What is under a suspended timber ground floor?
The oversite, the ventilated void, the sleeper walls, the wall plate and its damp proof course, and why every single one of those exists to keep the timber dry.
- Difficulty
- intermediate
- Time
- 10 min
- Read
- 5 min
- Safety
- warning
Short answer
Under the boards are the joists; under those is an open air space; under that is the oversite, a layer of concrete or membrane over the ground. Low honeycomb sleeper walls stand in the void to shorten the joist span, with a timber wall plate on top separated from the masonry by a damp proof course. Air bricks in the external walls let air cross the void. Everything in that list is there to stop the timber getting wet.
Most people meet this construction for the first time when a floor starts to bounce, or when a musty smell appears in a room, or when someone quotes for underfloor insulation. It is worth understanding the arrangement before anyone changes it, because nearly every serious problem in a suspended floor comes from something being added to the void or blocking its ventilation.
Safety
Step by step
- Start at the top and work down.Floor covering, then boards or sheet decking, then joists running one way across the room, then air, then the ground. That air gap is the defining feature — it is what makes the floor suspended rather than solid.
- Identify the oversite.The ground under the void is normally covered, historically with a weak concrete layer and in more modern work with a membrane. Its job is to stop moisture evaporating out of the soil into the void.
- Find the sleeper walls.Low walls of brick standing in the void, running at right angles to the joists. They exist to reduce how far the joists have to span, which is why they are placed at intervals rather than only at the edges.
- Notice that they are honeycombed.Sleeper walls are built with deliberate gaps between the bricks so air can pass through them. A sleeper wall built solid divides the void into compartments and stops the cross-ventilation, which is a classic cause of rot in one part of a floor and not another.
- Look for the wall plate and its damp proof course.A timber laid flat along the top of each sleeper wall spreads the load and gives the joists something to sit on. Under it there should be a damp proof course, because timber sitting directly on damp masonry is timber that will rot.
- Understand what the air bricks are doing.They are not there for the room. They ventilate the void, and they only work in pairs and sets — air has to be able to come in one side and leave the other. A single unblocked air brick on one wall does very little.
- Check what has been put in the void since.Rubble from an old fireplace removal, builders' waste, spoil from a drain repair, and insulation that has slumped off its supports and lies on the ground. All of these block airflow and bridge damp up to the timber.
Tips
- The single most useful diagnostic is whether the void smells. A dry, ventilated void smells of nothing much. A musty, earthy or mushroom smell coming up through the boards is worth investigating properly.
- Lifting one board near an external wall, at the end of a joist, tells you more than lifting one in the middle of the room. The ends near walls are where rot starts.
- If you are having underfloor insulation quoted, ask directly how the ventilation path is maintained and how the insulation is supported. Those two answers separate a good installation from an expensive rot machine.
Common mistakes
- Blocking or rendering over air bricks — It is done to stop draughts and it stops the ventilation the timber depends on. Rot follows quietly over a few years, and the person who blocked them has usually forgotten by then.
- Raising the outside ground level above the air bricks — A new patio, a raised bed or gravel piled against the wall drowns the ventilation and often bridges the damp proof course at the same time. It is one of the most common self-inflicted damp causes there is.
- Using the void as storage — It obstructs the airflow, it is a fire and vermin issue, and it makes any future inspection or repair far harder.
If it doesn't work
A springy patch in an otherwise solid floor
Cause: A joist end failing, or a sleeper wall or wall plate that has deteriorated below it — Fix: Lift a board over the springy area and look at the joist ends and the plate. Localised softness at an end near a wall points at damp rather than at the joist being undersized.
Musty smell in a ground floor room only
Cause: A poorly ventilated or damp void beneath that room — Fix: Check every air brick from outside for blockage, check the ground level against the wall, and look for anything obstructing the void inside. Restoring the airflow is the first move, before anything is treated.
Cold draughts around the edges of the room
Cause: The gap between the floor and the skirting, drawing air from the ventilated void — Fix: Seal the perimeter gap at the skirting rather than blocking the air bricks. The void is supposed to be ventilated; the room is not supposed to be connected to it.
Questions people ask
Can I insulate a suspended timber floor?
Yes, and it makes a large difference. The requirements are that the insulation is supported so it stays in contact with the underside of the boards, that it does not block the ventilation path, and that nothing traps moisture against the joists. That is a specification question and worth getting right.
How much clearance should there be under the joists?
Enough for air to move and for the timber to stay clear of the ground, and it is set by the design and the building control regime rather than by a number you can apply to an existing house. What matters practically is whether the void is clear and ventilated.
Why is one room solid and another suspended?
Very common in older houses. Kitchens, sculleries and back additions were frequently laid solid while the front rooms were suspended, and later work has often replaced one floor and not another.
Are there services in the void?
Usually. Gas pipes, water pipes and cables are routinely run through underfloor voids, which is another reason not to drive anything through the boards without knowing what is beneath.