Raft Foundation Guide: When to Use a Raft, Design Principles and Building Control Submission Requirements

Summary

A raft foundation replaces the traditional strip and pad system with a single reinforced concrete slab that distributes the building's load over the full footprint area, reducing the bearing pressure on the ground. By spreading the load widely, a raft can work on ground that would be inadequate for strip foundations.

Rafts are common in several scenarios: on shrinkable clay where strip depths would be impractical; on made or filled ground; on sites with variable ground conditions where differential settlement is a risk; and on low-bearing-capacity soils where the strip width required under Approved Document A tables becomes impractically large.

Raft foundations are not a cheap or quick solution. They require reinforcement, careful formwork, and more concrete than an equivalent strip foundation. They also require a structural engineer's design — there is no prescriptive table equivalent to Approved Document A Table A1 for raft foundations. However, on the right ground conditions, a raft is often the only cost-effective solution that provides uniform support across the building.

Key Facts

• No Approved Document A table — raft design requires a structural engineer's calculations; cannot be done from tables
• Reinforced concrete — all raft foundations are reinforced; RC35/37 or RC40 typically; mesh or bar reinforcement as specified
• Suitable for — low-bearing-capacity soils, variable ground, made/filled ground, shrinkable clay (shallow raft or stiffened raft)
• Edge thickening — raft edge beams (upstand or downstand beams) provide stiffness and improve load distribution at the perimeter
• Stiffened raft — raft with integral downstand beams under walls; better for heavier loads or more variable ground
• Beam and block floor — not a raft; beam and block is a suspended floor system; raft is a structural foundation
• Minimum thickness — typically 225–300mm for a flat raft; thicker under concentrated loads (columns, lintels)
• Cover to reinforcement — minimum 40mm cover to bottom reinforcement (exposed to ground); 25mm top; check engineer's specification
• Blinding layer — minimum 50mm of weak concrete (ST2 or lean mix) on levelled ground before raft reinforcement is placed
• Hardcore layer — compacted hardcore typically 150–300mm below the blinding; required to create a stable formation and raise floor level
• DPC/DPM — damp-proof membrane (1200 gauge minimum polythene) placed on blinding or under blinding, depending on design
• Building control — engineer's drawings and structural calculations must be submitted and approved before work commences

Quick Reference Table: Raft vs Strip Foundation Selection Factors

Detailed Guidance

When to Choose a Raft Foundation

A raft foundation is appropriate when:

1. Low bearing capacity soils If the ground investigation reveals soil bearing capacity below approximately 50–75 kN/m², strip foundations require very wide strips that become impractical. A raft spreads the load over a larger area, reducing the bearing pressure to within the soil's capacity.

2. Variable ground conditions When different parts of the site have different bearing capacities (e.g. soft clay near a pond, firmer sand further away), strip foundations will settle differently under different parts of the building — causing cracking and distortion. A raft is inherently more tolerant of variable ground because its stiffness redistributes loads from weaker to stronger areas.

3. Made ground and fill Brownfield sites often contain fills of varying composition, compaction, and depth. Strip foundations on fill are high-risk unless the fill is well-compacted engineered fill. A raft — particularly a stiffened raft with edge beams — is a more robust choice on made ground.

4. Shrinkable clay On high-shrinkage clays near trees, strip foundations must be very deep (sometimes 1.5m or more). An alternative is a shallow stiffened raft designed to span any locally heaving ground without cracking. This approach is detailed in NHBC Chapter 4.2 and requires structural design.

5. Contaminated ground Where ground contamination means the concrete must be specially formulated, and where minimising the volume of concrete in contact with the ground is desirable, a raft can sometimes reduce the perimeter of concrete exposed to contaminated soil compared to deep strip trenches.

Raft Foundation Types

1. Flat raft (simple slab) A uniform-thickness reinforced slab across the full building footprint. Simple to construct but less efficient structurally. Suitable for light loads on uniform, low-bearing soils.

2. Stiffened raft (beam and slab) A reinforced slab with integral concrete beams beneath wall lines (downstand beams) or upstand beams above the slab. The beams provide additional stiffness and improve load distribution. More common for housing on shrinkable clay or variable ground.

3. Cellular raft A raft with orthogonal grid of beams creating cellular boxes, maximising stiffness for minimum concrete volume. Used for heavier structures on poor ground. Less common in domestic work.

4. Piled raft A raft combined with piles to transfer loads to deeper, more competent soil layers. Used when the near-surface soil is too weak even for a raft and when ground investigation reveals suitable strata at depth. Most complex and expensive option.

