How to Clean Algae From Patio Stones: A Step by Step Guide

Introduction: Why removing algae matters and what you will learn

That slick green film on your patio is more than an eyesore, it is a safety and maintenance problem. Algae makes stones dangerously slippery when wet, causes stubborn dark stains, and traps moisture against joints, which speeds up cracking and erosion over time. Left untreated, small patches grow into full coverage, and repairs get more expensive.

This short guide shows you exactly how to clean algae from patio stones, step by step. You will learn how to assess the severity, pick a safe cleaning solution, use a pressure washer without damaging pavers, scrub and rinse effectively, tackle stubborn stains, and finish with sealing and simple maintenance to prevent regrowth. I will also list the tools and safety gear you need so you can get the job done fast and right.

Quick assessment: Identify algae, severity, and stone type

First glance, you want to confirm it is algae, not mold or moss. Algae looks like slimy green or black films that rub off when wet; moss is fuzzy and lifts easily. Do a fingertip swipe after wetting the area. If it smears, it is algae.

Next, note stone type. Porous stones like sandstone, limestone, and some older concrete pavers absorb water and stain easily. Dense stones such as granite and some modern pavers tolerate scrubbing and stronger cleaners. Check an inconspicuous spot before any treatment.

Also inspect joints. Are they sand, polymeric sand, or mortar? Loose sand will wash out under pressure, polymeric sand can be ruined by high pressure, mortar can crack.

Decision rule: light surface algae, nonporous stone, stable joints, go with a gentle approach, water, broom, mild detergent or low PSI wash. Heavy growth on porous stone or stubborn staining needs a more aggressive method, test a small area first.

Safety and tools you need before you start

Before you tackle how to clean algae from patio stones, suit up and assemble tools. PPE: chemical resistant gloves, safety goggles, a mask rated for dust and fumes, and waterproof boots. If you plan to use concentrated bleach or a pressure washer, add a respirator and ear protection.

Essentials: stiff broom, stiff nylon scrub brush with long handle, bucket, pump sprayer for cleaners, garden hose or pressure washer set to 1500 to 2500 PSI, and oxygen bleach or masonry patio cleaner. Useful extras: plastic sheeting to protect plants, a joint scraper, jointing sand, and a wet vacuum for rinse water.

Safe handling tips: test a small patch first, dilute cleaners per label, never mix bleach with vinegar or ammonia, rinse thoroughly, and work top to bottom to avoid streaks.

Simple methods for light algae: sweep, scrub, and soap

Start by dry sweeping the area with a stiff broom to remove leaves and loose grit. Work in small sections, about 3 by 3 feet, so you can focus on trouble spots. Mix warm water with a few tablespoons of mild dish soap in a bucket (roughly 2 tablespoons per gallon). For light growth, this simple soap solution is enough.

Dip a stiff bristled scrub brush and scrub in firm circular motions, concentrating on joints and textured stone. Let the soapy water sit for 5 to 10 minutes if the algae looks slimy, then scrub again. Rinse each section with a garden hose, not a high pressure washer, to avoid washing out joint sand. Repeat any stubborn areas; two or three short passes usually do the trick.

For spot treatment, sprinkle baking soda, add a little water to make a paste, scrub, then rinse. Wear gloves and eye protection while you work.

Effective cleaners explained: bleach, vinegar, and sodium percarbonate

Pick the cleaner based on severity and site conditions. For heavy, stubborn growth use household bleach; for light, recent algae use white vinegar; for eco friendly, effective lifting use sodium percarbonate, also called oxygen bleach. Here is how to mix and apply each.

Bleach
Dilution: 1 part household bleach to 10 parts water.
Steps: wet nearby plants, apply solution, scrub with a stiff brush, wait 10 minutes, then rinse thoroughly.
Precautions: avoids runoff onto plants and into storm drains, wear gloves and eye protection.

