How to Build a Retaining Wall: Materials, Cost & Planning
A 20 ft × 3 ft retaining wall needs roughly 198 blocks (12 in × 4 in units, including 10% waste), 1.5 cubic yards of gravel backfill; block units alone commonly run about $4–$20 per sq ft of wall face (approximately $240–$1,200 for a 20 ft × 3 ft wall). For segmental retaining wall systems, use the drainage aggregate and drainage detail required by the wall system or engineered design; CMHA guidance commonly uses at least 12 inches of free-draining gravel behind segmental facing units.
A retaining wall does two jobs: it holds back soil on a slope and it adds structure to your yard. Done right, a short, simple DIY retaining wall is a manageable weekend project. Done wrong, it leans, cracks, and eventually fails — usually because of poor drainage or a missing base.
This guide covers planning, materials, cost estimates, and the step-by-step process for building a block retaining wall. Use the calculator below for material quantities, or read on for the full guide.
Retaining Wall Calculator
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💡 Permit and engineering rules are local, and height is only one design factor — check local codes.
Do You Need a Permit?
Permit rules are local. As a reference point, the 2024 International Residential Code exempts retaining walls not over 4 feet from a building permit when measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge. Your jurisdiction may use a different code edition or more restrictive rules, so check locally before building.
Height is only one design factor. Soil, slopes, groundwater, wall geometry and surcharge loads can trigger professional design needs even on a shorter wall. Where local requirements do not specify otherwise, CMHA recommends engineered design for segmental retaining walls with total design height over 4 feet.
Choosing a Wall Material
The table below covers the basics. For a full breakdown of pros, cons, lifespan, and which material wins for different situations, see the retaining wall materials guide.
| Material | Materials/sq ft of wall face | DIY Friendly? | Notes |
|---|---|---|---|
| Interlocking concrete blocks | $4–$20 (block units alone) | Yes | Most popular DIY option, no mortar needed |
| Natural stone | $8–$15 | Moderate | Heavier, irregular — takes more time to fit |
| Timber / landscape ties | $5–$10 | Yes | Easier to cut, but service life depends on treatment, moisture and soil contact (roughly 20–40 years in favorable conditions) |
| Poured concrete | $12–$25 | No | Strongest; often used for taller or engineered walls — needs forms |
| Boulder wall | $10–$20 | No | Requires heavy equipment for placement |
For most DIY projects, interlocking concrete blocks are the way to go. They are engineered to stack and step back for stability, require no mortar, and come in multiple textures.
Materials You Will Need
For a typical 20 ft long × 3 ft tall block retaining wall:
| Material | Quantity | Estimated Cost |
|---|---|---|
| Wall block units (12 × 4 in face; block-only cost, 60 sq ft × $4–$20/sq ft) | ~198 blocks (180 before 10% waste) | $240–$1,200 |
| Cap blocks | ~17 | $50–$85 |
| Gravel backfill (¾" clean) | 1.5 cubic yards | $45–$80 |
| Base gravel (crusher run) | 0.5 cubic yards | $15–$25 |
| Landscape fabric | 50 linear ft | $20–$35 |
| Perforated drain pipe — size and placement per wall system or design | 24 ft | Varies |
| Construction adhesive | 2 tubes | $10–$15 |
Use our retaining wall calculator to get exact block counts and gravel amounts for your dimensions, and use its itemized estimate with your selected block price; the per-item costs above are rough planning figures, not a complete DIY total. Calculate gravel backfill separately with the gravel calculator.
Step-by-Step: Building a Block Retaining Wall
1. Plan and Mark the Layout
- Use stakes and string to mark the wall line.
- Call 811 to mark underground utilities (free in most states).
- Determine the highest and lowest points — the wall height varies along a slope.
2. Dig the Trench
- Width: block depth + 12 inches behind for gravel backfill.
- Depth: 6 inches below grade for the base (bury the first course below ground level).
- Level the bottom of the trench with compacted crusher run.
3. Lay the Base Course
- Spread 4–6 inches of compacted crusher run in the trench.
- Use a level on every block — the base course dictates the entire wall. If this row is off, every row above it will be too.
- Place blocks lip-side down for the first course.
4. Stack Remaining Courses
- Stagger joints like bricks — offset each row by half a block.
