drainage system

Planning Considerations

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.

Catch basin

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 its center. 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.

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.

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.

Measure 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.