Non return valves and basement sewer surcharge

Most basements that flood in this country are not flooded by a river or by rain running across the garden. They are flooded by their own drains. When the public sewer fills up and surcharges, the water finds the lowest connected fitting in the street, and if that fitting is your basement shower tray then that is where the sewer comes out. A non return valve for basement flooding is the cheapest defence against that, and on its own it is not enough.

The answer is usually a valve, a sump and a pump, with a maintenance regime somebody has actually agreed to carry out.

Why sewer surcharge floods basements rather than overland flow

A combined or foul sewer is designed to run part full. In heavy rain, or when a downstream blockage builds, the water level in the sewer rises above the level of the connections. Anything below that level is now at the bottom of a pressurised system.

Overland flood water has to get into the building, so it meets thresholds, doors and air bricks that you can see and defend. Surcharge arrives inside the building, through a pipe that was installed to carry water in the other direction, and it does so without warning. Those two mechanisms need different mitigation and they are often confused in householder submissions.

Ask your sewerage undertaker for its records of sewer flooding in the catchment before you design anything. The company holds that information, it is relevant to the design, and an authority will take an assessment far more seriously when it rests on the undertaker's own records rather than on an assumption.

How an anti flooding valve works and where it goes

An anti flooding valve sits in the drainage run and closes when flow reverses. Most types use a flap or a pair of flaps held open by forward flow and pushed shut by backflow, with some designs adding a powered closure and an alarm.

Position matters more than the device. A valve on the main connection protects everything upstream of it, including the ground floor, which sounds like an advantage until the valve shuts during a storm and the ground floor has nowhere to drain either. A valve on the basement branch alone protects the basement and leaves the rest of the house draining normally, which is usually the better arrangement in a house.

Three siting rules worth following. Put the valve where it can be reached and inspected without lifting a floor. Keep it out of the path of the main solids flow where you can. And never put a valve on a branch that has no alternative route for its own flow unless you have provided storage.

Standards and testing for backflow devices

Anti flooding devices for buildings are covered by BS EN 13564, which classifies them by type according to the closure mechanism and the duty. Specify the type as well as the product, because the type is what a building control body and a planning officer can check.

Approved Document H of the Building Regulations deals with drainage from below ground level where the sewer may surcharge, and it distinguishes between the simple case of a single dwelling and situations where more is at stake and a pumped system is expected. Read the current edition rather than a summary of it, because this is a point of detail that has been refined over successive revisions.

Whatever you install, say so on the drawings with a product type, an invert level and a maintenance note. A valve that appears only in the covering letter has a way of not getting fitted.

Sump, pump and storage sizing

Where the basement has its own sanitary fittings, the reliable arrangement is a sealed sump with a pump lifting the flow above the surcharge level of the sewer, discharging through a rising main with its own non return valve on the vertical section.

Design it for the day it fails, because it will.

1. Two pumps on alternating duty, so that one can fail without stopping the system.
2. Storage in the sump sized to hold the expected inflow through a credible period of power loss or pump failure, rather than the working volume between start and stop levels alone.
3. A high level alarm that somebody will hear, sited in the hallway rather than in the plant cupboard.
4. A standby supply, or at minimum a clear instruction that water use stops when the alarm sounds.
5. Access for maintenance that does not involve dismantling the laundry.

BS EN 12056-4 covers wastewater lifting plants and is the reference a drainage engineer will work to. For anything beyond a single en suite, this is a design job rather than a catalogue selection.

Maintenance and the failure mode nobody plans for

Valves fail open. Wipes, fat and grit hold the flap off its seat, the device looks fine from the outside, and the first anyone knows about it is the smell. An annual inspection with the flap lifted and the seat cleaned is the whole of the maintenance regime, and it takes ten minutes.

Valves also fail closed, which is the mode people forget. The valve has done its job, the sewer is surcharged, and now the shower upstairs is draining into a pipe with a shut door at the end of it. In a house with the valve on the basement branch only, nothing happens. In a house with the valve on the main connection, the kitchen floods instead of the basement.

Write the regime down, hand it to the occupier and, where the authority asks for it, offer it as a condition. A maintenance plan nobody holds is the same as no plan.

What a basement planning submission should state

Two documents carry the weight. Where the site is in Flood Zone 2 or 3, or on a site of one hectare or more, a Flood Risk Assessment is required, and it needs to deal with surcharge explicitly rather than stopping at the fluvial mapping. Where the works involve excavation below an existing building, many authorities, particularly in London, also require a Basement Impact Assessment covering groundwater, structural effect on neighbours and surface water.

The flood risk vulnerability classification in national guidance treats basement dwellings as highly vulnerable, which has consequences. Highly vulnerable development is not appropriate in the functional floodplain at all, and in Flood Zone 3a it needs the Exception Test. A habitable basement bedroom is a very different proposition from a basement plant room, and the classification is where that difference shows up.

Say in the submission what the protection is, what level it protects to, how occupants will know when it has operated and how they get out. Safe access and egress is assessed on the route, not on the front door.

Conditions that secure the protection

Expect, and where sensible offer, conditions along these lines: the backflow protection and pumped drainage to be installed in accordance with approved details before first occupation, a maintenance and management plan to be submitted and approved, finished floor and threshold levels fixed by reference to ordnance datum, and a flood warning and evacuation plan for the occupiers.

Conditions have to be enforceable, so write them in terms somebody can check on a site visit. A condition requiring protection to be provided is weak. A condition requiring a named device type installed at a stated level, with a maintenance plan, is not.

Where excavation involves dealing with groundwater as well as sewers, the consents needed for construction dewatering are a separate exercise and they take longer than people expect. Start them early.

This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required.

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