How to Clean a Flagstone Patio | A Complete, No-Flop Approach

A grimy flagstone patio can go from beautiful focal point to dingy eyesore fast. The wrong cleaning approach — especially cranking up a pressure washer or using harsh acids without care — can flake the stone, ruin its natural color, and leave you with a repair bill instead of a clean patio. The right approach is methodical, uses the gentlest effective method first, and always includes a spot test. Here’s exactly how to do it.

Before You Start: Sweep, Test, and Prep

Sweep all loose dirt, leaves, and debris with a stiff broom. Skip this step and you’re just grinding grit into the stone as you scrub. Then do a spot test in an out-of-the-way corner — especially if you plan to use bleach, acid, or a pressure washer — to make sure the stone holds up. Wet any nearby plants thoroughly before using chemicals to prevent chemical burn.

The Manual Cleaning Methods

For smaller patios (under 50 square feet) or light grime, manual scrubbing is the safest and most effective route. You have three solid options depending on what you’re cleaning.

Soapy water for everyday dirt: Mix warm water with a squirt of mild dish soap. Scrub with a stiff-bristle or 3-inch wire brush — a 1-inch brush is too narrow for a patio. Let the solution sit briefly but don’t let it dry on the stone. Rinse with a garden hose.

Bleach for moss and mold: Wet the stones and surrounding soil first (after rain is ideal). Apply with a tank sprayer, being careful not to overspray plants. Wait 10 minutes, scrub with a car-washing brush, then rinse thoroughly.

Household remedies for light stains: For mild dirt, dissolve 1 tablespoon of baking soda in 1 liter of water, let it sit an hour, then rinse. Let it sit 5 hours, then scrub and rinse.

When and How to Pressure Wash

For patios larger than 50 square feet or deep grime that scrubbing won’t touch, a pressure washer saves hours. But flagstone is fragile — high pressure destroys it. Start holding the wand far from the stone and move closer slowly until coatings lift and the stone’s natural color returns. Stop immediately if you see any flaking or shaling.

We have a full roundup of our top tested cleaners for flagstone patios here if you’d like product-specific recommendations.

Electric pressure washers are generally safer than gas models, which can be too powerful. Never let the wand linger in one spot, and never use the most powerful (red) tip unless you’re stripping old paint — even then, go cautiously.

Post-Cleaning: Joints, Drying, and Maintenance

Let the patio dry completely. For dry-laid patios, sweep stone dust into the joints, hose it gently to settle it, let dry again, and repeat as needed. For mortared joints, remove old mortar at least half an inch down, clean joints with a blower and hose alternately, then fill with a very dry mortar mix.

If you choose to seal the stone (some experts recommend against sealing exterior stone), use only a penetrating acrylic sealer with a matte finish. Glossy sealers ruin the natural look and can trap moisture. Clear snow promptly in winter to prevent ice from seeping into joints and causing freeze-thaw cracking.

Common Mistakes to Avoid

• Starting the pressure washer too close: Causes stone flaking; always start far and move closer slowly.
• Using a glossy sealer: Compromises the natural beauty of flagstone.
• Letting acid or bleach dry on the stone: Leads to staining; scrub and rinse before it dries.
• Ignoring the joints: Unfilled or dirty joints invite weeds and moss to return quickly.
• Using a 1-inch brush: Takes forever and misses grime; use a 3-inch wire brush for large areas.