How to Remove Mildew From a Concrete Patio
Lawn Tool Hub is reader-supported. We earn a commission from qualifying purchases made through links on this page, at no extra cost to you. Learn more.
The short answer
Apply a mildewcide cleaner — sodium hypochlorite based, or oxygen bleach for plant-heavy areas — at full label strength, let it dwell 10–20 minutes without drying, scrub with a stiff brush and rinse. Mildew returns wherever the concrete stays damp, so the lasting fix is sunlight and drainage: trim overhanging branches and clear the low spots where water sits.
Mildew Stain Remover for Concrete
A mildewcide formula that kills the growth in the pores rather than just rinsing the surface.
Mildew is not dirt — it is living growth with roots in the pores of the concrete. That is why sweeping and rinsing does nothing and why the spots come back in the same places every year. You have to kill it, then change the conditions that fed it.
Mildew, Algae or Stain?
| What you see | Likely cause | Treatment |
|---|---|---|
| Black speckles and blotches | Mildew / mold | Mildewcide cleaner, 10–20 min dwell |
| Uniform green film, slippery | Algae | Oxygen bleach, stiff brush |
| Brown or tan patches under trees | Leaf tannin | Oxygen bleach, two passes |
| White powdery haze | Efflorescence (salts) | Efflorescence remover, not bleach |
The Removal Steps
- 1Sweep and remove furniture. Pre-wet nearby grass and beds thoroughly.
- 2Mix the cleaner per label. Do not improvise stronger — over-strength chlorine will lighten colored concrete.
- 3Apply with a pump sprayer or watering can, working in shade or on an overcast day so it does not flash-dry.
- 4Dwell 10–20 minutes. Re-mist if the surface starts drying.
- 5Scrub with a stiff deck brush, then rinse from the high side down.
- 6Repeat on any spots that are still gray rather than clean — one pass rarely clears heavy growth.
Myth
More pressure removes mildew better.
The Fact
Pressure removes the top layer of concrete along with the visible growth. The spores stay in the pores and regrow within a season. Chemistry plus dwell time is what actually kills it.
Stopping It From Coming Back
- Get sunlight on the slab — trimming one low branch often does more than any cleaner.
- Fix drainage. Standing water after rain marks exactly where mildew will return.
- Keep leaf litter swept off in fall; decomposing leaves feed the growth.
- Move planters off the slab or set them on feet so water can escape underneath.
- Consider a breathable penetrating sealer, which reduces how much moisture the pores hold.
Frequently Asked Questions
- Will bleach damage concrete?
- Diluted per label and rinsed, no. Repeated full-strength applications can lighten colored or stamped concrete, so test a hidden corner first.
- How long does the treatment last?
- One to three years, depending entirely on sun and drainage. A shaded, poorly draining slab may need annual treatment.
- Is mildew on a patio dangerous?
- The main hazard is slipperiness. Wet mildew and algae films are genuinely slick — worth clearing before the season if you have steps.
- Can I use a mildewcide before sealing?
- Yes, and you should. Sealing over live growth traps it and it will show through as dark patches under the sealer.
The Gear We’d Buy
Everything referenced in this guide, in one place.
Mildew Stain Remover for Concrete
Top pickA mildewcide formula that kills the growth in the pores rather than just rinsing the surface.
Typically $20 – $35
Garden Pump Sprayer
Even application is what gives you even dwell time — a watering can leaves streaks.
Typically $20 – $40
Long-Handle Stiff Deck Brush
For agitating after the cleaner has done its dwell.
Typically $20 – $40
Lawn Tool Hub is reader-supported. We earn a commission from qualifying purchases made through links on this page, at no extra cost to you. Learn more.
Key Takeaways
- Mildew needs a mildewcide and dwell time; pressure alone just resurfaces it.
- Never seal over live growth — kill and rinse first.
- Sun and drainage decide whether it comes back.
- Oxygen bleach is the plant-safe substitute for chlorine near beds and lawn edges.
Reviewed by
The Lawn Tool Hub Test Bench
We buy, borrow and break lawn tools on our own yards — clay in the back, sand-cut Bermuda in the front — and write down what actually changed the result.
Keep reading
How to Clean a Patio Without a Pressure Washer
No pressure washer? A stiff brush, the right cleaner and 45 minutes will clear a patio of grime and green film without blowing out the joints.
How to Clean Outdoor Cushions
Mildew, sunscreen and bird droppings all need different treatment. Here is the method by fabric type and the drying step most people skip.
How to Seal a Paver Patio the Right Way
Sealing a paver patio only works if you clean, re-sand and time it correctly. Here is the order of operations, sealer types and how long each one lasts.
Top pick
Mildew Stain Remover for Concrete