Concrete Slab Foundation Calculator 2026
Raft foundations (mat foundations) comprise continuous reinforced concrete slabs extending across entire building footprint, distributing loads over maximum ground area. They suit weak soils (bearing capacity below 75 kN/m²), variable ground conditions, made ground, subsidence risk areas, or buildings with heavy irregular loads where strip footings would cause differential settlement. UK costs range £60-100 per m² including concrete, reinforcement, and installation.
Slab Thickness: Light rafts for single storey buildings use 150-200mm thickness, standard two storey construction requires 200-250mm, while heavy loads or commercial buildings need 250-300mm slabs. Thickness depends on building loads, soil bearing capacity, and structural engineer calculations per BS 8004 standards.
Edge Beams: Most rafts incorporate edge beams (300-600mm deep) around perimeter providing additional bending resistance and supporting external walls. Edge beam dimensions: light (300mm × 400mm) for single storey, standard (400mm × 500mm) for two storey, heavy (500mm × 600mm) for three storey or commercial loads.
Reinforcement: Raft slabs require steel mesh reinforcement both directions maintaining structural integrity and controlling cracking. Standard mesh types: A142 (6mm bars @ 200mm c/c) for light loads, A193 (7mm bars) for standard residential, A252 (8mm bars) for heavy loads, A393 (10mm bars) for commercial applications. Two layers (top and bottom) required for heavily loaded slabs.
| Building Type | Slab Thickness | Edge Beam | Reinforcement |
|---|---|---|---|
| Single Storey | 150-200mm | 300 × 400mm | A142 / A193 |
| Two Storey | 200-250mm | 400 × 500mm | A193 / A252 |
| Three Storey | 250-300mm | 500 × 600mm | A252 / A393 |
| Commercial | 300mm+ | 600mm+ deep | A393 or rebar |
Concrete C30/37 grade costs £85-105 per m³ delivered ready-mix. Steel reinforcement mesh: A142 £5-6/m², A193 £6-7/m², A252 £8-9/m², A393 £12-14/m². Typical 12m × 9m (108m²) raft with 200mm slab and light edge beams requires: 24.5m³ concrete (£2,085-2,570), 250m² mesh A193 (£1,500-1,750), excavation £500-800, labour £1,800-2,500. Total cost £5,885-7,620 or £54-71 per m².
Raft foundation is a continuous reinforced concrete slab covering entire building footprint, distributing loads over maximum ground area. Used for weak soils (bearing capacity below 75 kN/m²), variable ground conditions, made ground, high water table, subsidence risk areas, or where differential settlement could damage structure. Costs £60-100 per m² compared to strip footings £80-150 per linear metre.
Raft slab thickness ranges 150-300mm depending on building loads. Single storey buildings require 150-200mm, two storey 200-250mm, three storey or commercial 250-300mm. Edge beams add 300-600mm depth around perimeter for additional strength. Structural engineer specifies exact thickness based on soil conditions, building loads, and BS 8004 calculations. Minimum 150mm for any habitable structure.
Calculate main slab: Length (m) × Width (m) × Thickness (m). Example: 12m × 9m × 0.2m = 21.6m³. Calculate edge beams: Perimeter × Beam Width × Beam Depth. Example: 42m × 0.3m × 0.4m = 5.04m³. Total: 21.6 + 5.04 = 26.64m³. Add 10% wastage = 29.3m³. Use calculator for accurate estimates including complex shapes and multiple edge beam configurations.
Raft slabs require steel mesh reinforcement: A142 mesh (6mm bars @ 200mm c/c) for light single storey, A193 mesh (7mm bars) for standard two storey, A252 mesh (8mm bars) for heavy loads, A393 mesh (10mm bars) for commercial. Mesh placed centrally in slab or two layers (top and bottom) for heavy loads. Edge beams require separate reinforcement cages with 12-16mm main bars. Structural engineer specifies exact reinforcement meeting BS 4449 standards.
Raft foundations cost £60-100 per m² in UK including materials and labour. Breakdown per m²: concrete (£15-25), reinforcement (£8-15), excavation (£5-10), hardcore/insulation (£8-12), labour (£20-35), building control (£2-5). Total project costs vary: 100m² raft £6,000-10,000, 150m² raft £9,000-15,000. London and South East command 20-30% premiums. Poor ground conditions requiring deeper edge beams or additional reinforcement increase costs toward upper ranges.
Yes, UK Building Regulations require structural engineer calculations for all raft foundation designs ensuring adequate bearing capacity, slab thickness, reinforcement detailing, and edge beam specifications. Engineer considers site-specific soil conditions through ground investigation, calculates building loads, determines appropriate concrete grade, specifies reinforcement, and ensures BS 8004 compliance. Never construct raft foundations without approved structural calculations and building control approval risking structural failure, settlement damage, and enforcement action requiring costly remediation.