Suspended timber, beam and block, or solid concrete ground floors

Suspended timber floors

A suspended timber floor sits on joists spanning between sleeper walls or beams, with a ventilated void beneath maintained by airbricks in the external walls. It's still common in older South Devon housing and remains a valid choice for extensions on sites with variable or poor ground conditions, since it avoids the need for as much groundwork as a solid floor.

The ventilated void needs to stay clear and the airbricks unobstructed, since blocked ventilation is the single biggest cause of timber decay in suspended floors. Insulating between the joists needs to be done with a breathable material that doesn't block airflow beneath it.

Beam and block floors

Beam and block combines precast concrete beams with infill blocks to create a floor that's raised off the ground like a suspended floor but doesn't rely on timber, avoiding the rot risk while still allowing a ventilated void beneath for sites where ground conditions or drainage make a solid slab less suitable. It's a common choice on new extensions built onto sloping or variable ground.

It needs the same attention to under-floor ventilation as a timber suspended floor, since the void beneath still needs airflow to manage ground moisture, even though the floor structure itself isn't at risk of rot.

Solid concrete floors

A solid concrete ground floor, built up from hardcore, a damp-proof membrane, insulation and a concrete or screed finish, is now the most common choice for new extensions on stable, level ground. It gives good thermal mass, suits underfloor heating well, and doesn't need the ongoing ventilation maintenance a suspended floor requires.

It relies entirely on the damp-proof membrane being correctly installed and lapped with the wall's damp-proof course, since any breach in that membrane lets ground moisture into the floor build-up with no ventilation to help it dry out, unlike a suspended floor.

Matching an existing floor when extending

Where an extension joins an existing suspended timber floor, matching floor levels and construction types avoids awkward steps or a hidden step-down disguised under flooring. It's sometimes possible and often preferable to bridge a modern solid floor extension to an older suspended floor with a careful transition detail, but this needs planning at design stage rather than being resolved on site.

Differential movement between a solid concrete extension floor and an older suspended timber floor is also worth considering, particularly where a continuous flooring finish is intended to run across the junction.

• Check ground conditions before assuming solid concrete is the default choice
• Keep under-floor ventilation clear on suspended and beam and block floors
• Confirm the damp-proof membrane laps correctly with the wall DPC on solid floors
• Plan floor level transitions between old and new construction at design stage

Specifying the right floor

Ask your builder or engineer to confirm which floor type is being proposed and why, based on the ground conditions on your specific site, not just as a standard specification copied from another job. For solid floors, confirm the insulation thickness and damp-proof membrane detail; for suspended and beam and block floors, confirm the ventilation strategy.

If underfloor heating is planned, raise this early, since it affects the floor build-up thickness and is generally easier to accommodate in a solid concrete floor than a suspended one.

Common questions