Brick or flagstone set in mortar can transform a drab concrete slab into an attractive focus for relaxing and entertaining. Unless you have recently poured a new slab, test it for soundness before proceeding. Regardless of results—and no matter how new the concrete—use a metal straightedge to check the entire surface for high and low spots. Bricks and flagstones may rock on high spots and break loose. Water can collect in low spots and cause mortar to deteriorate.
Flatten high spots with a mason’s rubbing brick or with a silicon carbide wheel in an electric drill. Break up low spots covering more than 1 square foot with a cold chisel and fill them as you would a hole.
Find the area of the slab. (For a free-form slab, use the method shown on page 122.) You will need 4½ paving bricks to 1 square foot. For materials with nonstandard dimensions and for flagstones, consult a dealer. In either case, add about 5 percent for breakage and repair. Some flagstones are soft enough to be cut with a brickset; others are so hard that they must be scored with a masonry blade in a circular saw.
One cubic foot of mortar mix is enough to lay about 35 bricks or 12 square feet of flagstones. Buy additional mix to grout joints between bricks and flagstones.
Arranging the bricks or stones in a dry run is an essential first step. With bricks, orient the long sides with the slope of the slab. Doing so channels rainwater away from the house. Use a piece of ½-inch plywood to space a row of bricks along two adjacent sides of the slab, trimming bricks as needed (page 120) to fill the rows. Leave the dry run in place as a guide for a string marker.
In a dry run of flagstones, vary the sizes of adjacent stones to avoid long joint lines. Spaces between stones should range from ½ inch to 2 inches wide. Large gaps can be filled with pieces cut from stones that overlap.
Always leave the expansion joint between the slab and the house uncovered. At the end of the job, after the mortar has set, press polyethylene rope into the joint, then cover the rope with self-leveling polysulfide or silicone caulk. If the slab is divided by expansion joints, matching joints are required when veneering with brick but not with flagstones, which can withstand expansion forces that can crack or loosen brick.
You may want to install edging around brick or flagstone to protect vulnerable corners and cover the sides of the patio. Set pressure-treated 2-by-8s or 2-by-10s in the ground on edge, even with the veneer surface. Metal edging is available for free-form slabs. An application of masonry sealant helps prevent the growth of moss and mildew, which can be unsightly and slippery.
Setting brick in mortar
Use a large mason’s trowel to spread a ½-inch layer of mortar on the rough face of the first brick in the dry run. Make a shallow groove in the mortar with the point of the trowel. Set the brick at the edge of the slab and tamp it firmly with the trowel handle to level it. Working one brick at a time, use the ½-inch plywood spacer to position succeeding bricks. Level the bricks before proceeding. Stake a guide string to align bricks of the second course with the second brick of the dry run. Where a house wall prevents the use of a stake, tie the string to a brick and set it on a scrap of ½-inch plywood. Level the bricks in each course, then reposition the guide string for the next. Wait at least 24 hours before proceeding to the next step.
Grouting the joints
Fill a grout bag with a ½-inch nozzle about two-thirds full and roll the top to squeeze grout into the gaps between bricks. With a ½-inch joint filler, pack the grout into the gaps to a level slightly below the brick surfaces to make a drainage channel. Smooth the grout with the joint filler. Wait an hour, then remove ragged bits of grout with the trowel. Three hours later, smooth the joints with a wire brush, and hose the patio clean. Allow the grout to cure for several days. Then remove any grout stains on the bricks with a mild solution of muriatic acid and a wire brush. Hose away the residue.
Use leather-palmed gloves to protect your hands from rough edges of brick or stone. Irritants in mortar and grout call for a dust mask and gloves. Wear goggles when grinding or chipping at a slab, mixing mortar, cutting brick or stone, or when working with muriatic acid.
Laying out a dry run
Arrange the flagstones on the slab allowing ½- to 2-inch gaps between adjacent stones. Ignore any expansion joints except the one adjacent to the house wall. Where a stone hangs over the edge of the slab, mark a cutting line on it with chalk, using the edge of the slab as a guide. Where stones overlap each other, mark one of the stones, allowing space for the mortar joint between them. Cutting lines should be straight; you can approximate a curve with several short cuts.
Cutting a stone
Remove marked stones one by one and score each for cutting. To do so, hold a brick-set against the chalk line and tap it several times with a maul (above, left); then move the brickset along the line and tap again. For stones that are more than 1 inch thick, score a corresponding line on the other side by extending the first line down the edges of the stone and marking a connecting line on the back. Rest the stone on a board, with the scored line no more than ¼ inch beyond the edge. Then tap the overhang with the maul to snap it off (above, right). As each stone is cut, return it to its position on the slab.
Laying the stones in mortar
At a corner or along an edge, set a section of stones about 4 by 4 feet in area next to the slab as they were arranged in the dry run. Moisten the exposed slab with water, then trowel on a 1-inch-thick mortar bed. Position the stones on the mortar, seating them with a rubber mallet. Fill any large spaces with pieces of cut stone.
Making an even surface
Examine the flagstones just laid. If a stone sits too high, lift it aside and scoop out some of the mortar with a pointing trowel. If a stone is too low, add mortar and smooth it with the trowel to ensure a good bond. Finish each section by using a pointing trowel or a tongue depressor to remove any excess mortar that has pushed up between stones. Sponge stray mortar off the stones.
Grouting the joints
After the mortar has cured 24 to 48 hours, trowel grout into the joints. Then use a joint filler to compact the grout to a depth of