Build a Retaining Wall: Project Walkthrough

1. Decide exposed height first, and check whether this is a DIY wall at all

Measure the actual grade change the wall needs to hold — not the height that looks tidy, but the real difference between the grade on the high side and the grade on the low side where the wall will sit. Segmental block systems sold for homeowner installation are generally rated for a specific maximum exposed height, and once a wall's exposed height approaches roughly 4 feet, most jurisdictions require a stamped engineered design and a permit before you dig the first shovelful, because the soil pressure behind a wall increases sharply, not gradually, as height increases. A wall under that threshold still deserves a local code check before starting; a wall at or above it needs an engineer's sign-off, full stop, regardless of how confident the block manufacturer's installation guide sounds.

Run the retaining wall calculator with the exposed height you've settled on, along with your block's dimensions and how many courses will sit buried below grade — one full course below finished grade is the usual minimum for a segmental wall, more on a taller wall or on a slope that drops along the wall's run.

2. Dig to the depth the wall's own base and buried courses require

The retaining wall calculator's own numbers — buried course count times block height, plus base stone depth — set the floor of the excavation, not a rough guess at how deep a wall footing should look. Feed that same depth into the excavation spoil calculator along with the wall's length and a working width wide enough for the base trench plus the drainage stone zone behind the wall face, and check the swell factor for your actual soil — see soil swell and compaction for how much more loose volume clay produces than sand and gravel from the identical hole.

3. Base stone, then build up course by course with drainage stone behind

Compact a level base of crushed stone in the trench first — level along the wall's length even where the original grade above it isn't, since an unlevel base telegraphs into every course built on top of it. Run the gravel calculator for that base layer and separately for the drainage stone column that goes in behind the wall as each course goes up, since the two serve different jobs: base stone supports the wall's weight, while drainage stone exists so the wall never has to hold back a fully saturated slope — every drop that would otherwise sit trapped against the block finds its way down to the pipe at the base instead.

Run the french drain calculator for the perforated pipe that typically runs along the base of that drainage stone column, daylighting at a low point away from the wall — a retaining wall without a working drainage path behind it is holding back water pressure as much as soil pressure, and that's a load the wall's design didn't account for.

4. Backfill, cap, and finish grade

Backfill with the specified material in the zone immediately behind the wall (often the same crushed stone as the drainage column, not native soil, close to the wall face) and compact it in the same thin lifts the base stone needed, then cap the top course and finish grade so surface water sheds away from the wall rather than pooling and soaking straight down into the backfill zone.

Why one trench depth for the whole run is the wrong number at the downhill end

The retaining wall calculator's base-trench depth — buried courses times block height, plus base stone depth — describes one cross-section, not the whole wall, and it only matches the whole wall if the original grade the wall sits on is flat along its length. It rarely is. Take a 24-foot wall where the ground the base sits on drops about a foot over the last third of the run. To keep the base level — retaining wall footings are built dead level even when the grade above and below isn't — the buried portion at the downhill end has to grow by roughly two extra 6-inch courses to reach the same base elevation as the rest of the wall.

That means a trench depth of about 12 inches (one buried course plus 6 inches of base stone) along the uphill two-thirds of the run, stepping down to about 24 inches at the downhill third. Excavate and budget spoil removal off the 12-inch number because that's what the wall looks like measured from the front, and the downhill 8 feet alone adds close to a cubic yard of extra bank-measure digging beyond what the uniform-depth estimate accounted for — soil that has to come out from under a base course that's now two feet down, not one, and that the excavation calculator never saw because the trench depth entered into it was a single number for a wall that doesn't actually have one.

What goes wrong when the dig gets scoped off the front of the wall alone

Excavate the whole run to one trench depth because that's what the exposed wall face suggests, and the downhill end comes up short of base stone depth exactly where the wall needs it most — a base course founded on undisturbed but insufficiently deep native soil at that end is the beginning of an uneven settlement problem that shows up as a visible dip or lean in the wall months after backfill, not on install day when it would still be an easy fix.

The same reversal applies to the engineering check in step 1: starting excavation before confirming whether the wall's exposed height crosses the engineered-design threshold risks digging, pouring a base, and setting the first courses of a wall that then has to be redesigned or torn out because it was never reviewed against the actual soil pressure it would be holding back.