Concrete Masonry Units (CMU) Overview
CMU blocks (concrete blocks, cinder blocks) are efficient for building walls, foundations, and retaining structures. They’re:
- Faster to lay than brick
- Structurally strong
- Economical
- Easy to reinforce
This guide covers standard CMU wall construction for garden walls, retaining walls, and non-load-bearing partitions.
Block Types and Sizes
Standard CMU Sizes
| Nominal Size | Actual Size |
|---|---|
| 8” × 8” × 16” | 7-5/8” × 7-5/8” × 15-5/8” |
| 6” × 8” × 16” | 5-5/8” × 7-5/8” × 15-5/8” |
| 4” × 8” × 16” | 3-5/8” × 7-5/8” × 15-5/8” |
| 12” × 8” × 16” | 11-5/8” × 7-5/8” × 15-5/8” |
Standard course height: 8” (7-5/8” block + 3/8” mortar joint)
Block Types
Stretcher blocks: Standard blocks with two cores (hollow sections)
Corner blocks: Flat end for finishing corners
Half blocks: 8” long for ending courses
Bond beam blocks: U-shaped for horizontal reinforcement
Cap blocks: Solid top for finishing wall
Tools and Materials
Tools
- Mason’s trowel
- Levels (4-foot and torpedo)
- String line and line blocks
- Mason’s hammer
- Block splitter or masonry saw
- Jointing tool
- Story pole
- Wheelbarrow
- Mortar mixer (for larger projects)
Materials
Per 100 square feet of wall:
- 112 standard blocks (8×8×16)
- 8 cubic feet mortar
- Rebar (if reinforced)
- Grout (if reinforced)
Use our Concrete Block Calculator for exact quantities.
Foundation Requirements
Footing Specifications
CMU walls require solid concrete footings.
Minimum footing size:
- Width: 2× wall width (16” for 8” wall)
- Depth: Equal to wall width (8” minimum)
- Below frost line in cold climates
Reinforcement:
- Two #4 rebar running continuously
- Placed in lower third of footing
Footing for Retaining Walls
Retaining walls need larger footings:
- Width: 2/3 of wall height
- Key (groove) in center for drainage
- Drainage behind wall essential
Laying the First Course
Layout
- Snap chalk line on footing for wall face
- Dry-lay first course without mortar
- Check length - adjust block spacing or plan cuts
- Mark block locations on footing
Mortar Bed
- Mix mortar to proper consistency (see Mixing Mortar Guide)
- Spread full mortar bed on footing
- Furrow mortar with trowel
- Work in sections you can lay before mortar sets
Setting First Block
- Start at corner or end
- Set block into mortar bed
- Press down firmly to compress joint to 3/8”
- Check level in both directions
- Check alignment with chalk line
- Tap to adjust while mortar is plastic
Completing First Course
For each block:
-
Apply mortar to end (head joint):
- Stand block on end
- Apply mortar to flanges (not full end)
- Create good coverage on outer edges
-
Set block against previous block
-
Push firmly to compress head joint
-
Check level with adjacent blocks
-
Remove excess mortar immediately
Key points:
- Full mortar on bed joints (horizontal)
- Flange mortar on head joints (vertical)
- Keep blocks aligned to string
- Check level constantly
Building Up the Wall
Running Bond Pattern
- Offset each course by half block (8”)
- Creates strong interlocking pattern
- Start alternate courses with half block
Course-by-Course Process
Building corners first (lead corners):
- Build up corner 3-4 courses
- Step back each course
- Check plumb on both faces
- Creates reference for string line
Filling between corners:
- Stretch string line between leads
- Lay blocks to string
- Check each block for level
- Maintain consistent 3/8” joints
Story Pole
A story pole marks course heights for consistency:
- Mark pole at 8” intervals
- Check wall height at corners
- Ensures courses stay level across wall
Reinforcement
When Reinforcement Is Required
- Retaining walls
- Walls over 4 feet tall
- Seismic zones
- Any structural application
Vertical Reinforcement
Installation:
- Set rebar in footing (dowels)
- Thread blocks over rebar as you build
- Fill cores containing rebar with grout
- Typical spacing: Every 32-48” (per engineering)
Rebar sizing:
- #4 (1/2”) typical for residential
- #5 (5/8”) for taller walls
- Continuous from footing to top of wall
Horizontal Reinforcement
Bond beams:
- Use bond beam blocks (U-shaped)
- Place horizontal rebar in channel
- Fill with grout
- Typical spacing: Every 4 feet vertically
Joint reinforcement:
- Ladder-type wire reinforcement
- Placed in mortar joints
- Every 2-4 courses
- Less structural than bond beams
Grouting
Purpose
- Solidifies reinforced cells
- Creates structural unit
- Bonds rebar to block
Grout Types
Fine grout: Pea gravel aggregate, flows easily
Coarse grout: Larger aggregate, for bigger cores
Grouting Process
- Install cleanouts at bottom (for full-height grouting)
- Clean cells of mortar droppings
- Pre-wet blocks (dry blocks absorb water from grout)
- Pour grout to fill cells
- Consolidate with rod or vibrator
- Fill in lifts (maximum 4 feet at a time)
Low-Lift vs. High-Lift Grouting
Low-lift: Grout as you build, 4 feet maximum per lift
High-lift: Build wall complete, grout in one operation. Requires cleanouts and inspection.
