Mortared Flagstone Patio

Intro

Flagstone is fractured or cleft into flat slabs of various lengths, 2 inches or more thick, with random edges. The flagstone most commonly used for patios includes bluestone, limestone, redstone, sandstone, granite, and slate. Irregular shapes suit flagstone to both casual free-form and formal geometric design schemes.

Cut stone is flagstone finished with straight edges and square corners. It ranges in size from about 1 foot to 4 feet across and comes in different thicknesses.

Whatever type of flagstone you choose, it must be at least 2 inches thick to avoid breakage. A ton of stone covers about 120 square feet; order 5 percent more for breakage. Large stones cover a surface more quickly than smaller pieces but may prove harder to move, cut, and design.

Unlike ceramic tile, you can set flagstone in a sand base. A mortared installation, however, will give you years of maintenance-free service. A mortared patio requires a slab to provide a solid base. Cleft stone installations require an exterior mortar, generally Type M (which has high compressive strength) or Type S (high lateral strength).

Checklist

Time
16 to 20 hours for a 10x10-foot patio, not counting slab installation

Tools
Hammer, small sledgehammer, brick set, carpenter's pencil, mason's trowel, rubber mallet, mortar box, sponge, shovel, mortar bag, height gauge

Skills
Troweling mortar and setting flagstone, using a mortar bag, cutting stone

Prep
Install new slab or repair an existing one

Materials
Flagstone, mortar, 2x lumber

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Step 1

Lay out your pattern in a dry run next to the site. Mix enough mortar for about a 3x3-foot section, and trowel a 1-inch thickness on the slab. Then lift your stones from your trial run and set them in the mortar in the same pattern.

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Step 2

Set the larger stones first, keeping them in the pattern and using a height gauge to set them at consistent height. Push the stones down; don't slide them. Fill voids with smaller stones, cutting the stones to fit and leveling them with a rubber mallet.

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Step 3

Check the stones for level -- pull out low stones, add mortar, and reset them. Tap down the high stones. If tapping them down won't level them, lift them and scoop out just enough mortar to make them level. Clean off any mortar spills with a wet broom before you lay the next section. Don't wait until you've finished the patio -- the mortar will set on the first sections and you won't be able to get it off. Let the mortar cure three to four days, then mortar the joints.

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Step 4

Mix mortar in a mortar box and fill the joints using a pointing trowel or mortar bag. The bag squeezes mortar through a spout into the joints -- it's less messy and will reduce cleanup chores. Clean spilled mortar right away with a wet sponge. When the mortar holds a thumbprint, finish the joints with a striking tool. Cover the surface with plastic or burlap (keep burlap wet) and let it cure for three to four days.

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