Key Takeaways
- The industry-standard method is soft washing: a diluted bleach-based solution applied at low pressure, not scrubbing or pressure washing.
- A typical working solution is sodium hypochlorite diluted to roughly 3 to 6 percent, mixed with a surfactant made for roof work.
- Apply from the bottom up, let it sit for 10 to 20 minutes, then rinse from the top down with a garden hose or the lowest pressure washer setting.
- Protect landscaping before you start; sodium hypochlorite will damage plants on contact if it isn’t diluted and rinsed properly.
Cleaning asphalt shingles the right way means using a diluted bleach solution and low water pressure rather than scrubbing or blasting the surface. This method, called soft washing, is what the Asphalt Roofing Manufacturers Association recommends, and it’s also usually a requirement buried in your shingle warranty — several manufacturers void coverage if a roof is cleaned with high-pressure water instead.
What you’re actually cleaning off
The dark streaks most homeowners want to remove are caused by a cyanobacteria called Gloeocapsa magma, which feeds on the limestone filler in asphalt shingles. It’s not surface dirt, which is part of why scrubbing or plain water alone doesn’t remove it effectively — you need a cleaning agent that actually kills the organism, not just water that rinses loose debris away.
Mixing the cleaning solution
The standard professional approach uses three ingredients: sodium hypochlorite (the same active ingredient in household bleach, though roof-cleaning work typically starts from a higher-strength 12.5% concentration sometimes sold as pool shock), water to dilute it, and a surfactant designed for roof work that helps the solution cling to a sloped surface instead of running straight off. A common mix ratio is one part 12.5% sodium hypochlorite to three parts water, plus a small amount of surfactant, which works out to roughly a 3 to 6 percent working concentration on the shingle itself. Heavier growth like moss or lichen sometimes needs a stronger mix or a second application, but starting with a weaker solution and reapplying if needed causes far less risk to the shingles than starting too strong.
Don’t substitute dish soap for a proper surfactant
It’s a common shortcut, but dish soap doesn’t have the same clinging properties on a sloped, granulated surface and can leave residue that actually attracts more dirt over time. Roof-cleaning surfactants are inexpensive and sold specifically for this job.
Applying the solution step by step
- Protect landscaping first. Wet down any plants, shrubs, or grass near the downspouts and roof edges with plain water before you start, since sodium hypochlorite runoff can scorch vegetation on contact.
- Mix the solution according to the ratio above, adjusting concentration based on how heavy the staining or growth is.
- Apply from the bottom of the roof upward, working in manageable sections rather than the whole roof at once, using a pump sprayer.
- Let it sit for 10 to 20 minutes. This dwell time is what actually kills the algae, moss, or lichen; rinsing too early wastes most of the cleaning power of the solution.
- Rinse from the top of the roof downward using a garden hose or a pressure washer on its lowest possible setting, never a narrow or zero-degree nozzle.
- Flush the landscaping again with plain water immediately after rinsing the roof to dilute any runoff that reached plant beds.
- Inspect the result once the roof dries. Heavy or long-established staining sometimes needs a second application rather than a stronger first pass.
Choosing the right day to clean
Weather conditions on cleaning day affect both safety and results more than most people expect. Pick a mild, overcast or lightly sunny day rather than one with intense direct heat, since a hot roof surface causes the cleaning solution to evaporate faster than its recommended dwell time allows, which weakens the treatment before it has fully worked. Avoid windy days as well; wind carries sodium hypochlorite spray onto siding, windows, and neighboring property well beyond where you intend it to land, and it also makes ladder work and roof access less stable. A dry forecast for at least the next several hours matters too, since rain arriving mid-treatment dilutes the solution before it can finish killing the algae or moss, often leading to patchy, incomplete results that look like the cleaning failed entirely.
Safety considerations
Sodium hypochlorite is corrosive to bare or untreated metal, so pre-wet and continuously rinse any exposed flashing, gutters, or downspouts during application to avoid pitting or discoloration. It also produces fumes that can affect people with respiratory conditions, so work with good airflow and avoid application on windless, humid days when fumes linger near the work area. Wear eye protection and gloves, and never mix sodium hypochlorite with ammonia-based cleaners, which produces dangerous chloramine gas.
What not to do
Skip the pressure washer for the actual cleaning pass, even on a low setting — high-pressure water strips the protective granule layer off asphalt shingles, exposing the asphalt layer underneath to UV damage and accelerating the shingle’s aging. Scrubbing with a stiff brush causes similar granule loss. And resist the urge to walk the roof extensively while cleaning; wet shingles are slippery, and foot traffic on a soft, sun-warmed roof can dislodge granules on its own.
Tools worth having before you start
A pump sprayer sized for the job matters more than most people expect; a small handheld spray bottle works fine for spot-treating a single stain but is impractical for covering a full roof plane evenly. A garden hose with an adjustable nozzle covers the rinse step for most homeowners without needing a pressure washer at all. Non-slip footwear, a stable extension ladder with proper standoff stabilizers, and a roof harness anchored to a secure point are all worth the investment if you plan to do this more than once, since the fall risk doesn’t change just because the cleaning method is gentler on the shingles.
DIY-checkable versus call a professional
Mixing and applying a soft-wash solution is within reach for a homeowner comfortable working with basic chemicals and following label instructions carefully, provided the roof pitch and access are manageable from a ladder or safe walking areas. Steep roofs, multi-story homes, or roofs with extensive moss growth that need a stronger approach are usually better handled by a professional soft-wash contractor, who will also carry the right dilution equipment and insurance for roof-height chemical work.
Frequently Asked Questions
How long does the cleaning solution’s effect last on a shingle roof?
A proper soft wash with a correctly mixed solution typically keeps a roof looking clean for 3 to 5 years, though heavily shaded roofs with overhanging tree canopy tend to see algae or moss regrowth sooner. Some homeowners in high-growth areas opt for a lighter annual or biennial touch-up cleaning to extend those results further.
Is it normal for the shingles to look worse right after cleaning, before they dry?
Yes, wet shingles often look darker and blotchier immediately after treatment, and some residual staining can take a few days of sun exposure and natural weathering to fully fade. If staining is still clearly visible after the shingles are fully dry, that’s when a second application is usually warranted rather than assuming the first treatment failed.
Can cleaning solution damage a roof that already has cracked or curling shingles?
Existing damage like cracked or badly curling shingles can make a roof more vulnerable to any liquid intrusion, including cleaning solution finding its way under a lifted edge. If your roof already has visible shingle damage, it’s worth addressing those repairs before a full cleaning, or having a professional assess whether cleaning is advisable at all in that condition.