Raft foundation: meaning and measurement

Prepared by Eng. Ahmed Fathy, Projects Manager · Last reviewed: 8 November 2026

Definition

A raft is a thick reinforced concrete slab that extends under the whole building and spreads its loads over a large area of soil. It is used when the soil is weak, when isolated footings would cover more than half the area, or with a basement to resist water pressure.

Why it matters on a project

Choosing a raft or isolated footings changes concrete, steel and excavation quantities a great deal, and the choice comes from the soil report and load calculation, not preference.

Measurement and pricing

Concrete in cubic metres (length × width × thickness), steel in tonnes from the bar bending schedule, and waterproofing under and around it in square metres.

Worked example

A 20 x 15 m raft 0.8 m thick: concrete 20 × 15 × 0.8 = 240 m³, and at an estimated 100 kg/m³ the steel is about 24 tonnes, with the final figure from the bar bending schedule.

Common mistake

Casting the raft on uncompacted soil or without blinding and waterproofing, exposing it to uneven settlement or basement leaks.

Frequently asked questions

When do I need a raft instead of isolated footings?

When the soil report recommends it for low bearing capacity, when footings would cover more than half the area, or with a basement that must resist water pressure.

Is a raft always more expensive?

Not always. On weak soil it can be cheaper than large closely spaced footings with many ground beams, compared through a take-off of both options.

Related terms

Footings (foundations)Structural elements that transfer the building loads to the soil.
Soil investigation reportThe borehole study on which the foundations are designed.
WaterproofingLayers preventing water ingress into structural elements or spaces, such as bituminous membranes and cementitious coatings.
Bar bending schedule (BBS)A schedule listing the diameter, length, shape and number of reinforcement bars per element.

Raft foundation