How to Clean a Flagstone Patio

Start by looking at the joints

Before choosing a method, look between the stones. Flagstone patios are built two ways. Some have stones set in a mortar bed with mortared joints. Others have stones laid on sand or gravel with open joints filled with gravel, decomposed granite or ground cover. The joints decide how much water and pressure the patio can take.

Mortared patios tolerate more, as long as the mortar is sound. Patios with loose joints need a soft touch, because a pressure washer will blow the gravel or sand straight out and can undermine the stones.

Know your stone

Flagstone is a general name, not one rock. In LA yards it is commonly sandstone, slate, quartzite or limestone. Quartzite is hard and forgiving. Slate and sandstone are layered and can flake, also called spalling, when water and pressure get under a thin top layer. Limestone is calcium-based and reacts with acidic cleaners, so stick to neutral or mild alkaline products.

If you see thin chips peeling off the surface, treat the whole patio as delicate. More pressure will only speed up the flaking.

A gentle method that works

Sweep and blow off leaves, then pick out weeds by hand where you can. Pre-wet the stone. Apply a mild masonry cleaner or a diluted oxygen-based cleaner for algae and dark organic spots, and let it dwell without drying. Scrub the dirtiest stones with a stiff nylon brush.

Rinse with a wide fan tip held back from the surface, moving with the direction of the joints rather than straight into them. A rotating surface cleaner works great on flat concrete, but on irregular flagstone with lips and uneven edges it can catch, so wand work and brushing are often the better choice. Finish by rinsing the whole area so no cleaner is left in the joints.

Moss, algae and weeds

Shaded flagstone that stays damp through the rainy season, roughly November to March, grows moss in the joints and green or black algae on the stones. A cleaner with good dwell time does most of the work. Brushing loosens what is left.

Weeds in gravel joints come back from roots, so cleaning alone will not keep them away. Pull them, top up the gravel or decomposed granite, and consider whether a sprinkler is soaking that corner more than it needs. Our guide to moss and weeds in concrete joints covers longer-term control.

Mistakes that cause damage

The big ones are using a zero-degree or narrow tip, holding the nozzle close to a stone edge, and aiming straight into mortar joints. Each can chip stone, lift flakes or blow out old mortar. Using acid on limestone or on stones with calcium-rich mortar is another, since it can etch the stone and weaken the joints.

Do not try to clean over loose stones either. Water gets underneath and makes them rock more. Reset loose stones first, then clean.

Avoid letting cleaner dry on the stone in full sun, which can leave a haze, and keep runoff out of the street. Los Angeles Municipal Code section 64.70.02 prohibits discharging untreated wash water from paved-surface washing into the storm drain system.

What to expect afterward

Most flagstone comes back to an even, natural color, and it often looks darker and richer while wet. Some stones naturally vary in color, so a patio may look less uniform once the dirt film is gone. Deep rust, old oil and long-term tannin stains from leaves may lighten without fully disappearing.

Cracked or crumbling mortar, missing joint material and settled stones are repair items. A clean patio actually makes them easier to see, which is useful if you have been meaning to have a mason look at it.

Cost and planning

As of October 2026, our patio cleaning rate is $0.65 per square foot, with a $195 minimum per visit. A 400 square foot flagstone patio comes to $260. When Say Wash It cleans flagstone, we usually set aside more hand time than on smooth concrete because of the joints and edges, and the flat price is confirmed with you before work begins. The instant quote on this page gives you a starting number.