Guides / Fences and walls · Updated
How far apart should terraced retaining walls be?
From Allan Block, Keystone and Anchor installation manuals, checked November 2026.
Short answer
At least twice the height of the lower wall, and the upper wall no taller than the lower one. Two 2 ft walls need 4 ft of level ground between them. Closer than that, the walls lean on each other and an engineer has to design them together.
What the block makers say
| Block maker | What it says |
|---|---|
| Allan Block | "The height of the upper wall should be less than or equal to the height of the lower wall. The distance between the walls must be at least twice the height of the lower wall." Engineer for walls closer than that, more than two terraces, walls needing geogrid, or structures above. |
| Keystone | "The horizontal distance between walls must be at least (2H), 2 x design height of the lower wall." Grades around the walls must be level; otherwise geogrid is required. |
| Anchor | "The upper wall must be built a distance away from the lower wall of at least twice the height of the lower wall." Exceptions for weak soil or slopes above, below or between the walls. |
You may see 1.5 times quoted online. We could only trace that to an old brochure; every current manual we read says twice. One county we checked (Snohomish, WA) asks for benches at least as wide as the lower wall is tall, so check yours, but the block makers' 2H is the one to build to.
Gap by wall height
| Lower wall | Gap to the next wall |
|---|---|
| 1 ft | 2 ft |
| 1 ft 6 in | 3 ft |
| 2 ft | 4 ft |
| 2 ft 6 in | 5 ft |
| 3 ft | 6 ft |
| 4 ft | 8 ft |
Measured level, from the face of the lower wall to the face of the upper one.
Why a steep slope can't meet it
Split a slope into equal walls and the benches shrink at the same rate as the walls. Bench depth divided by wall height always equals the run divided by the rise. So the 2H rule works only on slopes of 1 in 2 or gentler: 4 ft of rise needs 8 ft of run at least, however many walls you use. Steeper than that, you need an engineered design, a longer run, or one engineered wall.
The terrace planner does this for your yard and shows the options from one wall to six. For a single wall's blocks and gravel, use the retaining wall calculator.