Raft foundation



Raft foundation design

Foundation rafts can often provide an economic solution when there are difficult ground conditions on the site. For example, if the ground is made up, or the ground has a very low bearing capacity. They can also sometimes be used when there are shallow mine workings below the site. However, this will depend on the depth and the thickness of the workings.

Rafts may also be an economic solution to reduce the foundation depth in shrinkable clay soils.  However, this is not always the case.   This type of raft needs a thick layer of stone sub base beneath it and this increases the cost.  Therefore, it is not always the best solution.

A foundation raft is designed to spread the weight of the building over as large an area as possible.  This reduces the bearing pressures on the soil. The raft is designed so that any small differential settlements do not cause any major cracking in the building.  However, it does not stop the settlement from taking place.  This may be imprtant where a new raft joins an existing building.  However, this type of movement can often be dealt with by suitable  detailing.

The raft consists of a reinforced concrete slab.  The slab normally has mesh reinforcement.  Sometimes stiffening beams have to be provided beneath loadbearing walls or around the edges. These beams are normally reinforced with steel cages.

Please note that if you need a bending schedule for the reinforcement, you can add this as an option.

We need the following information for a raft foundation design

architectural layouts or sketch drawings with dimensions of the building


for extensions we will also need details of the existing house foundations

a site investigation report or details of the ground conditions

details of any nearby drains and trees

Design information

All designs are carried out in accordance with BS 8110 and BS 8004:1986

The calculations provided will be suitable for Building Regulation approval

You should satisfy yourself that the excavation for the foundations will not undermine any adjacent foundations or neighbouring ground

Excavations may fall under the Party Wall Act.  Please seek specialist advice