You must ensure steel reinforcement is lapped at least 450mm (for both mesh and mild steel bars) and provided with 40mm concrete cover.
This stone should be mechanically compacted.
The raft design will normally need a hardcore sub-base to level the ground.
Very poor ground may mean you need a different solution such as piles.
A site investigation is usually needed for the engineer to understand what the ground is like.
These beams transfer the building loads through the rest of the slab and then evenly over the ground.
Internal stiffening beams are sometimes needed too.
The raft design usually has an 'edge beam' formed by a cage of steel reinforcement, which will need to be carefully assembled on site.
Here are important considerations if you're thinking about using a raft foundation: This is an alternative if you can't use a traditional strip or trench fill foundations.īut it's important to note that raft foundations aren't suitable in all cases and usually need to be designed by a structural engineer. This type of foundation spreads the load of the building over a larger area than other foundations, lowering the pressure on the ground. 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.