One of the attractions of interlocking blocks is that the shapes guide the builder, so walls can go up quickly with less mortar than conventional masonry. The details still matter, though, and your design should always come from a qualified engineer or architect. This is a general overview of what happens on site.

1. Prepare a level base

Walls are only as straight as the foundation under them. Make sure the foundation or plinth is level and clean, and that a damp-proof layer is in place (see our guide to foundations for interlocking block houses).

2. Set out and lay the first course

The first course is the most important. Set out the walls from your drawings, check diagonals to confirm the corners are square, and bed the first course on a thin, level mortar layer. Use a spirit level on every block and a string line along the wall.

3. Stack the following courses

Interlocking blocks locate themselves, so each course should drop into the one below. Stagger the vertical joints from course to course as the design shows. If a block does not seat properly, remove it and check for debris or a damaged edge rather than forcing it.

4. Reinforcement and grouting

Most interlocking block systems include vertical holes or ducts. Depending on the design, these are used for steel reinforcement and cement grout at corners, around openings and at intervals along the wall. Follow the engineer’s details exactly.

5. Openings and lintels

Plan door and window openings so they work with the block dimensions to reduce cutting. Provide lintels as specified and allow bearing length on both sides.

6. Services in the wall

The ducts can carry electrical conduit and sometimes small pipes. Plan runs before laying, as chasing finished walls weakens them.

7. Ring beam and roof

A reinforced ring beam at the top of the wall ties everything together and spreads roof loads. After it has cured, the roof structure can go on. A generous roof overhang protects the walls, as explained in protecting earth block walls from rain.

Check your blocks first

Consistent block size makes everything above easier. Blocks from a machine such as the Standard Straight Interlocking Block Press need to be well cured and uniform before they go in a wall. See also our overview of how interlocking soil blocks compare with stone.

• housing
• interlocking blocks
• wall construction