Overview
A retaining wall's biggest threat usually isn't the soil it holds back directly — it's water trapped behind it. This guide walks through retaining wall planning in the order that actually matters for longevity: wall material and reinforcement first, then drainage (the step most often under-planned), then the specific rock and aggregate materials used across broader site prep work.
Work through wall sizing and reinforcement first, then drainage, then general site material calculations.
Step 1: Size the Retaining Wall and Rebar
Retaining wall material needs depend on wall height, length, and block or concrete type, while reinforcement requirements scale with height — taller walls (generally above 3–4 feet) need rebar to resist the lateral soil pressure that concrete and masonry can't handle through compression strength alone.
The Retaining Wall Calculator estimates material needs from wall dimensions, and the Rebar Calculator calculates reinforcement spacing and quantity for the same wall.
Step 2: Plan Drainage Behind the Wall
Trapped water is the most common cause of retaining wall failure, since saturated soil is heavier and, in cold climates, subject to frost expansion — both multiply the lateral pressure pushing against the wall regardless of how well it's reinforced. A French drain redirects this water away before it builds up.
The French Drain Calculator calculates trench dimensions and gravel volume for drainage that should be planned alongside the wall itself, not added as an afterthought.
Step 3: Select Rock and Aggregate for the Application
Different site prep tasks call for different rock types. Rip rap's large, angular shape resists washing away on slopes and near water, river rock's smooth, rounded shape suits drainage and decorative ground cover, and crushed limestone compacts into a stable base layer for driveways and paths.
The Rip Rap Calculator sizes erosion-control rock, the River Rock Calculator sizes drainage and decorative rock, and the Limestone Calculator estimates base-layer volume.
Step 4: Calculate Sand and Convert to Tonnage
Sand serves multiple distinct roles — paver bedding, concrete and mortar component, or pipe backfill — each with its own depth and coverage requirement. Once total volume across any of these materials is known, converting to tonnage for bulk ordering requires each material's specific density, since gravel, sand, and stone don't weigh the same per cubic yard.
The Sand Calculator calculates volume for your specific application, and the Tonnage Calculator converts any calculated volume to weight using the correct material-specific density.
Step 5: Protect the Finished Wall with Sealant
Once construction is complete, sealant applied to the wall's surface reduces water absorption into the wall material itself, helping prevent freeze-thaw cracking and efflorescence — a maintenance step distinct from the structural drainage planned in Step 2.
The Sealant Calculator estimates coverage needed based on the wall's total surface area.
Key Terms
- Lateral pressure — the sideways force soil exerts against a retaining wall, which increases with soil saturation and height
- Rip rap — large, angular rock used for erosion control on slopes and near water
- French drain — a gravel-filled trench with a perforated pipe, used to redirect groundwater away from a structure
- Hydrostatic pressure — the pressure exerted by trapped water, a major cause of retaining wall failure when drainage is inadequate
- Efflorescence — the white mineral deposit that appears on masonry surfaces as water evaporates, reduced by sealant application
- Gravity wall — a retaining wall design relying on its own weight (rather than reinforcement) to resist lateral soil pressure, suitable only for shorter walls