How to Build a Curved Retaining Wall: Radius, Setback, Curves and Corners

To build a curved retaining wall, you drive a center stake, swing a string to mark the base course, and size that base radius from the radius you want at the top and the block’s setback. Setback changes the radius as the wall rises: outside curves get tighter toward the top and inside curves get looser, so you check the block’s minimum radius at the top of an outside curve and at the base of an inside one.

Inside curves vs outside curves

An outside curve bulges toward the low side, like a wall wrapping around the front of a raised patio. Its center point sits behind the wall, up in the retained soil. An inside curve wraps around the low side, like a wall cupping a lawn, and its center point sits out in front.

Most segmental block steps back a little on each course; our guide to retaining wall batter covers why. On a curve, that setback moves every course in relation to the center point:

• Outside curve: each course moves toward the center, so the radius shrinks as the wall goes up. Belgard’s outside-curve instructions call the top course the smallest radius in the wall.
• Inside curve: each course moves away from the center, so the radius grows. Belgard’s inside-curve page makes the base course the smallest.

The blocks behave differently too. Allan Block’s curve guide says to keep the fronts tight on an inside curve and fan the backs out with even gaps. On an outside curve the backs pinch, so you knock one or both rear “wings” off the blocks.

Work out the base radius before you dig

Start from the radius you want at the top course, measured to the block face, and use Belgard’s method:

1. Add 1/4 inch to the block’s per-course setback, then multiply by the number of courses.
2. Outside curve: add that to the top radius. Inside curve: subtract it. That’s the base course radius.
3. Mark the front edge of the trench 6 inches farther toward the low side: add 6 inches on an outside curve, subtract 6 on an inside curve.

Belgard’s worked examples, for a 1 1/8-inch setback and 8 courses:

Step	Outside curve	Inside curve
Radius you want at the top course	6 ft 0 in	10 ft 0 in
(1 1/8 in + 1/4 in) × 8 courses	11 in	11 in
Base course radius	6 ft 0 in + 11 in = 6 ft 11 in	10 ft 0 in minus 11 in = 9 ft 1 in
Front edge of the trench	6 ft 11 in + 6 in = 7 ft 5 in	9 ft 1 in minus 6 in = 8 ft 7 in

The minimum radius belongs to the block system, not to the wall. Allan Block’s commercial manual sets 4 feet at the top course for full-size units (2.5 feet for half-width) and publishes the base radius that keeps an outside curve’s top course from dropping under 4 feet:

Allan Block setback (full-size units)	4 ft wall	6 ft wall	8 ft wall	10 ft wall
3 degree	4 ft 9 in	5 ft 0 in	5 ft 4 in	5 ft 8 in
6 degree	5 ft 2 in	5 ft 6 in	5 ft 11 in	6 ft 4 in
12 degree	5 ft 6 in	6 ft 0 in	6 ft 6 in	7 ft 0 in

Laying out and building the curve

1. Sketch it on the ground with a garden hose or marking paint. Allan Block advises designing out tight radii, since gentle curves look better.
2. Set the center stake. From where the curve starts, measure square off the wall line by the radius and drive a stake: behind the wall for an outside curve, in front of it for an inside curve.
3. Mark and dig the trench. Tie a string to the stake, swing it at the trench-front radius and paint the arc. Our guide to retaining wall base depth covers how deep to dig and what goes in the bottom.
4. Mark the base course on the finished leveling pad with the same stake and string, then set the block fronts on that line, level side to side and front to back.
5. Keep the bond. Offset vertical joints by at least 1/4 of a block length from the course below. As the radius changes, use half or cut units to hold running bond, and glue cut pieces in place with concrete adhesive.
6. Check the bearing. Belgard says part of each block’s lip must sit against the units below on every course; where it doesn’t, glue the block down.
7. Blend into straight runs by laying out the curve plus the first couple of blocks of the next section, then adjusting one or two blocks.

