Raft Foundation Guide

A raft foundation is a stiffened concrete slab that spreads structural loads across most or all of the building footprint. Engineers specify rafts where near-surface soils are variable, soft, or prone to differential settlement, or where a continuous plate is more practical than deep strip trenches.

This page explains what raft construction involves. Strip-versus-raft comparison lives on its dedicated guide; cost drivers on foundation cost pages and /groundworks-costs. Mainline builds rafts to design — we do not choose raft versus piles for you.

review Groundworks Contractors in Wandsworth and Groundworks Contractors in Kingston.

When this topic usually comes up

When GI recommends a raft; when an extension or new-build drawing shows a reinforced slab with edge thickenings; or when service coordination under a raft needs early planning.

What it covers in practice

Typical works include formation preparation to level, blinding, reinforcement (often with edge beams or thickenings), concrete placement in a controlled pour, and curing protection.

Drainage and services that pass through or under the raft zone must be coordinated before steel is fixed. Soft spots at formation require designer instruction — do not “make up” with random fill.

Rafts are still shallow foundations; they do not replace piles where the engineer requires deep foundations.

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Common triggers and situations

Raft or stiffened slab on structural drawings
GI citing variable or soft near-surface soils
Edge thickenings or ground beams integral to the raft
Services routed under or within the slab zone
Differential settlement risk across the footprint

How teams usually progress from here

Prepare formation, fix steel, pour to specification, and protect the slab. Complete inspection holds and retain pour records.

Practical checklist

Raft drawing revision and schedules
GI alignment with raft choice
Service and drainage coordination
Pour access and curing plan
Inspection stages for formation and steel

What affects programme and complexity

Reinforcement tonnage, formation treatment, pour size, and service coordination drive cost. Compare whole packages, not slab £/m² alone — see raft foundation cost on the costs hub.

Typical next steps

Step 1: Prepare formation
What this step delivers: Levels and bearing accepted.
Step 2: Fix reinforcement
What this step delivers: Cover and detailing correct.
Step 3: Pour and cure
What this step delivers: Concrete to specification.
Step 4: Protect and hand over
What this step delivers: Ready for next build stage.

Frequently asked questions

Is a raft cheaper than piles?

Sometimes — not always. It depends on ground, slab specification, and dig. The engineer compares options; cost pages help budgeting conversations.

How is a raft different from a floor slab on strips?

A structural raft is designed to spread building loads. A suspended or ground-bearing floor slab on separate strips is a different system — follow the structural drawings.

How This Issue Is Normally Diagnosed and Repaired

Follow the typical path from problem identification through to resolution:

We provide these services across the UK, including

London Richmond Chiswick Wimbledon Kingston Fulham Hammersmith Ealing

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Have raft drawings? Request a foundation construction quote.

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