- The built-in lip on each block creates a slight setback, which leans the wall into the hill for stability.
- Check for level every 4–5 blocks.
5. Backfill with Gravel
- Line the back of the wall with landscape fabric to keep soil from clogging the gravel.
- For segmental block walls, fill behind the wall with clean, free-draining gravel at least 12 inches deep (CMHA guidance), using the material your wall system or engineered plans require.
- If your wall system or design calls for a drain pipe, lay a perforated pipe (holes down) at the base on top of the base gravel, sloped to daylight at the end of the wall. Pipe size and placement should follow the wall system or design.
6. Place Cap Blocks
- Glue cap blocks with construction adhesive (one bead along the back edge).
- Caps create a clean, finished top edge and lock the final course.
7. Finish
- Fold the landscape fabric over the top of the gravel backfill.
- Backfill the remaining space above the gravel with native soil.
- Grade soil away from the wall so surface water drains to the sides.
Retaining Wall Cost: DIY vs. Professional
The numbers below cover a standard concrete block wall. For a full cost breakdown by material and wall size — including all the drainage, geogrid, and permit line items most estimates leave out — see the retaining wall cost guide.
| Wall Size | Block Units Alone ($4–$20/sq ft) | Installed, Straightforward Modular Wall (approx. $15–$35/sq ft) |
|---|---|---|
| 20 ft × 2 ft (40 sq ft) | $160–$800 | $600–$1,400 |
| 20 ft × 3 ft (60 sq ft) | $240–$1,200 | $900–$2,100 |
| 40 ft × 3 ft (120 sq ft) | $480–$2,400 | $1,800–$4,200 |
| 50 ft × 4 ft (200 sq ft) | $800–$4,000 | $3,000–$7,000 |
These are broad comparison ranges for modular/interlocking concrete walls; the block-only column does not include base and drainage aggregate, caps, fabric, geogrid, delivery or tools, and the installed column is not block cost plus labor. Site preparation, drainage, reinforcement, engineering, delivery and difficult access can push the project above these ranges. Professional installation makes sense for taller walls, curved layouts, or hillsides with drainage issues.
Common Retaining Wall Mistakes
- No gravel backfill: Soil against the back of the wall traps water, causing hydrostatic pressure that pushes the wall outward.
- Skipping the base: Stacking blocks on bare dirt leads to settling and leaning within the first season.
- Not leveling the first course: Every error in the base row compounds as you stack higher.
- Building too tall without engineering:Height is only one design factor — soil, slopes, groundwater and surcharge loads matter too. Geogrid requirements depend on the wall system, wall height, soil, slopes and surcharge loads; follow the manufacturer's reinforcement chart or an engineered design rather than using one universal height threshold.
- Forgetting the drain pipe: Even with gravel, water needs a path to exit. Where your wall system or design calls for a drain pipe, install it as specified.
Frequently Asked Questions
- Do I need a permit to build a retaining wall?
- Permit rules are local. As a reference point, the 2024 International Residential Code exempts retaining walls not over 4 feet from a building permit when measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge. Check with your local building department first.
- How deep should the base course be buried?
- Bury the base course 6–8 inches below grade(1 inch per foot of wall height). Use 4–6 inches of compacted gravel underneath.
- What material is best for a DIY retaining wall?
- Interlocking concrete block (Allan Block, Versa-Lok) is the most DIY-friendly — no mortar, widely available, and handles typical soil loads well.
- How much does a DIY retaining wall cost?
- Block units alone commonly run about $4–$20 per sq ft of face area (approximately $240–$1,200 for a 20 ft × 3 ft wall). A complete DIY wall also needs base and drainage aggregate, caps, fabric, delivery and potentially reinforcement, so total material cost depends heavily on the wall system and site.
Related Guides & Calculators
- Retaining Wall Calculator — exact block and gravel quantities
- Retaining Wall Materials Guide — concrete block vs. timber vs. stone vs. poured concrete
- Retaining Wall Cost Guide — 2026 pricing by material and wall size, DIY vs. professional
- Gravel Calculator — for backfill and base gravel (see our driveway gravel guide for more on gravel types)
- Concrete Calculator — if you are pouring a concrete wall instead of stacking blocks (see our concrete cost guide)
- Fence Calculator — for fencing above retaining walls
- Paver Patio Guide — for the patio area above or below your wall
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