How do I tell if my ground floor is suspended timber or solid concrete?

Two completely different constructions sit under UK ground floors, and they behave in opposite ways. A suspended timber floor has a ventilated air space beneath it that must be kept ventilated or the joists rot. A solid floor has ground and, if it is modern, a damp proof membrane, and its problems are about that membrane rather than about airflow. Getting the identification wrong leads people to block the very vents their floor depends on.

What you'll need

• Torch
• Something to tap with
• Screwdriver, to lift a corner of carpet at an edge (optional)

Step by step

1. Walk on it and listen.A suspended timber floor has a slight give and a hollow, resonant note when you tap or drop something. A solid floor is completely dead underfoot and returns a dull thud.
2. Look outside for airbricks.Small vents low in the external wall, usually below the level of the floor inside. Their whole purpose is to ventilate the void under a timber floor, so their presence is close to conclusive.
3. Check the level relationship.Stand inside and sight the floor level against the airbrick position outside. Airbricks should be below the floor. If they sit above it, something is wrong — either they serve something else or the ground has been raised over them.
4. Look at an edge.Lift a corner of carpet at a skirting, or look where a floor covering has been cut around a pipe. You will see either floorboards or chipboard on joists, or a screed or concrete slab.
5. Check the age of the house.Broadly, pre-1930s houses usually have suspended timber ground floors; from the mid-twentieth century solid floors became normal; many houses have both, with a timber front room and a solid kitchen or rear extension.
6. Expect a mixture.It is very common for one house to have both, and for the boundary to be invisible under a fitted floor. Test each ground-floor room separately rather than assuming.
7. Act on the answer — suspended.Keep every airbrick clear and unobstructed, never seal the void, never fill it with insulation that blocks the airflow, and treat any blocked vent as urgent because joist ends rot quietly.
8. Act on the answer — solid.The question becomes whether there is a damp proof membrane and whether it is continuous with the wall's damp course. A solid floor with no membrane, or a screed poured above the wall's damp course, is a common cause of damp at skirting level.

Tips

• Before laying any new floor covering, test the floor for damp: a taped-down plastic sheet left for a few days shows condensation underneath if moisture is coming through a solid floor.
• Insulating a suspended floor is done between the joists with the ventilation path preserved beneath. Insulating it by blocking the void is a way to rot the floor out.
• If a room is noticeably colder at floor level and draughty at the skirting, a suspended floor is likely and draught-sealing the gaps at the perimeter — without touching the airbricks — makes an immediate difference.

Common mistakes

• Blocking airbricks to stop cold draughts — The void loses its ventilation and the joist ends, which are usually built into a damp external wall, start to decay. This is one of the most damaging things it is possible to do to an old house for the best of reasons.
• Laying an impermeable floor covering over a solid floor with no membrane — Moisture that used to evaporate through the floor is trapped and pushed to the edges, so damp appears at the skirtings and the adhesive fails.

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