The short answer
A ground-bearing slab sits on compacted fill and works well on good, level ground with shallow foundations. A suspended beam and block floor spans between the walls and suits most other situations: poor or sloping ground, clay with trees nearby, deep fill, and sites where speed matters.
Side by side
| Beam & block | Ground-bearing slab | |
|---|---|---|
| Ground conditions | Works on poor, sloping or variable ground | Needs good ground and limited fill depth |
| Speed | Laid in hours, usable straight away | Fill, compaction, steel, pour and curing |
| Weather | Little affected by rain once laid | Concrete pours are weather dependent |
| Drying out | Dry deck, only the screed needs drying | Slab and screed both need drying time |
| Ventilated void | Yes, keeps the floor dry and helps with ground gases | No void, relies on the membrane |
| Design | Layout and calcs from the supplier | Usually designed by the project engineer |
When a slab may be better
On firm, level ground with very little fill, a slab can be simple and economical, and some designs suit it, such as certain underfloor heating systems or very large open floors. If you are not sure which suits your site, send us your plans and we’ll tell you honestly.
When beam and block wins
- Fill depth would be more than the limit Building Control usually accepts under a slab
- Clay soils with trees nearby, where heave is a risk
- Sloping sites where the floor would be well above ground on one side
- Tight programmes, where you can’t wait for a slab to cure