Basement drainage, anti-flooding devices and pumping

A basement with a toilet, shower or washing machine in it drains below the level of the public sewer, and when that sewer surcharges in heavy rain the water has somewhere to go. Up your pipe. That is why Approved Document H expects sanitary appliances in a basement to be drained by a pumped system where surcharge is a risk, rather than by gravity with a valve, and it is why a basement conversion that was fine on the drawings can flood with foul water in its first wet autumn.

The planning and building control questions are separate, and both have to be answered.

Why a basement is different

Above ground, foul drainage works by gravity and the sewer is below you. Below ground, the sewer may be above you, at least when it is full. Sewers are designed to surcharge, meaning the water level in them rises above the pipe soffit during storms, and in combined systems that happens several times a year.

Anything connected below that surcharge level is exposed. A ground floor WC is protected by the height of the pan. A basement WC is not, and neither is a shower tray, a floor gully or a washing machine standpipe.

So the design question is simple to state. How do you let water out when the sewer is low, and stop it coming back when the sewer is high?

Surcharge in the public sewer

Start by finding out what you are connected to. Ask the sewerage undertaker for a sewer record, which shows the pipe, its size, its invert level and whether it is foul, surface water or combined. A pre-development enquiry will tell you more, including any history of flooding on the network.

Combined sewers are the ones that surcharge most often, because they take both foul flow and surface water runoff. Many older towns and most of inner London are on combined systems. If your record shows a combined sewer and your basement floor is below the cover level of the nearest manhole, assume surcharge is a real risk and design for it.

Where a Foul Sewage Assessment is required, that is where this evidence sits.

Non-return devices and what they cannot do

An anti-flooding valve, often called a non-return valve, lets flow out and closes against flow coming back. They cost little, they work, and they are limited.

The limits matter. A valve on a drain also serving appliances above ground will block those appliances while it is shut, so a surcharge event that lasts two hours means nothing upstairs can be used either. Valves need access for inspection and cleaning, because a trapped wipe holds the flap open and the device does nothing. And guidance treats them as suitable where the risk is low or the area served is not habitable, such as a garage, rather than as the answer for a basement bedroom and bathroom.

Sump and pump arrangements

The standard arrangement for a habitable basement is a sealed sump receiving the basement drainage, a pump or pair of pumps discharging through a small rising main, and that rising main looped up above the likely flood level of the sewer before it falls into the gravity drain. The loop is the part that actually stops backflow, because water cannot climb over it.

Specify, and check on site:

1. Two pumps rather than one, alternating, so that a failure is not an emergency.
2. A sealed sump with a vented cover, to keep foul air out of the room.
3. A non-return valve on the rising main, in addition to the loop.
4. A high-level alarm that is audible in the house, not only in the plant cupboard.
5. Access for maintenance without lifting the floor.
6. Pump capacity sized for the peak flow from the appliances served, including a bath emptying.

Macerating units serving a single WC are a different product with their own limits, and they are a poor answer where the basement is a self-contained flat.

Power failure and residual risk

Pumps need electricity, and the storms that surcharge the sewer are the storms that bring down power lines. That is the residual risk, and the honest answer is that you manage it rather than remove it.

Options run in rough order of cost. A battery-backed alarm so the occupier knows. A battery or inverter backup sized to run the pump for a few hours. A generator connection. And designing the basement so that a short period without pumping is tolerable, which usually means no floor-level drainage outlets and a raised threshold at the foot of the internal stair.

State the residual risk in the application rather than leaving it to be discovered. Where the property is in a flood zone, a Flood Risk Assessment has to address it, and the resilience measures discussed in our note on residual flood risk and property flood resilience belong in the same section.

Groundwater and the other water

Foul surcharge is one route in. There are three others, and a basement scheme has to deal with all of them.

Groundwater is the first. Water-resisting construction is designed to a recognised standard, and drained cavity systems, where water is allowed in behind the wall and collected in a perimeter channel, rely on a sump pump of their own. That pump is a separate installation from the foul one and should never be combined with it. Our note on evidencing groundwater flooding in a flood risk assessment covers how the risk is demonstrated.

Surface water is the second. External steps and lightwells collect rain and need their own drainage, often pumped, with a raised threshold or a grating and a channel at the door.

Overland flow is the third. In a storm that exceeds the drainage design, water runs along the street and down the first set of steps it finds. Raise the threshold, and do not put an air brick at pavement level.

Planning and building regulations

These are separate consents and they test different things. Building control applies the drainage requirements of the Building Regulations through Approved Document H, which is where the pumped system, the valve and the sump specification are assessed.

Planning may or may not be engaged. Excavating a basement wholly within the footprint of an existing house, with no external alteration and no change of use, may not require permission, though adding a lightwell, a separate entrance or a rear extension to the excavation usually does. Several authorities, particularly in London, have adopted basement policies and require a Basement Impact Assessment covering hydrology, hydrogeology and ground stability. Where a basement is to contain bedrooms and the site is in a flood zone, the vulnerability classification matters, and we cover that in basements, flood risk and sleeping accommodation.

This is general guidance, not legal advice, and a lawful development certificate is the only formal way to confirm that works are lawful. Check with your local planning authority before relying on it.

An illustrative case: a Victorian terrace with a converted cellar containing a shower room, drained by gravity through a non-return valve on the single outgoing drain. In a summer storm the combined sewer surcharged for about ninety minutes, the valve closed as designed, and the kitchen sink two floors up backed up instead. The remedy was a sealed sump, a twin pump set and an anti-flood loop, installed through the floor the owner had finished eight months earlier.

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