A ground bearing slab is a floor construction that rests directly on the ground without a raised subfloor or crawl space beneath it. The concrete slab sits on a compacted base layer and serves as both the structural floor and the interface with the earth. This construction method is widely used for ground floors in residential, commercial, and industrial buildings.
Application
Building a ground bearing slab begins with preparing and compacting the subgrade. A layer of compacted hardcore or gravel is placed on top to form a stable base. A damp-proof membrane (DPM) is then laid over the hardcore to prevent moisture from rising through the slab. Insulation boards are placed above the membrane, and the concrete slab is poured on top. The slab may be reinforced with steel mesh or fibre reinforcement depending on the expected loads.
The main advantage of a ground bearing slab over a suspended floor is its simplicity and cost-effectiveness. There is no need for floor joists, supporting walls, or ventilated void spaces. The slab is poured in a single operation and provides an immediately solid surface. It is also an excellent base for underfloor heating, as the concrete mass acts as thermal storage.
Moisture management is critical. Without a properly installed damp-proof membrane, ground moisture can migrate upward through the concrete by capillary action. This can cause problems such as mould growth, degradation of floor finishes, and structural dampness. The DPM must be continuous, with all joints lapped and sealed.
Insulation is typically placed below the slab so that the concrete can absorb and store heat from underfloor heating systems. Common insulation materials include EPS (expanded polystyrene), XPS (extruded polystyrene), and PIR boards. The required thickness depends on building regulations and thermal performance targets, typically ranging from 75 to 150 mm.
Related terms
- Foundation
- Concrete
- Insulation
- Reinforcement
- Damp-proof membrane
- Screed
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