A raft foundation is a reinforced concrete slab under the whole of a building or extension, 'floating' on the ground as a raft floats on water. This type of foundation spreads the load of the building over a larger area than other foundations, lowering the pressure on the ground.

This is an alternative if you can’t use a traditional strip or trench fill foundations. However it is important to note that raft foundations aren’t suitable in all cases and usually need designing by a structural engineer.

Here are important considerations if you’re thinking about using a raft foundation:

  • The raft design usually has an ‘edge beam’ formed by a cage of steel reinforcement, which will need carefully assembling on site. Internal stiffening beams are sometimes needed too. These beams transfer the building loads through the rest of the slab and then evenly over the ground.
  • A site investigation is usually needed for the engineer to understand what the ground is like. Very poor ground may mean you need a different solution such as piles.
  • The raft design will normally need a hardcore sub-base to level the ground. This stone should be mechanically compacted.
  • You must ensure steel reinforcement is lapped at least 450mm (for both mesh and mild steel bars) and provided with 40mm concrete cover. ‘Treading in’ the reinforcement during the concrete pour isn’t a suitable means of placing reinforcement - use the special ‘chairs’ or ‘soldiers’ to support the mesh.
  • The edge of the raft must be carefully detailed for the damp course and membrane position, which may need a ‘step’ forming in the concrete at the raft edge.
  • Insulation is usually placed over the top of the raft - take care to avoid cold bridging at the junctions with external walls.

Important! A structural engineer designing a raft foundation may not be fully aware of damp coursing, cold bridging or land contamination issues. Ensure that the building designer considers these details before constructing the raft - once the concrete is poured it may be difficult to overcome.

If in doubt, speak to your local authority building control surveyor. Use our free post code search tool to find contacts for your local LABC team.