Vinegar
Dilution: 1:1 vinegar to water for general cleaning; use straight 5 percent white vinegar for small patches.
Steps: spray or pour, let soak 15 to 30 minutes, agitate, then rinse.
Precautions: vinegar alters soil pH, protect vegetation and rinse surrounding soil.

Sodium percarbonate
Dilution: dissolve 2 to 4 tablespoons per gallon of hot water.
Steps: apply warm solution, let fizz for 15 to 30 minutes, scrub, then rinse.
Precautions: safer for plants than bleach, but still avoid concentrated runoff and follow manufacturer instructions.

For any method, test a small inconspicuous area first, and repeat treatments if needed when learning how to clean algae from patio stones.

Pressure washing the right way: settings and step by step

Pressure washing is great for heavy algae, but only when the stone can take the force. Test a small hidden area first. For soft stones like sandstone or limestone, use 500 to 1,200 psi and a wide 40 degree nozzle, keeping the wand 12 to 24 inches away. For slate or bluestone, 1,000 to 1,800 psi with a 25 degree nozzle is safer. For concrete pavers or poured concrete, 2,000 to 3,000 psi and a 25 degree nozzle will remove stubborn growth fast.

Safe sequence to follow
Sweep and wet the patio, apply a biodegradable algae cleaner, let it sit 5 to 10 minutes.
Start with the lowest pressure and the widest nozzle, work a 3 by 3 foot test patch.
Move the wand steadily at a 45 degree angle, top to bottom, keeping distance consistent.
Narrow nozzle or increase pressure only if needed; avoid spraying directly into joints.
Finish by rinsing and letting the area dry before sealing.

Stubborn algae and stains: targeted fixes and joint care

For small, stubborn patches, treat them in place. Mix one part household bleach with ten parts water, apply with a watering can or pump sprayer, wait five minutes, then scrub with a stiff nylon brush and rinse. For a gentler option, use white vinegar straight on the spot, scrub, then rinse. Use a stiff brush across the stone face for algae stains, but switch to a narrow joint brush for grooves and seams.

If moss or roots occupy joints, dig out the material with a screwdriver or pointing tool until you reach clean stone. Remove root growth by pulling or cutting back to the source, then refill joints, either repointing with fresh mortar for flagstone, or sweeping in new joint sand for pavers and compacting it well. Finish by applying a sealant or polymeric jointing product to prevent repeat growth.

Prevent algae from returning: drainage, sunlight, and sealing

Once you know how to clean algae from patio stones, the smart move is to stop it coming back. Start with drainage. Ensure the patio slopes away from the house, about 1/8 to 1/4 inch per foot, clear nearby gutters, and add a gravel trench or French drain where water pools. Replace loose joint sand with polymeric sand to limit moisture in the joints.

Increase sunlight by pruning trees and shrubs so the patio gets four to six hours of direct sun. Thin the canopy rather than removing large branches, and cut back groundcover that hugs the stones.

Seal the surface with a penetrating silane or siloxane sealer for porous stone, or use a solvent acrylic sealer for stronger color enhancement. Apply with a pump sprayer, follow label directions, and test a small area first.

Maintenance schedule: sweep weekly, rinse monthly, treat shady spots with an algaecide yearly, and deep clean then reseal every one to three years.

Conclusion and final insights: quick checklist you can use today

Quick recap: learning how to clean algae from patio stones comes down to choosing the right method for your stone, preparing the area, using the correct cleaner, and protecting plants and yourself.

Checklist you can use today
Inspect the stone type and damage, note loose joints and stained spots.
Test any cleaner in a hidden area for 24 hours.
Choose a method: pressure wash for large, durable pavers; stiff brush plus oxygen bleach for softer natural stone; diluted household bleach, one part bleach to ten parts water, for heavy concrete algae.
Protect foliage with water and plastic, wear gloves and eye protection.
Scrub or wash, rinse thoroughly, let the stones dry completely.
Consider a sealant to slow regrowth.

Start gentle, then step up intensity based on results and skill level.