Quick answer: Create a low point in the floor void, fit a slim automatic pump (such as the APP RS32EA, APP BPS100EA or AMA Drainer 301, depending on space), and discharge outside through a non-return valve to a suitable point. Where there’s no internal access, an external pump chamber such as the FPS Iguazu 635 can collect water through a pipe from the void. But unless the property has flooded, find out why the water is there first. It could be a leak.
Many older UK homes have suspended timber floors: floorboards on joists over a ventilated void, with airbricks in the outside walls. When water gets into that void it’s out of sight, and it can sit there for weeks.
Why water under the floor matters
Standing water in a floor void isn’t just a damp problem. The guide Retrofitting for Flood Resilience (Edward Barsley, RIBA Publishing) describes how floodwater damages suspended timber floors: hydrostatic uplift can lift floorboards, absorbed water makes boards swell and bow, and moisture trapped around the timbers leads to mould and rot. Drying times depend on the timber and how well the void is ventilated.
First: is it actually a flood?
If the property has just flooded, the source is obvious. If not, don’t start by pumping. Water in a floor void without a flood usually has a cause that a pump will only hide:
- A leaking pipe. Water supply, heating or waste pipes running through the void. Turn off every tap and appliance and watch the water meter: if it still moves, you have a leak.
- Leaking drains. A cracked drain or waste connection can discharge into the void.
- Gutters and downpipes. A blocked gutter or broken downpipe saturating the wall and ground next to the house.
- Raised ground levels. Patios, paths or flowerbeds built up above the airbricks or the damp-proof course.
- High groundwater. Water that rises seasonally in wet winters and falls again in summer.
Pumping out a leak just runs up your water bill and leaves the damage happening. Fix the cause. If it turns out to be groundwater, or the property is at real risk of flooding, then a pump becomes the right answer.
What the guidance recommends
Retrofitting for Flood Resilience lists a sump and pump system as a measure for suspended floors, alongside resistance measures such as automatic airbricks. It describes how:
- sumps are typically positioned beneath suspended timber floors, with water directed to them by perimeter drainage channels;
- outlet pipes are ideally positioned above 600mm on the external wall;
- a sump and pump won’t reduce the extent of floodwater outside, but can limit the rate at which water rises inside;
- most pumps are electric and need a back-up power source;
- systems can be fitted with alarms, and should be inspected regularly so channels and outlets stay clear.
The same guide also recommends making floor finishes easy to lift, so the void can be reached and dried.
For keeping water out of the void in the first place, see our guide to anti-flood air bricks.
Option 1: a sump and pump inside the void
This suits most homes where you can get into the void through a floor hatch.
- Find the lowest point of the oversite (the ground or concrete under the floor).
- Form a small sump there, deep enough for the pump to sit below the surrounding level so water drains to it.
- Fit a slim automatic pump sized to the space.
- Run the discharge through the wall to a suitable point outside, with a non-return valve so water can’t run back. See why water flows back through a puddle pump.
- Leave an access hatch so you can check and maintain it.
Floor voids are often shallow, so the pump’s height and the sump depth have to fit under the joists. Three pumps cover most situations:
| Pump | Size | Switches on / off | Flow | Best for |
|---|---|---|---|---|
| APP RS32EA | 160 × 170 × 295mm | On at 50mm, off at 25mm | 130 L/min | Narrow sumps; fixed float can’t snag |
| APP BPS100EA | Compact float, top discharge | On at about 30mm, off at about 3mm | Low-level duty | Shallow pooling on a flat oversite where water must be kept very low |
| AMA Drainer 301 | Arm float for narrow chambers | Factory set on 145mm, off 70mm (adjustable) | Up to 166 L/min | Deeper sumps; integral non-return valve; 10mm solids |
The RS32EA and BPS100EA are also sold as complete kits with hose and fittings: the Narrow Sump & Pit Pump Kit and the Ultra-Compact Sump & Pit Pump Kit, which is only 134mm wide.
For more on pumps that fit tight spaces, read best pump for narrow spaces and chambers and what is a compact float switch?
Option 2: an external chamber when there’s no inside access
Sometimes the void can’t be reached from inside: a finished floor you don’t want to lift, a tenanted property, or a void too shallow to work in. In some set-ups, a pump chamber is installed outside instead, with a pipe running from the floor void through the wall into the chamber.
The FPS Iguazu 635 is a 190 litre packaged pump station with an automatic AMA Drainer 301 already fitted. It accepts a 110mm pipe connection, so a pipe can be laid from the void to the chamber. Points to plan for:
- The chamber inlet must be lower than the void, so water falls to it by gravity.
- The chamber is set in concrete to stop it floating when the ground is wet.
- A recessed, sealed lid keeps surface water and debris out.
- Options include a high water alarm and battery back-up, which match the guide’s advice on alarms and back-up power.
Our guide to installing a packaged pump station for flood control and what a sump pump is and how to install one cover the installation in more detail.
Discharge and electrics
- Where does it go? To a surface water drain, soakaway or garden area that falls away from the house. Never to the foul drain without permission, and never where it will run back towards the building.
- How high? The guide suggests outlets ideally above 600mm on the external wall, so floodwater outside can’t easily get back in.
- Electrics: have a qualified electrician fit a suitable outlet, protected by an RCD, positioned above any likely flood level.
- Power cuts: if you’re relying on the pump in a flood, consider back-up power. See what water pump can I use without mains electricity.
Keep it working
A pump hidden under the floor is easy to forget. Test it a couple of times a year by pouring water into the sump, check the float moves freely, and clear any silt. Our list of common sump pump mistakes shows what we see go wrong most often.
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