Raft foundation

A continuous reinforced-concrete slab foundation that distributes the building's loads over the whole footprint, «floating» on weak or uneven soils. Below, a blinding layer and a waterproofing separate it from the ground; a ventilated cavity detaches it from rising damp. It is the building's continuous foot.

The raft is the foundation that gives up isolated bearing points (footings, beams) to become a single large continuous slab under the whole building. Its logic is to reduce the pressure on the soil by spreading the load over a wide area: where the ground is weak, soft or uneven, the raft «floats» and evens out the settlements, avoiding cracks from differential settlement.

A building's load, concentrated on columns and walls, generates high pressures on the soil. Splitting it onto separate footings works as long as the ground is good; on low-bearing-capacity soils, however, the footings would become huge and differential settlements (one support sinking more than another) would crack the structure. The raft solves this by joining all the supports into a single rigid slab: the average pressure drops, the settlements even out and the building moves «all together».

In contact with the ground, the raft must defend itself from water. A blinding layer creates a regular casting surface and protects the bottom reinforcement; over it a waterproofing (membrane or bentonite, which swells to seal) stops the water and, where needed, resists the water-table pressure and the rise of radon gas. A ventilated cavity (dome formwork) finally detaches the lived-in level from the ground, creating a ventilated gap that drives off rising damp.

The raft is a major pour: thicknesses from 30 to 80 cm and more, with double reinforcement meshes top and bottom. The challenges are the heat of hydration (a massive pour heats up and, cooling unevenly, can crack) and shrinkage. They are managed with low-heat concretes, bay pours with studied joints, moist curing and, where required, shrinkage joints. The bottom cover, guaranteed by spacers on the blinding, protects the reinforcement from corrosion for the whole life of the work.