Concrete Block Walls: How to Build Load-Bearing Walls

Concrete Blocks: A Proven Material for Load-Bearing Walls

Concrete blocks (also called CMU — Concrete Masonry Units, or “parpaings” in French) remain the most widely used material in France for building load-bearing walls of individual houses. Affordable, strong and easy to work with, they account for roughly 50% of new builds. This guide details the complete laying technique, from the first course to the ring beams.

Types of Concrete Blocks

Tools Required

Preparing the Mortar

The bedding mortar is the bond between the blocks. Its mix ratio must be consistent to ensure uniform strength.

Standard mix for bedding mortar:

• 1 part cement (CEM II 32.5)
• 3 parts 0/4 mm sand
• Approximately 0.5 parts water (adjust to consistency)

Step 1: The First Course (Levelling Course)

The first course is the most important. It determines the plumb and level of the entire wall.

1. Spread a thick mortar bed (2 to 3 cm) on the foundation strip
2. Lay the first corner block precisely (check level in both directions)
3. Lay the opposite corner block
4. Stretch a string line between the two corners
5. Lay the intermediate blocks following the string line
6. Check the level and alignment of each block
7. Adjust with the rubber mallet

Step 2: Laying Subsequent Courses

Staggering Rule

The vertical joints of each course must be offset by at least one third from the course below (ideally by half a block, i.e. 25 cm). This staggering (called “bonding”) ensures the mechanical connection between blocks.

Laying Technique

1. Spread a 2 cm mortar bed along the top of the lower course
2. Butter the end of the block to be laid (vertical joint)
3. Press the block against the previous one
4. Tap with the mallet to adjust
5. Check level, plumb and alignment with the string line
6. Remove excess mortar as you go

Joint Thickness

Step 3: Vertical Ring Beams (Columns)

Vertical ring beams are reinforced concrete columns built within the wall thickness. They are mandatory:

• At every corner of the building
• On each side of openings (doors, windows)
• Every 4 metres maximum of wall length
• At every wall junction

Construction:

1. Use corner blocks (U or L-shaped) to create the formwork
2. Place vertical reinforcement (minimum 2 or 4 HA10 bars + HA6 links every 20 cm)
3. Pour concrete (C25/30) and vibrate with a poker

Step 4: Horizontal Ring Beams

Horizontal ring beams encircle the building and distribute loads. They are mandatory:

• At each floor level (bottom and top ring beam)
• At the top of the walls (crown ring beam, under the roof structure)

Construction:

1. Lay lintel blocks (U-blocks) along the full wall length
2. Place horizontal reinforcement (minimum 4 HA10 bars + HA6 links every 20 cm)
3. Ensure reinforcement continuity at corners (minimum 50 cm laps)
4. Pour and vibrate the concrete

Step 5: Openings (Lintels)

Each opening (door, window, patio door) requires a lintel to carry the loads above.

Lintel Options

Key rules:

• The lintel must bear at least 20 cm on the wall on each side
• Lintel dimensions must be calculated by the structural engineer
• Provide temporary propping during concrete hardening

Common Mistakes to Avoid

1. Neglecting the first course: if it is not perfectly level, every subsequent course will be wrong
2. Joints too thick or uneven: they weaken the wall and create thermal bridges
3. Forgetting to stagger joints: a wall without bonding has no mechanical strength
4. Building too high too fast: do not exceed 1.20 m height per day to allow mortar to set
5. Neglecting ring beams: they are mandatory and essential for stability

Indicative Budget for 100 m² of Walls

The saving compared to a professional quote (€6,000 to €12,000) is significant, but the work takes time: allow 2 to 4 weeks for two people to build the walls of a single-storey house.