Both suspended systems avoid the problems of a ground-bearing slab on made ground or shrinkable clay, but they behave very differently to build.
Block and beam
Precast concrete beams with infill blocks give a strong, quick, dimensionally stable floor that is installed in a day or two and can be worked on immediately. It handles clay heave and variable ground well because the load goes to the foundations, not the fill.
It needs crane or telehandler access for the beams and a ventilated void beneath, and it adds significant weight to the substructure.
Suspended timber
Timber joists on sleeper walls or hangers are lighter, cheaper on smaller spans, and easy to build with hand tools where access is tight — a genuine advantage on rear extensions with no side access.
The trade-off is that the timber depends on a permanently dry, well-ventilated void, and it is less forgiving of a damp site.
Insulation and ventilation
Both need continuous insulation and a cross-ventilated void with clear airbricks that are never blocked by later landscaping. Perimeter insulation and continuity with the wall insulation is where heat is most often lost.
Underfloor heating is easy to accommodate above either system, but the build-up and total floor thickness needs planning before the foundations are set out.
Common questions
Which floor is cheaper?
On small spans with restricted access, timber usually is. On larger areas with plant access, block and beam is often competitive and faster.
Do both need a ventilated void?
Yes. Cross ventilation through airbricks keeps the void dry, and it must not be blocked by later paths or planting.
Which is better on clay?
Both suspended systems are appropriate where a ground-bearing slab would be at risk from heave, but block and beam is the more common choice on difficult ground.
Can I use underfloor heating with either?
Yes, with the appropriate insulation and screed or panel system planned into the floor build-up from the start.