How to Remove Mold, Mildew, and Algae from Patios and Decks
The fastest method is to apply a bleach-based or oxygen-bleach solution, scrub, and rinse. But the cleaner and pressure you choose must match your surface material—concrete, wood, or composite—because the wrong approach can etch stone, shred wood fibers, or void a warranty. If you’ve ever pressure-washed a wood deck and ended up with fuzzy splinters, you already know why this first check matters.
Surface and Safety Checks Before You Start
Run through this quick decision aid before mixing anything. If any check fails, adjust your plan before proceeding.
| Check Item | What to Look For | Pass / Fail |
|---|---|---|
| Surface material identified | Sealed concrete? Pressure-treated pine? Cedar? Composite like Trex? | Pass only if you are sure |
| Weather forecast clear | At least 24 hours of dry weather after cleaning | Pass if no rain expected |
| Hidden area test done | Applied cleaner to 1 sq ft, waited 10 minutes, rinsed | Pass if no discoloration or damage |
| Nearby plants protected | Plastic sheeting or pre-wetting grass/shrubs before spraying chemicals | Pass if protection is ready |
| Tools gathered | Bucket, garden hose with spray nozzle, stiff-bristle brush (nylon for wood, polypropylene for concrete), long-handle scrub brush, eye protection, rubber boots | Pass if all on hand |
| Pressure washer readiness | Confirm surface can handle it: concrete up to 3000 psi; wood max 1200 psi with wide fan tip | Pass if you own or are renting one |
If you fail a check, stop and fix that issue. A cleaner that dries on hot concrete can leave white efflorescence, and a pressure washer on composite can delaminate the surface permanently.
Choosing the Right Cleaner for Your Surface
The critical decision point is whether your surface is porous and how it reacts to bleach. Concrete tolerates diluted chlorine bleach well; wood and composites often need oxygen bleach or dedicated cleaners.
Concrete and Stone Patios
Concrete, brick, and natural stone pavers are alkaline and handle diluted chlorine bleach without etching.
- Mild growth – Mix 1 cup household bleach per gallon of water. Spray on, let sit 10–15 minutes, scrub with a stiff brush, rinse with a garden hose.
- Heavy algae or deep mildew – Use a 1:3 bleach-to-water ratio or a dedicated concrete cleaner. Apply with a pump sprayer, scrub, and pressure-wash at 2500–3000 psi using a 25° to 40° nozzle. Keep the wand moving. Wear rubber boots and eye protection.
Concrete example: On a south-facing concrete patio shaded by a fence, algae turned the slab green within two months. One bleach treatment removed 90% of visible growth; a second application a week later, plus a stiff-bristle broom pass, handled the rest.
Failure mode: If the solution dries on concrete before you rinse, it leaves chalky white residue. Work in sections no larger than 50 sq ft on warm days and rinse as soon as the dwell time is up.
Wood Decks (Pressure-Treated, Cedar, or Redwood)
Bleach can strip natural oils and gray the wood. Oxygen bleach (sodium percarbonate) kills mold safely.
- Mix – Follow package directions (typically 1 scoop per 2 gallons warm water).
- Application – Wet the deck first, apply with a garden sprayer, let sit 15 minutes (do not let it dry). Scrub with a nylon brush along the grain, rinse thoroughly.
- Stubborn spots – Use a commercial deck cleaner followed by a rinse. Avoid bleach if the wood is unfinished or has a clear sealer.
Wood deck example: A 10-year-old pressure-treated deck with black mildew streaks cleaned in two 20-minute sessions using oxygen bleach. After rinsing, the wood lightened by two shades and mildew did not return for 18 months.
Failure mode: Pressure washing wood above 1200 psi raises the grain, creating a rough surface that traps dirt. Stick to a garden hose for rinsing, or keep the pressure washer at low setting with a fan tip held at least 12 inches away.
Composite Decking (All Brands)
Most composite manufacturers prohibit chlorine bleach because it can cause fading, delamination, or void the warranty. Use only oxygen bleaches or composite-specific cleaners.
- Procedure – Mix a composite-safe cleaner per the label. Apply with a pump sprayer, let sit 5–10 minutes, scrub with a soft bristle brush, rinse. Never use a pressure washer; use a low-pressure hose or wide-angle nozzle.