Finishing
Control Joints
- Prevent random cracking
- Place every 20-25 feet
- At changes in wall height
- At door and window openings
Joint Tooling
- Tool joints when mortar is thumbprint hard
- Horizontal joints first, then vertical
- Consistent pressure for uniform appearance
- Concave joint most weather-resistant
Cap Options
Solid cap blocks: Pre-made concrete caps
Mortar cap: Sloped mortar bed
Stone caps: Flagstone or cut stone
Cast concrete: Poured-in-place cap
Retaining Wall Considerations
Drainage Is Critical
Behind every retaining wall:
- 4” perforated drain pipe at base
- Gravel backfill (12” minimum)
- Filter fabric to prevent soil clogging
- Weep holes through wall (every 4-6 feet)
Engineering Requirements
Retaining walls over 4 feet typically require:
- Engineering drawings
- Building permit
- Inspection
- Structural reinforcement
Stepped Footings
For sloped sites:
- Footing follows grade in steps
- Level sections minimum 4 feet
- Steps no more than 2 feet
Common Mistakes
| Mistake | Problem | Prevention |
|---|---|---|
| No footing | Wall settles, cracks | Always pour adequate footing |
| Blocks not level | Crooked wall | Check every block |
| Insufficient mortar | Weak joints | Full bed, proper flanges |
| No reinforcement | Structural failure | Follow code requirements |
| Poor drainage (retaining) | Wall failure | Gravel backfill, drain pipe |
| Grouting dry blocks | Weak grout bond | Pre-wet blocks |
Building Codes
General Requirements
Most jurisdictions require:
- Permit for walls over 3-4 feet
- Engineering for retaining walls
- Inspection at specified stages
- Compliance with seismic requirements (if applicable)
Check Local Codes For
- Maximum height without engineering
- Required reinforcement
- Footing specifications
- Setback requirements
Cost Estimates
Materials Per 100 Square Feet
| Item | Quantity | Cost Range |
|---|---|---|
| 8” CMU blocks | 112 | $180-280 |
| Mortar | 8 bags | $40-60 |
| Rebar (if reinforced) | 50 linear feet | $30-50 |
| Grout (if reinforced) | 6 bags | $30-50 |
| Materials total | $280-440 |
Add professional labor: $400-600 per 100 sq ft
DIY Suitability
Good DIY Projects
- Garden walls under 3 feet
- Raised planters
- Small retaining walls (2-3 feet, well-drained)
- Non-structural partitions
Hire a Professional For
- Walls over 4 feet
- Retaining walls over 3 feet
- Any structural application
- Foundation walls
- Seismic zones
Calculate Your Block Wall
Ready to plan your project? Use our free Concrete Block Calculator for accurate material estimates.
Pro Tip: Stack your blocks near your work area before you start, organized by type. Nothing slows a project like hunting for the right block while mortar sets up. Also, rotate blocks so the best faces point outward—manufacturers mark the “show” side, but quality varies. A few minutes of sorting saves hours of regret looking at ugly blocks in your finished wall.