Sub-Floor Damp and Floor Void Investigation
Most London terraces stand on suspended timber floors over a void. What happens in that void decides whether the joists last another century or fail quietly under the carpet.
A suspended timber ground floor is a deck of floorboards on joists, carried on sleeper walls and built into the external walls at their bearing ends, over a floor void that is supposed to be ventilated by airbricks. When that void is wet or unventilated, the result is wet rot at joist ends, dry rot spreading along sleeper walls, wood-boring beetle, springy or dipping floors, musty smells and mould on skirtings. It is also a common hidden source of humidity in the rooms above.
What causes a wet floor void
- Blocked, buried or painted-over airbricks, or airbricks removed when a patio or extension was built
- External ground or paving raised above the damp-proof course and the void floor (the oversite)
- Leaking supply, waste or heating pipes run through the void
- Defective or leaking drains and gullies alongside the wall
- Bridged DPC at sleeper walls where no slate or felt was laid
- Builders’ rubble, soil and debris left in contact with timber
- Earth-retaining walls on sloping sites in Surrey and south London
How we investigate
- Moisture readings and probing of floorboards, skirtings and accessible joists from above
- Lifting one or two boards at the threshold, bay or the wettest reading to inspect the void directly, by agreement
- Inspection of joist ends and bearing ends built into external walls, the most common failure point
- Checking sleeper walls, wall plates and the oversite for standing water, fungal growth and bridging
- Counting and testing airbricks and confirming the cross-flow of air through the void
- Thermal imaging and pressure testing where a heating or supply leak is suspected
- A drain camera survey where a drain runs alongside the wet area
Structural concerns
If joists have lost section to rot or beetle, the floor may need structural repair. We describe the extent and location of decayed timber, the moisture conditions driving it and the ventilation and drainage faults to correct. Where a structural engineer is needed to design the repair, the report says so plainly; we do not guess at structural adequacy. Repairs are quoted separately by our team or your own contractor.
Lenders and purchases
Floor voids are a standard part of our pre-purchase damp surveys and damp and timber reports wherever boards can be lifted. Where the vendor refuses access, the report records the limitation.
We investigate across Greater London and Surrey. See areas we cover.
Frequently Asked Questions
My floor is springy near the bay window. Is that serious?
Bay joist ends are a classic wet rot location because the bay walls are thin and the ground outside is often raised. It needs inspecting rather than guessing.
Do you have to lift floorboards?
To inspect the void properly, usually one or two boards at the right spot. We agree it with you first and the making-good goes in the scope.
Can a wet floor void cause mould upstairs?
Yes. Moisture evaporating from a damp oversite raises humidity in the rooms above, especially where airbricks are blocked.
Is dry rot in a floor void an emergency?
Active dry rot should be dealt with promptly because it spreads through masonry to other timber. Our report shows the extent so the repair can be scoped correctly rather than over-sold.
Do you repair joists?
No. DML investigates and reports. Timber repairs, ventilation and drainage works are quoted separately by our team or your builder.
Related services
Related reading: Rising damp treatment: what to expect from start to finish · How to identify rising damp: signs and causes
Worried about what is under the floor?
Independent sub-floor and timber investigations across London and Surrey.
Last reviewed: 23 October 2026. Independent investigation and written reports. Repairs are quoted separately.