Interlocking blocks: how mortarless construction works

Interlocking blocks have shaped tops and bottoms, keys and grooves, so each course locks into the one below. Walls go up with little or no mortar between blocks, and in hollow types the reinforcement and grout run through vertical holes. The idea works with compressed earth blocks and with concrete blocks.

How the interlock works

A conventional brick wall relies on a bed of mortar to hold courses in place and to take up differences in brick size. An interlocking block uses its shape instead. Raised parts on one block sit in recesses on the block above, which stops courses sliding and keeps the wall in line. Because the joints are dry or very thin, the blocks must be made to tight, consistent dimensions.

Where interlocking saves

• Mortar: little or none between courses, depending on the system.
• Time: laying is quicker once the first course is set.
• Skill: keeping a wall plumb is easier when the blocks guide each course, so crews learn quickly.
• Finish: uniform faces can be left exposed or plastered thin.

What makes or breaks an interlocking wall

1. Block accuracy. A block a few millimetres too tall throws out every course above it. The mould and the press decide this.
2. The first course. It must be laid dead level on a true base, usually in mortar. Every course above copies its errors.
3. Engineering. Openings, corners, ring beams and reinforcement should follow an engineer’s design, especially in seismic zones.
4. Protection. Earth-based interlocking blocks need the same care as other CSEB walls: a good plinth and roof overhangs.

Types of interlocking block

• Solid interlocking soil blocks for walls in homes and low-rise buildings.
• Hollow interlocking blocks with vertical holes for reinforcement and grout.