Raft Foundations: When and Why to Use Them

A raft foundation spreads load across an entire reinforced concrete slab rather than concentrated strips. They are the right choice in specific ground conditions and the wrong one in many others.

What a raft is

A continuous reinforced concrete slab covering the building footprint, sometimes thickened around edges or under columns. Building loads distribute across the whole raft area rather than concentrating on narrow strips.

When rafts win

• Heave-prone clay: Spreads load uniformly, accommodates differential movement
• Variable ground: Made ground, mixed fill — raft averages out the variation
• Low bearing capacity: Soft soils where strip foundations would need to be very wide anyway
• Cellular structures: Buildings with many walls already approximating a raft layout
• Light buildings on poor ground: Sheds, conservatories
• High water table: Floats above seasonal variation

When rafts lose

• Concentrated heavy loads from columns (better with pad foundations or piles)
• Good bearing ground (strips are cheaper)
• Sloping sites (rafts have to step or terraces — complex)

Design considerations

• Typical residential raft: 250–450mm thick, two-way reinforcement
• Edge beams or downstands to handle perimeter wall loads
• Thermal break around perimeter for Part L compliance
• Insulation under raft for habitable ground floors
• DPM (damp proof membrane) below

Cost

• Domestic raft (50–100m²): R160,000–R400,000
• Commercial raft: R1,600–R2,800 per m² depending on thickness and steel

Often cheaper than equivalent piled solution; sometimes cheaper than deep trench-fill.

Common errors

• Under-specified reinforcement on clay sites
• Inadequate edge thickening for wall loads
• Missing radon barrier in radon-affected areas
• Poor cold-joint detailing

MCFAR designs rafts where ground conditions justify them.

Frequently Asked Questions