Calculators / Hardscape
How to terrace a slope
Put in how much the yard rises, over how far, and how wide it is. You get the number of walls, their height, how deep the level benches come out, and whether they can stand on their own.
Updated · How we check our numbers
Your terraces
2 walls, 2 ft each
2 walls 2 ft tall with level benches 10 ft deep. That's 504 blocks and 63 caps for 30 ft walls.
- Wall height
- 2 ft3 ft 2 in from footing
- Bench depth
- 10 ftneeds 4 ft
- Blocks
- 504+ 63 caps
- Gravel
- 11.5 tonsbase and drainage
Estimate for planning only, not professional advice. Check the manufacturer’s instructions, your local code and a licensed professional before you buy, build or install. Use at your own risk. Terms.
One wall in detail2 ft tall, 30 ft long
slope = 4 ft up in 20 ft = 1 in 5
each wall = 4 ft ÷ 2 = 2 ft; bench = 20 ft ÷ 2 = 10 ft
2H rule: 10 ft ≥ 2 × 2 ft = 4 ft
courses = 6 up + 2 buried = 8 of 4 in; 32 in + 6 in base = 3 ft 2 in from footing
blocks = 30 a course × 8 × 2 walls × 1.05 = 504
soil moved ≈ 5.6 cu yd, cut from the back of each bench to fill the front
2 walls 2 ft tall across 20 ft of slope.
How many walls
Equal walls, even slope- 1 wall4 ft each, one level yardToo tall to DIY
5 ft 2 in from the footing; over 3 ft exposed or 4 ft from the footing.
- 2 walls (shown)2 ft each, 10 ft benchesDIY range
Each wall stands on its own.
- 3 walls1 ft 4 in each, 6.7 ft benchesEngineer check
Allan Block wants an engineer for more than two terraces.
- 4 walls1 ft each, 5 ft benchesEngineer check
Allan Block wants an engineer for more than two terraces.
- 5 walls9½ in each, 4 ft benchesEngineer check
Allan Block wants an engineer for more than two terraces.
- 6 walls8 in each, 3.3 ft benchesEngineer check
Allan Block wants an engineer for more than two terraces.
Run a 4 in perforated pipe behind every terraced wall (Allan Block), out to daylight at least every 50 ft. Never let the upper wall's pipe empty above or behind the lower wall (Anchor).
No driveway, shed, patio or fence posts close behind a wall unless it's engineered for them. That's a surcharge, and it needs a permit and a design at almost any height.
Call 811 a few days before you dig. Terracing moves a lot of soil.
How we work it out
wall height = rise ÷ walls; bench depth = run ÷ walls; independent when bench ≥ 2 × wall height
The 2H rule. Allan Block, Keystone and Anchor all say terraced walls act on their own only when the gap between them is at least twice the lower wall's height and the upper wall is no taller. Closer than that, the lower wall carries the upper one and an engineer has to design them together. Allan Block also wants an engineer for more than two terraces.
Why steep slopes don't work. With equal walls on an even slope, bench depth divided by wall height is just run divided by rise, however many walls you build. So the rule holds only on slopes of 1 in 2 or gentler: 5 ft of rise needs at least 10 ft of run.
How tall. We keep each wall to 3 ft showing and 4 ft from the bottom of the footing. IRC R404.4 requires a design past 4 ft of soil, IRC R105.2 exempts walls up to 4 ft from the footing from permits, and Allan Block's table puts small block in clay at about 3 ft without geogrid. Some towns draw the line at 3 ft. Each wall gets a buried course (1 in per foot of height, 6 in at least) and 6 in of gravel base.
Where the numbers come from
- Allan Block Residential Landscape Retaining Wall Guide (terraces p.47, base p.24, drainage p.20)
- Allan Block Commercial Manual (terraced walls p.60)
- Keystone Construction Manual (terraced walls p.59)
- Anchor Diamond Pro wall installation guide (terraced walls)
- City of Grand Prairie, TX: residential retaining walls (IRC R404.4)
- City of Port Townsend, WA: retaining walls under the IRC (R105.2)
- USDA NRCS: Conservation in Your Backyard, Terracing
Guides
Walls and grade
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