BEWI Claymaster (Clay Heave Protection) is a product designed to protect against clay heave.

Shrinkable clays are soils containing a large proportion of mineral particles of less than 0.002mm in diameter.

This produces an extremely cohesive soil type when wet, but shrinks to a crumbly, crazed structure when dry. The expansion and contraction process during wet and dry periods is known as ‘clay heave’. This can cause severe problems with foundations.

Claymaster works by surrounding the pile cap with a layer of low density, compressible, expanded polystyrene, reducing pressure on the ground beams as the surrounding clay expands and contracts.

Key Benefits

  • Solves clay heave problems
  • Easy to install
  • Resistant to moisture
  • Lightweight
  • Durable
  • Negligible compression during concrete casting
  • No need for specialist equipment
  • A+ rated in BRE Green Guide to Specification

NHBC Requirements

Thickness of Claymaster to comply with NHBC requirements
Soil heave potential Underside of beam Against side of foundations & beams
NHBC void dimension Equivalent Claymaster NHBC void dimension Equivalent Claymaster
High 150mm 250mm 35mm 75mm
Medium 100mm 175mm 25mm 50mm
Low 50mm 100mm 0mm 0mm

Product specification

Board Size Standard Thicknesses
2400mm x 1200mm 50, 75, 100, 150 and 200mm


Which thicknesses are available to order in Claymaster?

Standard thicknesses are 50, 75, 100, 150 and 200mm. Other thicknesses may be available to order.

When is Claymaster used?
Claymaster is a compressible-fill material used to reduce the pressure against foundations due to clay heave