How to Bleach Concrete Patio: Step-by-Step Guide

Mix 8 oz of household bleach (sodium hypochlorite) into 1 gallon of water, apply it to your damp concrete patio, let it sit for 5 to 10 minutes, scrub with a stiff brush or rinse with a pressure washer, and then rinse thoroughly with clean water. That process removes mildew, algae, and most organic dark staining from concrete, and it's what I reach for first when a patio looks dingy, green, or covered in black spots.

That said, bleach only works on biological and organic discoloration. Before you mix anything, it helps to spend two minutes figuring out what you're actually dealing with, because rust stains, efflorescence, and oil won't budge with bleach no matter how long you leave it on. The sections below walk through the whole process, from safety gear to rinsing, plus what to do when bleach isn't enough. Can you clean a patio with bleach?

Safety prep before you open the bleach

Bleach is effective but it's not something to be casual with. A few minutes of prep prevents damage to your plants, your skin, and nearby surfaces.

• Wear chemical-resistant gloves (nitrile or rubber) and safety glasses. Bleach splashed in the eyes is a real hazard, especially when you're scrubbing.
• Put on old clothes you don't mind ruining. Even diluted bleach can strip color from fabric.
• Work in a well-ventilated area. Outdoors is already better than inside, but avoid doing this on a still, windless day in a enclosed courtyard.
• Wet down any plants, grass, or shrubs near the patio with plain water before you start. The pre-soak dilutes any bleach runoff and protects roots.
• Cover nearby flower beds with plastic sheeting if they're very close to the work area.
• Never mix bleach with vinegar, ammonia, rust removers, toilet bowl cleaners, or any acid-based concrete cleaner. Mixing bleach with an acid produces chlorine gas — which is genuinely dangerous. This also means you can't use vinegar as a rinse or follow-up treatment right after bleaching.
• Make your bleach solution fresh each time you use it. Bleach loses strength quickly once diluted, so a day-old mix won't perform as well.

Figure out what's actually causing the discoloration

Bleach is highly effective against organic and biological growth but does nothing for mineral or metal-based staining. Identifying the problem first saves you a wasted afternoon.

Mold, mildew, and algae (bleach works great here)

These are the most common culprits on shaded or damp patios. Mold and mildew usually show up as black, grey, or greenish patches, often in low spots or along edges where water sits. Algae is typically a slick green or dark green film. All of these are organic growth and respond well to sodium hypochlorite.

General grime and organic staining (bleach works)

Leaf tannins, dirt, and general darkening from foot traffic and weathering are usually organic in nature. Bleach lightens these well, especially when combined with scrubbing.

Efflorescence, white, chalky deposits (bleach won't help)

If you see a white, powdery, or chalky film on the concrete surface, that's efflorescence, mineral salts that have migrated to the surface as moisture evaporates. Bleach has no effect on it. A dilute acid solution (like a 1-part-vinegar-to-4-parts-water rinse as a test, or a proper concrete efflorescence remover with phosphoric or hydrochloric acid) is what actually dissolves these deposits. Just make sure you fully rinse the bleach away and let the concrete dry before switching to any acid-based approach, because mixing the two is dangerous.

Rust stains (bleach won't help, may make it worse)

Orange or reddish-brown stains near metal furniture legs, planters, or pipe outlets are iron oxide (rust). Bleach doesn't remove rust and can actually make it look more vivid orange. You need a rust-specific concrete remover, more on that in the troubleshooting section.

Grease and oil (bleach is limited here)

Dark, oily spots from a grill or car overhang are hydrocarbons. Bleach won't dissolve them. A degreaser or a baking soda paste scrub works better as a first step, though bleach can help clean up residual discoloration afterward.

The bleach process, step by step

This is the method I use for mildew, algae, and general organic darkening. It works on plain concrete patios and sealed concrete. If you have stamped or colored concrete, test in a small hidden area first because pigmented concrete can be sensitive to bleach at higher concentrations.

What product to use

Standard household bleach (around 5.25 to 6.15% sodium hypochlorite) works well. Clorox also makes an Outdoor Bleach line specifically formulated for exterior mold and mildew work, which is what I'd recommend if your patio is heavily affected. Avoid splash-free or scented bleach for this, they often have additives that leave residue on concrete. Whatever you buy, check the label for sodium hypochlorite as the active ingredient.

Dilution

The right mix is 8 oz (one cup) of bleach per 1 gallon of water. This lands you in a safe and effective concentration range, strong enough to kill biological growth, dilute enough that you're not risking surface damage or creating an overwhelming chemical environment. Don't go straight bleach on concrete. It's overkill, harder to rinse, and can leave streaks or interfere with the surface over time. Mix the solution in a plastic bucket (not metal) or a garden sprayer.

Application

1. Wet the concrete surface with plain water first. This helps the bleach solution spread evenly and prevents it from being immediately absorbed into dry concrete before it has time to work.
2. Apply the bleach solution using a garden sprayer, a mop, or a stiff-bristled brush. Work in manageable sections — around 50 to 100 square feet at a time — so the solution doesn't dry out before you rinse.
3. Make sure you get even coverage. Uneven application is the most common reason patios end up with lighter patches or streaks.
4. Let the solution dwell on the surface for 5 to 10 minutes. For heavy mildew or stubborn algae, you can push to 10 minutes, but watch the surface — if it starts to dry out, mist it lightly with more solution. Don't let it sit for 30+ minutes and walk away.
5. While the solution is dwelling, agitate the surface with a stiff deck brush. This is the step most people skip, and it makes a real difference on textured concrete. For heavy growth, put some elbow into it.
6. Rinse thoroughly with clean water. Start at the highest point and work down toward drains or the lawn edge.

