Laying Out Sites for Slabs

Although concrete is very strong in many respects, it has limited tensile (lateral) strength. A slab without a firm, uniform base almost certainly will crack and heave unevenly, leaving you with a difficult repair job. This happens with driveways, for example, where the sand base underneath washes away, allowing the weight of vehicles to crack the slab. If possible, place concrete slabs on undisturbed soil—soil that has never been dug up. In many areas, this means digging down until you reach clay. If you must pour a slab on top of recently dug soil, as would be the case with backfill from a foundation dig, pack the soil by watering it for several days and giving it time to settle. Then compact the earth, using a hand tamper for small areas or a rented vibrating tamper for larger areas.

Rent a Transit - For most jobs, a line level or water level is all you’ll need to lay out a level or accurately sloped slab. For large jobs, consider renting a surveyor’s transit. This is a small telescope-like device that sits on a tripod. By sighting through it, you can determine level points across a distance. Plant the tripod legs in the ground at a point where you easily can see all the slab stakes. Adjust the transit so it remains level as it is rotated. Have a helper hold a marked pole beside each stake and raise it up and down until you can sight the mark through hatchmarks in the transit. Mark where the pole bottom touches the stake.

Install perimeter drainage systems. When rain falls on a large concrete slab, the low ends of sloped sections receive a lot of water quickly. In many cases, this can be solved simply by digging a border and planting flowers or shrubs in a bed covered with wood chips. For more severe puddling, dig a trench and fill it with gravel. For even more drainage, dig a trench at least 12 inches deep and install sloping perforated drainage tile in gravel, as shown. Each of these options can be installed after the slab is poured.

Install a catch basin and dry well for severe drainage problems. For large areas where you anticipate a lot of water that has no place to go, consider sloping the ground toward a catch basin located in the center of the area. The water caught there can be piped out, either into a city sewer system or into a dry well. A dry well is a large hole filled with stones and topped with a layer of gravel. It holds water until it can percolate into the ground.

Check for square using the 3-4-5 method. Drive in a stake at one corner of the slab, such as against the foundation. Working from that fixed point, check for square as you drive stakes for the other corners. Use the 3-4-5 method. Measure 3 feet along one side and make a mark or drive a stake. Then measure 4 feet along the line and mark the spot with a piece of tape. Make sure both measurements begin at exactly the same point. Measure between the 3- and 4-foot marks, moving the line until this distance is exactly 5 feet. You’ll then have a square corner. If you have room or have a large slab, use multiples of 3-4-5: 6-8-10 or 9-12-15.

Water Level - With a water level, you easily can find the same level for two places far from each other, even if you have to turn a corner or two, step down an incline, or climb a couple of stairs. This simple tool consists of two calibrated plastic tubes attached to a garden hose. Drain spigots and fill spigots let you fill the tubes and hose with water. The tool works on the principle that water seeks its own level.

Use a square for walks. To square one side of a long, narrow slab, such as a walk, use a carpenter’s framing square. Square the line from a step, slab, or foundation. Position the second line by measuring off the desired width so the two lines are parallel.

Check by measuring diagonals. As a final check that the slab perimeter is square, measure from corner to corner in each direction. The two measurements should be the same. Make sure you line up the same edge of the tape measure when you do this. Don’t use this method in place of the 3-4-5 method, only as a double-check.

Excavating for Slabs

If your soil is soft, your back is strong, and you have plenty of time, you can dig the hole for a small slab by hand (or you can hire young and willing laborers). For large jobs or in places where the digging is tough, rent a small digging machine or hire an excavating contractor. The process is easy; the digging often is not. You’ll need to remove all organic matter—not only the sod, but also roots 1/2 inch or more in diameter. Many building codes require you to dig down to undisturbed soil (subsoil that has never been dug up).

Tools: Shovels, spade, baby sledgehammer, mason’s line, line level, chalk line.

Mark a curved line. Where your plan calls for a curve in the slab, lay a charged garden hose (turn on the water with the nozzle shut) in position to mark the slab perimeter. Take into account the width of the stakes and forms. Pour flour or sand over the hose. Lift it up and you’ll have an easy-to-follow curved line.

Spare your back as you dig. You’ll spare yourself a lot of pain and fatigue by using your knee, not your back, to push the shovel into the soil. As you dig, simply position your knee against your lower hand and push forward with your knee. You’ll be surprised how much extra force it provides.

1. Mark and trench the perimeter. Mark the location of the slab edges by sprinkling flour or sand over the mason’s lines strung when you laid out the project. Remove the line, but leave your stakes in place. Use the marks as a guide for digging a shallow trench about 1 foot wide. Dig the trench so it extends about 3 inches beyond the outside edge of the slab.

2. Mark slab height on the house. If the new slab butts against an existing structure, snap a chalk line on it to establish where the surface of the slab will meet the structure. Patio surfaces should be about 1 inch below door thresholds to keep rain and snow out of your house. Outdoor slabs should be about 1 inch above the lawn surface. If the slab does not abutt the house, make a mark on one of the corner boards indicating the top of the slab.

3. Set lines and drive stakes. Reattach the mason’s lines. Where they meet the house, set them level with the chalk line that marks the slab top. Pull the line taut and level it using a line level. Mark that spot on one of the outside corner stakes. Measure down from that mark V4 inch per running foot of slab (the distance from the house to an outside corner) and make a second mark. Tie the line securely to the second mark. Repeat this procedure on the other side. Drive in form stakes at a point outside of the line at a distance from the line equal to the thickness of the forming lumber. Be sure the tops of the form stakes are a little below the level of the line.

Caution! When reattaching the mason's lines, be careful not to wrap the lines the wrong way around the stake. If you have any doubts, recheck for square.

4. Excavate the interior. Remove the sod and topsoil to reach the desired depth. You may want to use the sod elsewhere in your yard. If so, undercut it horizontally about 2 inches beneath the surface and cut it into easy-to-handle 8x16-inch sections. Save enough to resod around the edges of the new slab. If you plan to place sand or gravel beneath the slab, excavate at least 5 inches deep. Check the bottom from time to time as you dig by laying a straight 2x4 or 2x6 on edge so its top barely touches the mason’s line. If you remove too much soil in some spots, fill them with sand or gravel—not loose soil. Use a flat spade or square shovel to shave away the final inch of soil from the bottom and sides of the excavation.

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