliver high pressure water (Agitation) to remove stubborn stains.

However, for soft-washing, you are removing heat and agitation and solely relying on Chemistry and Time for cleaning.

  • You will use a strong oxidizer (Usually Sodium Hypochlorite) along with an appropriate surfactant to help remove and lift the stains associated with algaes, mosses, and other bacterial growth on the outside of the home.

  • After applying your chemicals to a roof, it will need to dwell for an appropriate amount of time (usually 10-15 minutes) to fully remove staining. The key here is to keep it wet and allow the chemicals to do their job.

This combination allows you to properly clean a roof while minimizing risk and decreasing the likelihood of damage.

So What’s the process?

First, you’ll need to decide what equipment you’ll use to apply your chemistry.

When using Sodium Hypochlorite (12.5%), at a minimum you’ll need to be applying at roughly a 1:4 mix ratio of SH to Water. Some tougher cleaning may require up to a 1:1 ratio. Because of this, a down-stream injector won’t be sufficient for roof cleaning. Even other popular tools like the X-jet may not be able to achieve the high percentage mix that you need to properly clean a roof.

For cleaning with the Roofec XCS500, I recommend starting at 1:4 mix (Water turned on 100%, SH turned on ½ way) and roughly a 1:100 mix (2-4 GPH) of surfactant.

Here is the step by step guide to clean an Asphalt Roof:

  1. Speak with the customer and ask them what their specific concerns are
    • Figure out and manage their expectations
    • This will help you determine what is important to them so you can make sure you deliver good results!
  2. Perform a visual inspection of the entire property.
    • Take Before Photos (DO THIS EVERY TIME!)
    • Note any pre-existing damage and notify the customer
    • Take special note of any potential liabilities (delicate plants, electronics around the home, etc.)
  3. Tape off, move, cover and pre-soak with water:
    • If Cleaning siding as well, tape off:
      • Outdoor outlets
      • Video Doorbells
      • Other electronics
    • Stained wood
    • Delicate Plants
    • Grills and outdoor furniture
    • Anything else that may be damaged by overspray
  4. Pre-Soak all surrounding vegetation with water
    • Soak both the leaves and roots well with water
    • This will help insure that any overspray gets diluted and doesn’t damage plants
    • If working alone, use a sprinkler to keep landscaping wet during the job
  5. Apply chemicals to roof with light pressure (600 PSI or less)
    • 3-6% SH + Light Surfactant (Start with less chems and only increase as needed)
    • Avoid over-application of chemistry (no need for unnecessary run-off)
      • “If it’s wet, it’s working”
    • Apply chemicals starting at the gutter line and working your way up (when possible)
    • Work your way around the home
  6. Make sure to keep surrounding vegetation wet while working on the roof

  7. Allow 10-15 minutes dwell time

  8. Re-apply if necessary
    • Any remaining black staining will need a second application
    • Once staining has turned brown, it will come down with a good rinse
  9. If needed/desired, Treat roof with a bleach neutralizer to help mitigate damage from run-off when rinsing

  10. Rinse entire roof with water, starting at the peak, working down to the gutter line (one side at a time)
    • Typically spend 2X the time rinsing as you do applying chemicals
  11. Post-soak ground level with water
    • all surrounding landscaping/vegetation
    • Any other surfaces that were subjected to any overspray
    • Pay particular attention to where downspouts drain
  12. Remove any taping and tarping you placed before the job

  13. Perform visual inspection of entire property
    • Take After Photos
    • Return any moved items to their original places
    • Walk entire property and view it from the customer’s point of view
      • How does the end result look?
      • Is anything out of place?
      • Is there anything else that should be added on to the job?
      • Was anything missed?
  14. Talk to customer and deliver the invoice

While some of this may seem pretty basic, the important thing to remember is to

The best way to get consistent results is to have a process that you can repeat on every home. This not only ensures consistency but also will save you if/when a customer has a complaint. Even if a customer calls you back in 3 months with a problem you'll be able to know exactly what you did and how you did it because you do it the same way every time!