Raft slab
How it works
Section titled “How it works”A raft is a continuous reinforced concrete structure under the whole building, spreading the load over a large area. That reduces the pressure on the ground and reduces differential settlement.
Why it suits a light structure
Section titled “Why it suits a light structure”Even movement
Section titled “Even movement”An accurate, stiff frame does not tolerate differential settlement well. The raft “bridges” local weaknesses in the ground.
Small thickness
Section titled “Small thickness”Because the load is low, the slab can be thinner than for a heavy building.
It is the floor base as well
Section titled “It is the floor base as well”The top of the slab is the base of the floor, so no separate screed base is needed.
A flat surface
Section titled “A flat surface”Good for a panelised structure, because it gives one well-controlled plane — provided the pour is accurate. Setting out and foundation tolerances.
What has to be solved
Section titled “What has to be solved”Insulation
Section titled “Insulation”Insulation under the slab and at its edge, to handle the thermal bridge. Leave it out and significant heat is lost at the perimeter. Slab edge.
Ground moisture and vapour
Section titled “Ground moisture and vapour”Damp-proofing beneath the slab and separation towards the structure. Damp-proofing at the base.
Radon protection under the slab where it is warranted. Radon protection.
Anchorage
Section titled “Anchorage”Resisting the tension arising at the ends of the shear walls. The weight of the slab helps, but the local connection has to be designed.
Service penetrations
Section titled “Service penetrations”The position of pipes passing through the slab has to be known in advance, because it is hard to do afterwards.
When it is not the right choice
Section titled “When it is not the right choice”- on ground of very good bearing capacity, where a strip footing is substantially cheaper,
- on a sloping site, where a raft needs a lot of earthwork,
- where the building is temporary, or a removable foundation is needed.
The point
Section titled “The point”A raft is not only a foundation but a build-up: insulation, moisture protection and floor base at once. So designing it is a building-physics task too.