Allan Block — What Makes It Different
Allan Block is the most widely sold interlocking retaining wall system in North America — available at Home Depot, Lowe's, Menards, and most masonry supply yards. The Standard block is 12 inches wide, 8 inches per course height, and about 18 inches deep. What sets it apart from generic concrete blocks is the built-in setback geometry: each block has an angled rear lip that automatically positions the next course 1.25 inches back from the one below. Stack six courses and you have a wall that leans 7.5 inches into the hillside — no shimming, no math, no mortar. That built-in batter is what gives Allan Block its structural integrity and makes it genuinely DIY-friendly for walls under 4 feet tall.
Allan Block comes in several products:
- AB Standard (AB Retaining Wall Block): 12" wide face × 8" per course × ~18" deep, ~35 lbs. The workhorse for most landscape walls. This calculator defaults to Standard specs.
- AB Junior: 6" wide × 4" course height × ~9" deep, ~15 lbs. Smaller walls, garden borders, curved walls.
- AB Capstone (Coping): 12" wide × 3.5" tall × 15" deep. Finishes the top course flush — no exposed aggregate, clean look.
- AB Courtyard Collection / AB Fieldstone: Decorative face textures, same structural specs as Standard.
Step-by-Step: Measuring for Your Allan Block Wall
1. Measure wall length: Run a tape along the base of where the wall will sit. For curved walls, measure along the curve (blocks can accommodate gradual curves without cutting). Note whether your wall is a straight run, L-shape, or curved — this affects how you order and stagger blocks at corners.
2. Determine visible wall height: How tall do you want the visible wall? A 3-foot wall holds back moderate slopes and is the sweet spot for DIY — most codes don't require permits for walls under 3–4 feet, and no geogrid is needed in most conditions. Measure from the soil level at the base to the top of the finished cap.
3. Add the buried base course: Allan Block requires burying the first course 4–6 inches below finished grade. This anchors the wall against forward movement (sliding) and erosion. The calculator adds one full buried course automatically — you'll excavate deeper than the visible height suggests.
4. Check for obstacles: Downspouts, utility lines, tree roots, and existing structures all affect wall placement. Call 811 (USA) before digging to mark underground utilities.
Drainage — Not Optional
Every Allan Block wall needs drainage — no exceptions. Water is the #1 killer of retaining walls. When it rains, soil becomes saturated and hydrostatic pressure builds behind the wall. At 62 lbs per cubic foot, even a small accumulation of waterlogged soil exerts thousands of pounds of force on a 20-foot wall. Allan Block's built-in setback helps, but drainage eliminates the pressure entirely rather than just resisting it.
The standard drainage assembly consists of: a 12-inch-wide column of 3/4-inch clean crushed stone directly behind the blocks (running the full height of the wall), a 4-inch perforated corrugated drain pipe at the base embedded in that stone, and a sheet of landscape fabric on the hillside side of the stone to prevent soil migration into the aggregate. The drain pipe must slope at least 1/4 inch per foot and daylight (exit) at a lower point — never dead-end it. This system typically adds $150–300 to a 20-foot wall and pays for itself by preventing a $2,000 wall failure.
When You Need Geogrid
Geogrid is a plastic mesh reinforcement that extends horizontally from the back of the wall into the hillside. It works by anchoring the wall to a mass of compacted soil behind it, essentially making the wall behave like a gravity structure with a much larger footprint. Allan Block recommends geogrid for:
- Walls taller than 4 feet (6+ visible courses)
- Any wall with a surcharge load behind it: driveways, parking areas, structures, swimming pools, or steep slopes
- Walls in clay-heavy or expansive soil that holds water
- Walls on slopes greater than 3:1 (horizontal:vertical)
When geogrid is needed, Allan Block's design tables call for a layer every 3 courses (every 24 inches of height), extending 6 feet into the hillside. If your wall falls into any of the categories above, stop here and consult a local civil engineer before building — the cost of an engineering review ($300–600) is far less than the liability of a wall failure on a surcharge application.
Block Count Math — How We Calculate It
Here's the exact math so you can verify the calculator's output:
- Visible courses: Ceiling(wall height in inches ÷ 8) — rounds up to the next full course
- Total courses: Visible courses + 1 (buried base course)
- Blocks per course: Ceiling(wall length in feet × 12 ÷ 12) = wall length in linear feet rounded up (each AB Standard block is exactly 12" wide)
- Gross block count: Blocks per course × total courses
- With waste: Gross × 1.10 (10% waste for cuts, broken blocks, corners)
- Capstones: Ceiling(wall length in feet) — one capstone per linear foot
- Gravel (cubic yards): (length × height × 1 ft wide) ÷ 27
- Drain pipe (linear feet): Wall length + 4 (extra for outlet runs)
Example: 24-foot wall, 3.5 feet visible height → 6 visible courses + 1 buried = 7 courses; 24 blocks per course; gross = 168; with waste = 185 blocks; 24 capstones; 3.1 cu yd gravel; 28 ft drain pipe.
Materials Needed
Enter dimensions above to see results
Allan Block Pro Tips
- Bury the first course 4–6 inches below grade — this is the wall's foundation
- Level the base course perfectly — errors compound upward with every course
- Install drainage before you backfill — you can't retrofit it later
- Compact backfill in 6-inch lifts with a plate compactor — hand tamping isn't adequate
- Apply construction adhesive to the top two courses and capstones to prevent displacement
- Walls over 4 feet tall require geogrid and usually a building permit
- Call 811 before digging to locate underground utilities
Frequently Asked Questions
Q: How many Allan Block blocks do I need per square foot?
A: Allan Block Standard blocks (12 inches wide × 8 inches per course) cover approximately 0.667 square feet of wall face each, so you need about 1.5 blocks per square foot of wall face. Add 10% for waste and cutting, plus an additional buried base course equal to one full row of blocks per linear foot of wall.
Q: What is the built-in setback on Allan Block?
A: Allan Block Standard has a built-in setback of approximately 1.25 inches per course. This means each course is automatically set back from the one below due to the block's angled design — no shimming or adjustment needed. A 3-foot-high wall will lean into the hillside about 5.5 to 6 inches at the top, providing natural stability without mortar.
Q: Do I need geogrid for my Allan Block wall?
A: For walls 4 feet and under with level or gently sloping backfill and no surcharge (no driveway, structure, or heavy load behind the wall), most Allan Block walls can be built without geogrid. Walls over 4 feet, walls with surcharge loads, walls on steep slopes, or walls in expansive clay soil typically require geogrid every 3 courses (every 24 inches of height). When in doubt, consult a local engineer — permits are often required for walls over 4 feet.
Q: Can I build an Allan Block wall without concrete or mortar?
A: Yes. Allan Block is a dry-stack interlocking system — no mortar or concrete is needed for the main wall. The only adhesive used is construction adhesive (such as Loctite PL 500 Landscape Block Adhesive) on the top two courses and capstones to prevent displacement from frost heave or impact.
Q: How much does an Allan Block wall cost per square foot?
A: Allan Block Standard blocks retail for approximately $3.50–5.00 each at big-box stores. A 3-foot-high × 20-foot-long wall requires about 110–130 blocks, costing $385–650 just for blocks. Add drainage gravel ($80–150), drain pipe ($30–60), capstones ($80–130), adhesive ($20–40), and landscape fabric ($20–40) for a DIY material total of roughly $600–1,100 — about $10–18 per square foot. Professional installation runs $30–80 per square foot for the same wall.