We have been exploring savings on a house build for a client on the use of trench or foundation blocks for the buried part of the masonry walls for the property.  For those who don’t know their trench blocks from their normal blocks, a trench block is a lightweight interlocking block brought on the construction market recently.  They come in standard strength and hi strength grades and have hand holds built into the block for carrying.

These blocks are greater in size than normal blocks with a face dimension size of 440mm x 215mm and are available in a range of thicknesses ranging from 255mm to 300mm, and 355mm. One trench block is equivalent in size to two aggregate blocks, or 12 bricks, plus wall ties and infill.

One supplier is Forterra (under their thermalite brand), but there are lots of other suppliers available.


The benefi ts to the builder of using blocks below ground are:

  • Speed of build.
  • Lightweight and safe.
  • Less waste as blocks can be cut accurately.
  • Lower cost.

Independent tests have shown that you can build foundations at least twice as fast by using trench blocks and four times as fast as building with common brick.

Blocks are available in a weight less than 20kg with handholds and will comply with CDM Regulations which cover health and safety issues on site for repetitive handling.

Trench Block Detail

The use of trench blocks as a form of construction incorporates a blockwork foundation below ground level with a traditional brick and block cavity wall above. The cavity can be partially filled or full filled as required. This type of construction offers a structure with good sound and thermal insulation qualities.

foundation 1


A double blockwork cavity filled wall is an alternative solid block foundation construction. The blocks below ground level are laid to form a cavity which is then filled up to ground level with a cement/aggregate mix. The cavity can be partially filled or fully filled as required.  Filling the cavity prevents water standing in the base of the wall allowing deterioration of the masonry construction over time.

foundation 2