Check with your community's building department before setting out to build a retaining wall. Many codes require a permit for any structure that holds back what amounts to thousands of pounds of earth, and most limit the height of an amateur-built retaining wall to 3 feet. If your slope needs a higher wall or requires extensive grading, call in a masonry or landscape contractor -- or terrace the slope with two or more lower retaining walls.
1. Dig a trench into the slope to a depth that's just below the frost line for your region (local building authorities can give you that measurement). Make the trench wide enough for the wall you plan to pour; dig enough space, too, for you to work.
2. Start forms. Cut 3/4-inch plywood into panels 3-1/2 inches taller than the height your wall will extend above the ground (the earth below provides the form for the footing). Coat plywood with motor oil for easier removal. Nail studs of 2x4 lumber to the plywood, spacing them 24 inches apart.
3. Assemble the form with end pieces of plywood and interior spreaders of 2x4 wood. Set the form in place, make sure it's level and plumb, then brace it with outriggers and stakes. Push the form into place with one foot while you drive the stakes.
4. Add rerod. To strengthen the wall, drive lengths of rerod into the ground every 18 inches. Tie rerods to the spreaders with No. 8 or 9 wire. Tie horizontal rerods to the vertical ones every 18 inches. If you plan to cap the wall with concrete, let the vertical rerods protrude an inch or so above the spreader's bottom edges.
5. Pour the concrete. We built a wheelbarrow ramp to the top of the form. As you pour, have a helper tamp the concrete to squeeze out air bubbles. After the concrete has set slightly, smooth its surface with a float.
6. Finish. After the concrete cures, remove the forms, install perforated drainpipe as shown in the illustration at upper right, then backfill. Cap the wall with brick, wood, or precast concrete coping, if desired. Use the terrace created by the wall for gardening or other activities.