- Regular maintenance – Wash monthly with mild dish soap and water to prevent algae buildup.
| Surface | Best Cleaner | Mix Ratio | Tools | Key Warning |
|---|---|---|---|---|
| Concrete / Brick | Chlorine bleach + water | 1:4 light growth, 1:3 heavy | Pump sprayer, stiff brush, pressure washer (optional) | Protect plants; avoid drying on surface |
| Wood (treated or untreated) | Oxygen bleach (sodium percarbonate) | Per package (~1 scoop/2 gal) | Garden sprayer, nylon brush, hose | Do not let solution dry; max 1200 psi |
| Composite | Composite-specific cleaner or mild soap | Per label | Pump sprayer, soft brush, low-pressure hose | Never use bleach; no pressure washer |
Step-by-Step Cleaning for Concrete Patios
Use this sequence for concrete, brick, or stone with moderate mold and mildew.
- Clear the area – Move furniture, grills, planters. Sweep away leaves and debris so the cleaner reaches the surface.
- Pre-wet surrounding plants – Soak grass and shrubs with water to dilute overspray and reduce chemical burn.
- Mix your cleaner – In a 5-gallon bucket, combine 1 quart household bleach with 3 quarts cool water. Hot water releases bleach fumes too quickly.
- Apply the solution – Use a pump sprayer to coat the entire patio. Work from the far end toward your exit so you don’t walk through wet cleaner.
- Let it sit – Wait 10 minutes. On hot sunny days, shorten to 5 minutes to prevent drying.
- Scrub – Use a stiff-bristle pole brush on stained areas, focusing on joints and corners where mold builds. For heavy algae, scrub aggressively; the solution alone may not penetrate thick growth.
- Rinse thoroughly – Spray with a garden hose until all soap residue is gone. Rinse any plants that got splashed.
- Inspect – If dark patches remain, repeat steps 3–7 after the surface dries. Most concrete needs two passes for stubborn algae.
Likely causes for incomplete removal: The solution dried on the surface (reapply and work in smaller sections), the growth was too thick (increase bleach ratio to 1:3 but no stronger), or tree sap/dirt shielded the mold (scrub more aggressively). If after two attempts the stains remain, stop and consider a professional.
Success check: After 24 hours, the surface should be free of green, black, or slimy patches. Rub a white cloth over a dry spot—if no transfer appears, cleaning is complete. If green returns within two weeks, the roots survived and you may need a commercial mold-killing product.
When to Call a Professional
Do not spend more than two full DIY attempts on the same problem. Escalate to a licensed deck or concrete restoration contractor if:
- The area is larger than 500 sq ft and you lack a pressure washer or time to hand-scrub.
- Multiple layers of paint, sealer, or stain need stripping before mold removal.
- Wood is rotted or concrete is spalled (flaking) – structural issues, not cosmetic.
- Mold returns within three months despite proper cleaning – this may indicate a moisture problem under the surface (poor drainage, a leak). A contractor can test moisture levels and recommend slope correction or a vapor barrier.
If any of these apply, stop DIY and get an assessment. The cost of a professional deep clean is usually less than the damage from a failed DIY that leaves hidden moisture behind.
Frequently Asked Questions
Can I use vinegar instead of bleach?
Vinegar can kill mild surface mold on concrete, but it is less effective on algae and deep mildew. For wood, undiluted white vinegar is safe but may need multiple applications. For composite, vinegar is not recommended because the acidity can dull the finish.
How often should I clean my patio or deck to prevent mold?
Once a year is usually enough. In shady, damp climates, every six months may be necessary. A mid-season rinse with a garden hose can delay growth.
Will a pressure washer remove mold without chemicals?
A pressure washer alone can blast off surface mold and algae, but it may not kill roots in pores or cracks. Bleach or oxygen bleach is still needed to prevent rapid regrowth. On wood, high pressure can damage fibers.
Is it safe to pour leftover cleaning solution down the drain?
No. Bleach solutions should be diluted and poured onto concrete or ground away from storm drains. Oxygen bleach breaks down into oxygen and water, so it is safer but still avoid dumping in large quantities.
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Sir Cleans a Lot is a professional home cleaning specialist with over 10 years of hands-on experience. He has helped thousands of homeowners tackle stubborn stains, eliminate mold, and keep their homes spotless using practical, science-backed methods. When he’s not testing the latest cleaning products or researching stain removal techniques, he’s sharing his expertise to make cleaning easier for everyone.