Building a Block Wall: Basic CMU Steps

Concrete Masonry Units (CMU) Overview

Concrete block walls go up faster than brick, but the layout still has to be right. The footing, first course, reinforcement, and grout all matter. Start here if you are building a small non-structural CMU wall.

Important: Structural, retaining, or engineered walls may require plans, permits, and inspection.

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

1. Snap chalk line on footing for wall face
2. Dry-lay first course without mortar
3. Check length - adjust block spacing or plan cuts
4. Mark block locations on footing

Mortar Bed

1. Mix mortar to proper consistency (see Mixing Mortar Guide)
2. Spread full mortar bed on footing
3. Furrow mortar with trowel
4. Work in sections you can lay before mortar sets

Setting First Block

1. Start at corner or end
2. Set block into mortar bed
3. Press down firmly to compress joint to 3/8”
4. Check level in both directions
5. Check alignment with chalk line
6. Tap to adjust while mortar is plastic

Completing First Course

For each block:

1. Apply mortar to end (head joint):

   • Stand block on end
   • Apply mortar to flanges (not full end)
   • Create good coverage on outer edges

2. Set block against previous block

3. Push firmly to compress head joint

4. Check level with adjacent blocks

5. 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):

1. Build up corner 3-4 courses
2. Step back each course
3. Check plumb on both faces
4. Creates reference for string line

Filling between corners:

1. Stretch string line between leads
2. Lay blocks to string
3. Check each block for level
4. 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:

1. Set rebar in footing (dowels)
2. Thread blocks over rebar as you build
3. Fill cores containing rebar with grout
4. 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

1. Install cleanouts at bottom (for full-height grouting)
2. Clean cells of mortar droppings
3. Pre-wet blocks (dry blocks absorb water from grout)
4. Pour grout to fill cells
5. Consolidate with rod or vibrator
6. 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

Need exact block counts? Use our Concrete Block Calculator.

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Pro Tip: Stack blocks near the wall and sort the best faces first. That saves time while the mortar is still workable.