Soil block walls are strong in compression, but they dislike standing water. That makes the foundation and the first few courses the most critical part of the building. The advice below is general. Foundation depth and size depend on your soil, the building loads and local regulations, so always get them designed by a qualified engineer.

Know your ground

Look at what is under the site. Firm, well-drained ground is the easiest to build on. Black cotton soil and other expansive clays move as they wet and dry, and may need special foundation details. Soft or filled ground needs careful assessment.

Strip foundations

For many single-storey houses, a reinforced or mass-concrete strip foundation under each wall is the common solution. It spreads the wall load over a wider area of ground. The trench must reach firm soil, be level, and be free of loose material and standing water before concrete is poured.

Raise the plinth

Keep the lowest soil blocks well above the finished ground level. A raised plinth built from concrete or another water-resistant material keeps splashing rain and ground moisture away from the earth walls. Heights of several hundred millimetres are common, and your engineer will specify what suits your site.

Damp-proof course

A damp-proof course (DPC) between the plinth and the first soil block course stops moisture rising into the wall. Make sure it is continuous at corners and joints, and that the mortar bed under the first course is level and well finished.

Manage water around the building

• Slope the ground away from the walls on all sides.
• Provide a paved apron or a gravel strip around the house to reduce splashback.
• Use gutters and downpipes, discharging well away from the foundation.
• Keep gardens and watering points away from the base of walls.

Paving and drainage

Hard surfaces around the building direct water away. Concrete paving blocks are a practical option. See our guide to making cabro paving blocks.

Where to go next

With the foundation done, move on to building the walls, and plan the roof early so overhangs and gutters are part of the design from the start.

• damp proofing
• foundations
• housing