In tight curves, Allan Block notes full-size blocks can leave gaps between courses, and you may need to trim part of the bottom notch. Caps get cut to the curve last; our guide to retaining wall caps shows how.

Geogrid on curves

A curve changes how geogrid lays. Allan Block requires 100 percent coverage around a curve, with the strong direction perpendicular to the wall face.

• Inside curve: the strips fan apart behind the wall and leave wedge-shaped gaps. Butt the front edges together, mark the gaps, and cover them with filler pieces on the next course (or the one below).
• Outside curve: the strips overlap. Lift the overlapping grid and separate the layers with 3 inches of approved fill. Never compact directly on geogrid.

Grid lengths and spacing come from the wall design; see geogrid retaining walls.

Building 90-degree corners

Allan Block says most curves in its system need no cutting. Corners need a corner unit or a cut block on every course.

Outside corners

• With corner units: Allan Block uses a corner block on every course, alternating right- and left-hand units, and builds out from the corner. Belgard alternates its corner units so the long side switches sides each course, sets them back to match the wall’s batter, and glues them from bottom to top.
• Without corner units: Belgard’s outside-corner method splits a unit in half and sets it at the corner with both split faces out, removing the rear lip if needed so it lies flat. The other half goes on the next course, facing the other way, and every corner unit gets concrete adhesive.
• Geogrid: Allan Block’s corner guide runs each layer back into the hill with its strong direction perpendicular to the face, so each new layer goes perpendicular to the one below.

Inside corners

• Allan Block: cut half of the raised front lip off one block per course with a diamond saw or chisel, set it perpendicular to the adjoining block, and reverse the cut block’s position on each course so the corner interlocks.
• Belgard: its inside-corner steps set one block at the corner and a second perpendicular to it. On course two, finish one side on bond before the other, and extend the base and second course past the corner so it won’t settle. Glue the corner units.
• Geogrid: extend each layer past the corner by at least 25 percent of the wall height, alternating direction each layer. Allan Block’s example: a 12-foot wall divided by 4 means 3 feet past the corner.

When to call a pro

A short wall on a gentle curve is fair DIY. Call a pro when:

• The wall is taller than about 4 feet or needs geogrid. Our guide to retaining walls over 4 feet covers engineering and permits.
• The radius is near the block’s minimum, or the wall snakes through several curves.
• A driveway, patio or building sits near the top of the wall.

If you’d rather have the curve worked out on paper first, our online landscape design plans lay out the wall line, curves and heights, starting at $349.

Frequently Asked Questions

What is the minimum radius for a curved retaining wall?

It depends on the block system. Allan Block’s full-size units can go as tight as 4 feet at the top course and its half-width units 2.5 feet. Check the minimum at the top of an outside curve and at the base of an inside curve.

What’s the difference between an inside and an outside curve?

An outside curve bulges toward the low side, its block backs pinch, and it gets tighter as it rises. An inside curve wraps around the low side, gaps open between the block backs, and it gets looser as it rises.

How do you build a 90-degree corner on a retaining wall?

Outside corners use corner units or split blocks that alternate direction every course and get glued with concrete adhesive. Inside corners overlap a modified block at the corner on each course, reversing direction each time.

Do you have to cut blocks to build a curve?

Often not. Allan Block says most of its curves can be built without cutting, though outside curves may need rear wings knocked off. Tight curves need cut or half units to keep the joints offset.

Sources: Allan Block, Building Curves in a Retaining Wall (Residential Installation Manual); Allan Block, Curves and Serpentines (Commercial Installation Manual); Allan Block, Building Corners (Commercial Installation Manual); Belgard, Technical Resource Guide: Outside Curves (p. 38); Belgard, Technical Resource Guide: Inside Curves (p. 39); Belgard, Technical Resource Guide: Outside 90-Degree Corners (p. 40); Belgard, Technical Resource Guide: Inside 90-Degree Corners (p. 41)

Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.