Many older London houses have ground floors made of timber boards on joists, with an open space underneath. That space, the floor void, is where the incoming water supply, the kitchen pipes and often the heating pipes run. It is also where a leak can go unnoticed for a long time, because the water drops onto the ground below the house instead of coming through a ceiling.
This guide explains how suspended timber floors are built, what a leak underneath looks and smells like, how to inspect the void safely, and how the evidence is used to trace the pipe. If your floor is solid and tiled, our guide to finding a leak under a tiled floor is the better starting point.
What is a suspended timber floor?
In a suspended floor, the floorboards are nailed or screwed to timber joists. The joists usually rest on small brick walls, called sleeper walls, and on the main walls of the house. Below them is a void, which can be anything from a few centimetres to a crawlable space, over a base of earth or a thin layer of concrete.
The void is meant to be ventilated. Air bricks in the outside walls let air flow underneath, which keeps the timbers dry. This matters for leaks, because a well-ventilated void can dry out a small leak and hide it, while a poorly ventilated one lets moisture build up quickly.
What are the signs of a water leak under the house?
- A musty or earthy smell in the ground floor rooms, often strongest near skirting boards or in cupboards.
- Floorboards that feel soft, bouncy or springy in one area.
- Boards that are cupping, lifting or showing dark stains at the joints.
- Mould or damp staining at the bottom of walls or on skirting boards.
- Condensation or a damp feel in rooms that used to be dry.
- A sound of running or dripping water with everything turned off.
- A water meter that keeps moving with no water in use, but stops when the internal stopcock is closed.
- Low water pressure at the kitchen tap or elsewhere downstairs.
Some of these, especially the smell and the mould, can also come from damp that has nothing to do with a pipe. Our guide to telling damp from a leak explains the difference.
Which pipes run under a suspended floor?
Most houses of this type have the water supply entering at the front, often under the hallway, and running under the floor to the stopcock and the kitchen. From there, pipes may run to a downstairs toilet, a utility room, an outside tap and up to the first floor. Heating flow and return pipes to ground floor radiators usually run between the joists too, sometimes in narrow microbore pipe. Waste pipes from a downstairs toilet or kitchen may also cross the void.
Knowing roughly where these run makes the inspection faster. The stopcock, the radiators and the kitchen sink are useful landmarks, because pipes have to reach each of them.
How do you get into the floor void safely?
Some houses have an access hatch, often in the hallway, a cupboard under the stairs or a kitchen floor. If there is no hatch, a board can sometimes be lifted, but it needs care.
- Choose the right board. A short board that is already screwed down, or one that was lifted before, is the easiest. Tongue and groove boards are harder to lift without damage.
- Check for pipes and cables first. Water pipes and electric cables often run just under the boards, close to where they are fixed. Never drive a saw, chisel or bar in blind.
- Do not cut through joists or notch them to make access.
- Use a torch and a mirror, or a phone camera on a stick, rather than climbing in. A shallow void is not a safe place to crawl, and older voids can contain debris, sharp objects and old wiring.
- If you smell gas when the board comes up, stop, leave the property and call the National Gas Emergency Service on 0800 111 999.
What should you look for in the void?
- Standing water, or a base that is wet in one area and dry elsewhere.
- Drips on the underside of boards or on pipes.
- Water marks or dark staining on joists and sleeper walls.
- White, powdery salt deposits on brickwork, which show where water has soaked in and dried.
- Fungal growth on timber, which can look like white strands, cotton wool or a brown crust.
- Damaged pipe insulation, or signs that rodents have chewed plastic pipes.
Look along the line of any pipe you can see. Water tends to run along the pipe and drop from the lowest point, which may be some distance from the leak itself.
How are moisture and humidity readings used?
A single wet area tells you there is water. A set of readings tells you where it is coming from. Leak detection engineers use them in two ways.
- Timber moisture readings. A pin-type moisture meter pushed into the boards and, where reachable, the joists shows how wet the timber is. Readings taken in a grid across the floor build a map, and the wettest point is usually closest to the source. Timber that stays above roughly 20 percent moisture content is commonly regarded as at risk of fungal decay.
- Air humidity in the void. A humidity reading inside the void, compared with the room above and the air outside, shows whether the void is unusually damp. A logger left in place for a day or two can show whether the humidity rises when the heating or water is used.
Readings need interpreting. A void can be damp because the air bricks are blocked, the ground is wet, or the outside ground level has been raised above the damp proof course. A good survey checks those too, so a ventilation problem is not mistaken for a leak, or the other way round.
How is the pipe run traced?
Once the evidence points to a leak, the job is to find the exact point without lifting the whole floor.
- Isolation and pressure testing of the cold, hot and heating pipework separately shows which system is leaking.
- Acoustic listening on the boards picks up the sound of water escaping from a pressurised pipe, and works best on metal pipework.
- Tracer gas introduced into a drained pipe rises through the gaps between boards and can be detected at the surface.
- Thermal imaging is useful for heating and hot water pipes, which show as warm lines between the joists.
- An inspection camera through a small opening lets the engineer see the pipe at the suspected point before any more boards come up.
The aim is to mark one or two boards to lift, rather than a whole room.
What are the risks to the joists?
Timber that stays wet can decay. Wet rot is usually confined to the wet area and stops when the timber dries. Dry rot is more serious, because it can spread through masonry to other timber, and it needs specialist treatment. Joist ends that sit in damp walls, and the timber plates on top of sleeper walls, are the most vulnerable places.
After the leak is repaired, the void needs to dry. Clear the air bricks, remove any standing water, and recheck moisture readings over the following weeks. If timber is soft, crumbles or shows fungal growth, have it inspected before the floor is covered again.
When should you call a leak detection specialist?
Call a specialist when your meter shows water being lost inside the house, when a floor is soft or smells musty and there is no obvious cause, or when you can see water in the void but not where it is coming from. Our under-floor leak detection is priced at £300 to £700, and every survey includes a written report. See our trace and access page or call 020 3912 0200.
Frequently asked questions
How do I know if there is a water leak under my floorboards?
Look for a musty smell, soft or lifting boards, damp skirting or a water meter that moves with everything off. A torch and mirror through an access hatch will often show whether the void is wet.
Can a leak under the house cause structural damage?
It can damage the timber floor structure if it goes on for a long time, by causing wet or dry rot in joists and wall plates. Finding and fixing it early, then letting the void dry, limits that risk.
Why can I not see any water, even though the meter is moving?
In a suspended floor, water can drain into the ground under the house, so there may be nothing to see from above. That is why meter tests and void inspections are so useful in these houses.
Do I need to lift all the floorboards to find the leak?
No. Pressure tests, acoustic listening, tracer gas and moisture mapping can usually narrow it down to one or two boards before anything is lifted.
Should I run a dehumidifier after an under-floor leak?
A dehumidifier helps dry the room above, but the void itself mostly dries through ventilation. Make sure the air bricks are clear and check moisture readings over the following weeks.
