What Is Soft Washing and How Does It Work?
Soft washing applies a biocidal cleaning solution at under 500 PSI, roughly garden-hose pressure, to kill algae, moss, and lichen on asphalt shingles at the cellular level. It is the method the roofing industry recommends for shingle cleaning, and it does not strip granules or void manufacturer warranties. Here is how it works and why it is the correct choice.
How Soft Washing Works
Soft washing uses a low-pressure delivery system (typically a 12-volt electric pump or downstream injector) to apply a biocidal cleaning solution at pressures under 500 PSI comparable to a garden hose. The solution is primarily sodium hypochlorite (pool shock, sodium hypochlorite at 10 to 12.5 percent concentration diluted to 1 to 6 percent application strength) combined with a surfactant to help the solution cling to the roof surface. The solution is applied, allowed to dwell for 10 to 20 minutes, and then rinsed at low pressure with water. The active ingredient kills algae, moss, and lichen at the cellular level.
Why Soft Washing Works Better Than Pressure
The biological growth on roofs (algae, moss, lichen) is alive killing it through chemistry is more effective than physically blasting it off with pressure. High-pressure washing removes the visible growth but leaves behind the root structure and spores that allow rapid regrowth. Soft washing kills the organisms completely, and the results last significantly longer. ARMA’s recommendations for asphalt shingle cleaning explicitly state that low-pressure washing with a chemical solution is the correct approach not high-pressure washing.
Chemical Safety Considerations
Sodium hypochlorite is an effective biocide that breaks down into salt and oxygen it does not persist in the environment long-term. The primary concern is the potential impact on plants, grass, and aquatic environments during the cleaning. Professional soft-wash operators pre-wet and post-rinse landscape areas to dilute any runoff, use concentrations appropriate for the growth level (not higher than necessary), and avoid application before heavy rain that would wash concentrated chemical into storm drains. (For more on this, see Questions to Ask Before Hiring a Roof Cleaning Company.)
Working With a Contractor on This Project
Getting the right outcome on any home services project starts before the work begins. A few consistent practices make the difference between a smooth project and a frustrating one.
Get at least two or three quotes. A single quote gives you no basis for comparison. Two or three quotes reveal the market rate for the work, expose scope differences between bidders, and let you assess contractor communication quality before any commitment is made. The lowest bid is not automatically the best value if the scope is thin or the materials are underspecified.
Ask for licenses and insurance before work starts. Verify current general liability insurance and, for any work involving workers on your property, workers compensation coverage. Licensing requirements vary by state and trade; confirm the specific requirements in your jurisdiction. A licensed contractor who carries appropriate insurance protects both parties if something goes wrong during the project.
Get the scope in writing. A written proposal that specifies materials, labor scope, timeline, what is included (permits, cleanup, disposal), and warranty terms is the foundation of a professional engagement. Verbal agreements on any of these points invite misunderstanding. For any project over $500, written scope is non-negotiable.
Clarify the payment schedule upfront. A reasonable payment schedule ties major payments to project milestones: a deposit at signing, a payment at material delivery or project start, and a final payment at completion after a walkthrough confirms satisfaction. A contractor who requires full payment upfront before work begins is a red flag in any trade.
Making a Confident Decision
The information in this article is designed to help you ask better questions of the professionals you work with, evaluate their answers with more confidence, and understand the decisions involved in this type of home services project. It is not a substitute for professional assessment of your specific home.
Every home is different. Roof slopes, local weather, tree exposure, soil conditions, and existing system condition all affect what the right solution is for your specific situation. The general principles here apply broadly, but a professional who assesses your home in person will give you guidance that accounts for the details that general articles cannot.
When you are ready to move forward, getting at least two or three quotes from licensed, insured, local contractors is the most important practical step. It lets you compare approaches, assess contractor quality, and confirm that the solution proposed matches your specific situation and budget.
Roof Cleaning Cost at a Glance
Roof size: Larger roofs cost more due to more area to clean.
Roof pitch: Steeper roofs cost more for special equipment and safety measures.
Growth severity: Heavy moss, algae, or lichen means more work and higher cost.
Overall range: $300 to $600, averaging $450.
Ranges here are national ballpark figures, not quotes. Actual pricing varies by region, home condition, materials, and contractor. How we source cost data.