Things You'll Need
Mortar or chipping chisel
Hand brush or whisk broom
Portland concrete type I-II
Masonry sand (3 parts to every one part of concrete)
Hose connected to water spigot
Flagstone replacement piece
Siloxane sealer (if other stones are sealed)
Flagstone steps can loosen over time, or excessive moisture can get in, weaken and crack them, requiring replacement. Replacing a flagstone step may seem like a daunting task; however, with the right tools, some sand and Portland cement mix you can quickly replace the step. Be sure to select a stone close in color to the one you are replacing and have the stone supply company cut your stone to size.
Removing the Old Stone
Put on protective goggles and work gloves.
Tap out the old stone using a chisel and hammer. Angle the chisel slightly as you tap to get under the step that needs replacing.
Chip away all old mortar--on joint edges and bed--once the stone has been removed.
Brush all debris from the area you will be setting the new stone. Crumbled old mortar, dust or debris will prevent proper adherence of the new mortar mix.
Attaching New Flagstone
Mix concrete and sand (three parts sand to one part cement) in a wheelbarrow. Add water according to directions. You do not want the mix wet, but rather on the dry side.
Wet with water and a sponge the concrete bed where the step will be placed. This will help the mortar mix adhere better.
Trowel the mortar mix onto the concrete slab to a depth of a half-inch and onto the back side of the flagstone.
Set the flagstone into the mortar mix, pressing it into place. Use a level to make sure the stone is straight and flat.
Gently wipe off any mortar residue from the face of the stone with sponge and water.
Using the tuck point, tuck each joint with additional mortar mix, filling each joint to the top of the stone, scraping gently with the trowel for a smooth, even finish.
Allow the flagstone to set and cure overnight.
Apply the stone sealer with a roller brush according to the product's directions, if the rest of your steps have this protector.
Allow to dry overnight.
Measure the thickness of your other stone steps before purchasing your replacement. Flagstone steps are typically 1¼ to 1½ inches thick.
Don’t do this job if rain is expected within 24 hours, as this can weaken the set mortar.