Ground Preparation Before a Raft

The preparation below the raft is critical and is often where mistakes are made:

Step 1: Site investigation A raft design requires ground investigation data: soil description, bearing capacity, depth to acceptable bearing stratum, shrinkage potential of clays, presence of fill, groundwater level. Do not proceed without adequate ground investigation.

Step 2: Bulk excavation to formation level The formation level is the level at which the hardcore or blinding will start, typically 300–450mm below finished floor level (depending on construction depth).

Step 3: Compacted hardcore

• Minimum 150mm compacted granular fill (Type 1 MOT or equivalent) at formation level
• Compact in maximum 150mm layers
• Edge restraint required to prevent granular fill from spreading during compaction
• Do not over-compact near existing structures or drainage (vibration risk)

Step 4: Blinding concrete

• Minimum 50mm of ST2 (weak) concrete over hardcore
• Provides a clean, level, dry working surface for reinforcement placement
• DPM may be placed on top of blinding or between blinding and hardcore depending on engineer's specification

Step 5: DPM

• 1200 gauge polyethene sheet minimum
• Lapped joints minimum 300mm
• No punctures; any damage must be taped before concrete is placed

Step 6: Reinforcement placement

• Per engineer's drawing and specification
• Check cover using appropriate bar chairs or spacers
• Bottom reinforcement: typically 40mm cover to ground face
• Top reinforcement: typically 25mm cover
• Check lap lengths (per engineer's design, typically 40× bar diameter)

Step 7: Concrete pour

• Pour and compact all in one continuous operation where possible
• Vibrate carefully — avoid touching the DPM with the vibrator needle
• Level to the top of the raft; allow for floor finish depth if screed or tiles are to follow
• Cure: protect from frost, rain, and drying wind for minimum 3 days

Building Control Submission Requirements

Building control will require the following before approving a raft foundation:

1. Ground investigation report — borehole logs, trial pit descriptions, soil bearing capacity data
2. Structural engineer's design — raft type, dimensions (thickness, beam depths/widths), reinforcement layout (plan and sections)
3. Structural calculations — demonstrating the raft can carry the building loads without exceeding soil bearing capacity and limiting differential settlement
4. Concrete specification — grade, cement type, admixtures, exposure class
5. Construction sequence notes — any specific requirements for compaction, blinding, DPM installation

The building control officer will inspect: the prepared formation before blinding, the reinforcement before pouring, and the poured concrete after curing. Do not pour blinding without formation inspection approval, and do not pour the raft without reinforcement inspection approval.

Frequently Asked Questions

Can I use an Approved Document A table to design a raft foundation?

No. Approved Document A Table A1 only applies to strip foundations for traditional masonry construction. There is no equivalent prescriptive table for raft foundations. A raft always requires a structural engineer's design with calculations.

Is a ground-bearing slab the same as a raft foundation?

No. A ground-bearing floor slab (often called a GBS) is a floor slab that bears on the ground but is not a structural foundation. It is not designed to transfer building loads to the ground. A raft foundation is structurally designed to carry the building loads. They may look similar but serve different purposes. A raft also typically has edge beams and thicker sections under walls; a floor slab is usually a uniform thin slab.

How thick should a raft foundation be?

This depends on the structural design, but typical domestic raft foundations are 225–300mm for a flat raft. Stiffened raft beams (downstand under walls) are typically 450–600mm deep below the slab. The engineer's calculation determines the required thickness based on applied loads and ground conditions.

Can a raft foundation be used for a timber frame building?

Yes. Raft foundations are used under both masonry and timber frame buildings. The load distribution is different — timber frame buildings tend to have more concentrated loads at stud and post positions — but the raft design can accommodate this. The structural engineer will design the raft to suit the specific structural frame.

Regulations & Standards

• Building Regulations 2010 Approved Document A — Structure; confirms raft design requires engineer's input (no table equivalent)

• BS 8004:2015 — Code of practice for foundations; raft design principles

• BS 8110-1:1997 — Structural use of concrete — Code of practice for design and construction; applies to reinforced raft design [verify current status relative to BS EN 1992]

• BS EN 1992-1-1 (Eurocode 2) — Design of concrete structures; increasingly used for raft design

• NHBC Standards Chapter 4.2 — Stiffened raft options for clay shrinkage sites

• CDM Regulations 2015 — groundworks safety obligations

• Approved Document A — GOV.UK — Building Regulations structure guidance

• BS 8004:2015 via BSI — foundations code of practice

• NHBC Chapter 4.2 — foundations in shrinkable soils

• strip foundation design — comparison with strip foundation design and Table A1 use

• cdm regulations groundworks — CDM obligations for groundworks

• excavation safety trench support — excavation safety during groundworks

• building regs part h drainage — drainage design coordinated with raft foundation