Dwell time, scrubbing, and pressure washing options

The 5-to-10-minute dwell time is a practical sweet spot. If you are wondering how long to leave sodium hypochlorite on your patio, stick with the dwell-time guidance in the next section and reapply only if the surface starts to dry how long to leave sodium hypochlorite on patio. Some guidance extends this to 15 minutes for heavily soiled surfaces, but the key rule is that the solution should stay wet on the surface the whole time, if it dries, it can leave residue and uneven results. In direct sun or dry heat, you may need to reapply partway through.

For scrubbing, a stiff-bristled deck brush (the kind with a long handle) is your best tool without power equipment. Put real pressure on it and scrub in a back-and-forth motion, not circles, to avoid swirl marks on smoother concrete. Let the bleach do the chemical work and the brush do the mechanical work together.

If you have a pressure washer, use it on the rinse step rather than the application step. Apply the bleach solution by hand or sprayer, let it dwell, scrub if needed, then pressure wash at a medium setting (around 1200 to 1500 PSI works well on standard concrete) to blast off the loosened growth and bleach residue. Avoid concentrating the nozzle in one spot too long on older or pitted concrete, it can etch the surface.

Rinsing, neutralizing, and avoiding streaks

Rinsing is where a lot of people rush and then end up with a streaky or blotchy patio. Rinse the entire treated area generously with clean water, working from the top of any slope toward the drain. Don't stop at a quick spray, you want enough water flow to fully flush the bleach residue off the surface. If the concrete surface still feels slippery after the first rinse, go again.

You don't need to neutralize bleach on concrete as a mandatory step the way you might with muriatic acid. Bleach breaks down relatively quickly, and a thorough water rinse is the standard finish. That said, if you want to be cautious (especially near plants), a second flush of plain water over the surrounding soil after you're done helps dilute any runoff that reached the edges.

To avoid streaks: apply evenly, keep the solution wet during the dwell period, and rinse in long consistent strokes rather than random bursts. Streaks usually happen when bleach dries unevenly on the surface before it's rinsed, or when you apply too much in one spot. Working in sections and rinsing each section before moving on helps a lot on large patios.

Keeping it clean longer

Once the patio is clean and dry (give it a full 24 to 48 hours), applying a concrete sealer is the best thing you can do to slow regrowth. Sealers reduce water absorption, which is what mildew and algae thrive on. Even a basic penetrating sealer makes a noticeable difference. Trim back any overhanging trees or shrubs that keep sections of the patio in constant shade, shade and moisture are the main reasons mildew keeps coming back.

Alternatives to bleach

Bleach is the most effective option for biological staining on concrete, but there are real reasons to consider alternatives, pets that walk on the patio, nearby vegetable gardens, or just a preference for less aggressive chemistry.

If you want to avoid bleach entirely, oxygen bleach (sodium percarbonate-based products like OxiClean) is my go-to recommendation. It works more slowly than sodium hypochlorite and needs a longer dwell time, but it's considerably gentler on surrounding vegetation and still targets organic growth effectively. Mix per the product directions, apply to wet concrete, let it sit 15 to 20 minutes, scrub, and rinse. You may need a second application for heavy mildew.

Vinegar is fine for light surface mildew or as a DIY test for efflorescence, but it's genuinely limited on heavy algae or dark mildew staining on concrete. It also can't be used right after bleach, if you want to switch to vinegar for any reason, rinse the concrete thoroughly and wait for it to dry completely before applying any acid-based solution.

Troubleshooting: when bleach doesn't fully work

Patchy or uneven results

This is almost always an application problem. If some areas lightened and others didn't, you likely had uneven coverage or the solution dried in spots before rinsing. Re-wet the whole surface, apply another round of fresh bleach solution with careful attention to even coverage, scrub thoroughly, and rinse completely. A garden sprayer gives much more consistent coverage than a mop for large areas.

Stains fading slowly or not at all

If organic staining is very deep-set or the concrete is heavily textured, one treatment may not be enough. A second application after the first one dries is fine and usually handles persistent mildew. If after two rounds you're still not seeing improvement, consider whether the stain is actually organic, rust, efflorescence, and oil won't respond to bleach no matter how many times you apply it.

Rust stains that bleach won't touch

Use a dedicated rust remover for concrete, products containing oxalic acid or a similar rust-dissolving agent are formulated for this. Follow the product directions carefully and do not apply on a surface that still has bleach residue (rinse, let dry). Rust removers are acid-based and the mixing hazard with bleach is real.

Efflorescence that bleach left behind or made visible

If your concrete has a white, chalky film after bleaching, that's efflorescence, not bleach residue. The bleach removed the biological growth that was masking the mineral deposits. An acid-based efflorescence remover (phosphoric or hydrochloric acid, diluted per label directions) is what you need. Again, fully rinse the bleach and let the concrete dry before applying anything acid-based.

Surface etching or texture change

If the concrete surface looks rougher or pitted after treatment, that's unlikely to be from diluted bleach at the recommended concentration. More commonly, this happens from using muriatic acid or from aggressive pressure washing at too close a range. Bleach at 8 oz per gallon is generally safe for concrete surfaces when rinsed properly.

When to consider muriatic acid

Muriatic acid (dilute hydrochloric acid) is sometimes used for heavy-duty concrete cleaning, especially for efflorescence or to etch concrete before sealing or painting. It is significantly more aggressive than bleach and requires a higher level of safety precautions: full eye protection, acid-resistant gloves, and careful neutralization with a baking soda solution after treatment. Never use muriatic acid on the same visit as bleach without complete rinsing and drying in between. If your patio has staining that survives two rounds of bleach treatment and you've ruled out rust and efflorescence, a professional cleaning service with the right equipment and chemistry is worth the call rather than jumping straight to muriatic acid without experience.