How to plan a block retaining wall
- Stake out the wall and measure its length along the face. Add the lengths of each straight run.
- Measure the height you want above the finished ground in front of the wall, not counting the cap.
- Pick your block, or enter the height, length, depth and weight from its product sheet.
- Set the base and wall rock depths from the manufacturer's installation guide. The defaults follow Allan Block's.
- Check the height notes and the table below. Clay soil, a slope or a driveway above the wall all lower the height a wall can reach without geogrid.
How the materials are calculated
Courses = (height above ground + minimum buried depth) ÷ block height, rounded up
Blocks per course = wall length ÷ block length, rounded up
Blocks = blocks per course × courses × (1 + extra)
Caps = wall length ÷ cap length, rounded up
Base stone (cu ft) = wall length × base width × base depth
Wall rock (cu ft) = wall length × courses × block height × wall rock depth
Trench depth = base depth + buried depth
Worked example
A 24 foot garden wall 24 inches high in AB Classic blocks needs (24 + 6) ÷ 8 = 3.75, rounded up to 4 courses, so 8" of block sits below grade. Each course takes 288 ÷ 18 = 16 blocks, and 16 × 4 = 64, plus 5%, is 68 blocks, about 2.6 tons. The 24 by 6 inch base takes 24 cubic feet of stone, and 1.4 feet of wall rock over 2' 8" of block takes 89.6 cubic feet. Together that is 4.21 cubic yards, or 6.8 tons. Dig the trench 14" deep, and lay 16 caps with 1 tube of adhesive on top.
How tall a gravity wall can be
A gravity wall holds the soil back with its own weight and setback, without geogrid. Allan Block's Table 1.3 gives these maximum heights for AB Classic, measured above grade with the cap included:
| Above the wall | Clay | Silty sand | Sand or gravel |
|---|---|---|---|
| Level ground | 3 ft 2 in | 4 ft 7 in | 5 ft 2 in |
| Surcharge (100 psf) | 1 ft 4 in | 3 ft 1 in | 3 ft 6 in |
| 3:1 slope | 2 ft 4 in | 4 ft 1 in | 4 ft 7 in |
The surcharge row assumes a solid surface such as concrete, asphalt or pavers above the wall. The heights exclude seismic loads, and Allan Block says final designs must come from a local registered engineer. Under the 2021 IRC (R404.4), a wall that is not braced at the top and holds back more than 48 inches of soil, or more than 24 inches while it also resists another load, must be engineered. Check with your building department, since local codes vary.
About the defaults
- Blocks: AB Classic is 8 × 12 × 18 inches and about 75 lb with a 6 degree setback. Versa-Lok Standard is 6 × 16 × 12 inches and 82 lb with a 3/4 inch setback per course.
- Buried depth, 6 inches: the minimum from both Allan Block and Versa-Lok for level ground in front.
- Extra blocks, 5%: Allan Block's recommendation.
- Base, 24 by 6 inches: Allan Block digs a 24 inch wide trench with at least 6 inches of wall rock as the base, and Versa-Lok's ICC-ES report calls for a 6 inch crushed stone leveling pad.
- Wall rock: both makers use at least 12 inches of drainage stone behind the blocks, and that is the Versa-Lok default. Allan Block's estimate is 1.4 feet, because it also fills the hollow cores of its blocks.
- Stone weight, 1.62 tons per cubic yard: Allan Block's estimating figure of 120 lb per cubic foot.
- Cap length: Versa-Lok's 0.8 caps per foot works out to 15 inches. No cap length is published for AB Classic, so its 18 inch default is an assumption; check your capstone.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block length to get blocks per course, multiply by the number of courses including the buried one, and add 5%, which Allan Block recommends. A 24 foot wall 24 inches high in 8 inch AB Classic blocks takes 16 blocks per course and 4 courses, so 64 blocks, or 68 with the extra.
How deep do you bury the first row of retaining wall blocks?
Allan Block buries at least 6 inches, or 1 inch for each foot of wall height if that is more. Versa-Lok's guide says at least 6 inches with level ground in front, and one seventh of the wall height where the ground in front slopes. Because blocks come in whole courses, the example wall ends up with 8" below grade.
How tall can a block retaining wall be without reinforcement?
It depends on the block, the soil and what sits above the wall. Allan Block's table puts an AB Classic gravity wall with level ground above at 3 ft 2 in in clay, 4 ft 7 in in silty sand and 5 ft 2 in in sand or gravel, cap included, and much lower under a driveway or a slope. Separately, the IRC requires an engineered design above 48 inches of retained soil, or 24 inches when the wall also carries another load.
How much gravel goes behind a retaining wall?
Allan Block and Versa-Lok's ICC-ES report both call for at least 12 inches of drainage stone behind the blocks, and Allan Block also fills its hollow cores with the same wall rock. For the example wall that is 3.32 cubic yards of wall rock, plus 0.89 for the base: about 6.8 tons at 1.62 tons per cubic yard.
Does a retaining wall need a drain pipe?
Allan Block requires one for walls over 4 feet tall and for walls built in silty or clay soils. The pipe goes at the back of the trench for the full length of the wall, and has to be vented to daylight or to a storm sewer.
How many caps and how much adhesive do I need?
Divide the wall length by the cap length. Versa-Lok's guide estimates 0.8 caps per foot of wall, which is 20 caps for 24 feet. Allan Block uses one 29 ounce tube of adhesive for every 60 feet of capped wall.