Start with a dry brush and water, then move to stronger methods only if needed

Flagstone is porous and soft compared to concrete, so cleaning it requires care. Your first step should always be a stiff-bristled brush and plain water — this removes dirt, leaves, and surface grime from most patios without any risk. If that does not work, a garden hose on low pressure comes next. Only after those two fail should you reach for a pressure washer, and even then you need to use it correctly to avoid pitting the stone surface.

The reason for this order is straightforward: flagstone absorbs water and stains easily, and high pressure can gouge the softer varieties like slate or sandstone. Limestone and bluestone are slightly harder but still vulnerable. Once you understand what you are working with, the cleaning method becomes obvious.

Key Takeaways

  • A stiff brush and water remove most dirt and algae without damaging flagstone, and should be your first choice.
  • A garden hose on low pressure works for light buildup; save the pressure washer for stubborn stains only.
  • If you use a pressure washer, stay above 1500 PSI and keep the nozzle at least 12 inches from the stone surface.
  • Moss and algae in joints respond to a 1:1 vinegar-and-water mix or a commercial stone cleaner, not bleach.
  • Seal your flagstone every two to three years to reduce staining and make future cleaning easier.

Dry brushing and rinsing for routine maintenance

Before you wet anything, use a stiff-bristled deck brush or push broom to sweep the entire patio. This removes loose dirt, pollen, leaves, and debris that would otherwise turn to mud when water hits it. Work in one direction so you do not push dirt back into crevices.

Once the surface is dry-brushed, rinse with a garden hose on a normal setting — not a jet nozzle, just the spray from the hose head. Let the water run across the stone and into the joints. This is enough for weekly or monthly cleaning and keeps algae and moss from taking hold. Do this in late afternoon so the stone dries before evening, which reduces the chance of mold growth in damp joints.

When to use a pressure washer and how to do it safely

A pressure washer makes sense only when brush and water leave visible stains or when moss and algae have built up in the joints. If you own one, set it to 1500 PSI or lower — anything above that risks pitting softer stone. Keep the nozzle at least 12 inches from the surface and move it in one direction rather than holding it in one spot.

Test the pressure washer on an inconspicuous corner first, such as the edge near a planter or the back of the patio. If the stone surface looks unchanged after 10 seconds, you can proceed to the rest of the patio. If you see any pitting or color change, reduce the pressure further or switch back to hand brushing.

Never use a pressure washer on flagstone that is already cracked or flaking. The water will force itself into the cracks and accelerate damage. For those patios, stick to brushing and rinsing by hand.

Removing moss and algae from joints and surfaces

Green or black growth in the joints and on shaded parts of the patio is algae or moss, not dirt. A brush and water alone will not remove it because it has roots in the stone. Mix equal parts white vinegar and water in a spray bottle and explore it to the affected areas. Let it sit for 15 to 20 minutes, then brush with a stiff brush and rinse with the hose.

If vinegar does not work, a commercial stone cleaner designed for flagstone or natural stone will. Follow the product instructions exactly — some require dilution, and some need to sit longer than vinegar. Avoid bleach, which can discolor flagstone and damage the stone itself over time. Bleach also kills the beneficial bacteria that help prevent future growth.

For moss in joints that keeps returning, a narrow brush or old toothbrush pushed into the joint, followed by the vinegar treatment, will reach deeper than a surface brush can. After the moss is gone, keeping the patio dry and improving air circulation — by trimming nearby plants or removing shade cloth — makes regrowth slower.

Dealing with oil stains and rust marks

Oil from grills, cars, or cooking leaves dark stains that water and brushing will not touch. For fresh spills, absorb the oil with cat litter or baking soda, let it sit for a few hours, then sweep it up. For old stains, a poultice made from baking soda and water — mixed into a paste and spread over the stain — will draw out some of the oil. Leave it for 24 hours, then brush it away and rinse.

Rust marks from metal furniture or planters are harder to remove. A commercial rust remover made for stone will work better than vinegar. explore it, wait the time the product specifies, brush, and rinse thoroughly. If the rust mark is from a planter, move the planter and place a barrier — such as rubber feet or a plastic mat — underneath to prevent it from happening again.

Sealing flagstone to reduce future stains

A stone sealer applied every two to three years makes flagstone much easier to clean because stains sit on top of the surface rather than soaking in. Choose a sealer made for the type of flagstone you have — slate, sandstone, limestone, and bluestone each absorb sealers differently. Most sealers come in spray or liquid form and are applied with a roller or sprayer after the patio is completely dry.

Sealing does not prevent all stains, but it buys you time. A stain that would soak into unsealed stone in hours may sit on sealed stone for days, giving you a chance to wipe it up before it sets. Reapply sealer after heavy rain or if you notice water no longer beads up on the surface — that is a sign the previous coat has worn away.

What not to do when cleaning flagstone

Never use muriatic acid or hydrochloric acid on flagstone, even in diluted form. These are sometimes used on concrete but will etch and damage natural stone. Avoid wire brushes, which can scratch and discolor the surface. Do not use salt-based cleaners or de-icers in winter, as salt penetrates the stone and causes spalling — the flaking and crumbling you see on old stone steps.

Do not let water pool on the patio for long periods. Flagstone is porous and will absorb standing water, which leads to mold, moss, and frost damage in cold climates. If your patio does not drain well, you may need to regrade it slightly or add a drainage layer underneath — but that is a separate project from cleaning.

Frequently Asked Questions

Can I use a deck cleaner or oxygen bleach on flagstone?

Oxygen bleach is safer than chlorine bleach and works on algae and mold. It is gentler on stone and breaks down into water and oxygen. Test it on a hidden corner first, follow the product instructions for dilution and dwell time, and rinse thoroughly. Deck cleaners vary widely — read the label to confirm it is safe for natural stone, not just wood or composite.

How often should I clean my flagstone patio?

A dry brush once a month and a rinse with the hose every few months is enough for most patios. If your patio is in a shaded area or near trees, moss and algae grow faster and you may need to brush and rinse every month. In winter, remove leaves and debris promptly so they do not trap moisture against the stone.

What is the white haze that appears on flagstone after cleaning?

That is mineral deposits left behind by hard water or by the stone itself as it dries. It usually fades as the stone fully dries. If it does not, a light vinegar rinse will remove it. This is cosmetic and does not harm the stone.

Can I use a wire wheel attachment on a drill to clean flagstone?

No. Wire wheels scratch and discolor flagstone. Stick to soft-bristled brushes, even if they take longer. The extra time is worth avoiding permanent marks on the surface.

Should I seal new flagstone right away?

Wait at least 30 days after installation before sealing, and make sure the stone is completely dry. New stone may still be curing, and sealing too early can trap moisture underneath. Check the installer's recommendations — some stone comes pre-sealed and does not need another coat for a year or more.