Drainage Terms Explained

When you speak to drainage companies or engineers you will hear lots of technical phrases, jargon and acronyms. We will try and explain their meaning in an easy to understand way.


Aco drainage is a brand of linear drainage, a u-shaped drainage channel with removable metal or plastic grating, used to take surface water away from driveways, car parks and patios.

Air Admittance Valve

An Air Admittance Valves (AAV) or Durgo Valve is a clever system that opens to allow air to enter a soil vent pipe when there is negative pressure, but remains closed the rest of the time.

Buchan Trap

The Buchan Trap was installed in Victorian homes in London and uses a water trap to prevent sewer gases entering a property. It is no longer needed and is prone to blocking with wet wipes.

Catch Pit

A catch pit (also known as an interceptor trap) is an empty chamber built to 'catch' silt and other debris that is carried along a pipe by rainwater. They help prevent pipes from getting blocked.

Chemical Drain Cleaners

They might clear clogged hair from your sink, but they won't clear a blocked manhole. Chemical cleaners can be dangerous if incorrectly used and should not be used if you have a septic tank.


A cesspit is a sealed, water tight underground tank that collects wastewater and sewage. There is no processing or treatment of the waste. Cesspits need to be emptied regularly by a tanker.

Combined Drainage

In a combined sewer, sewage and surface water goes into one pipe. Most of central and inner London has combined sewers. During heavy rainfall, combined sewers can exceed capacity.

CCTV Drain Survey

A drain survey is an inspection of a drainage system, using cctv cameras which are inserted into your drains with a push rod. The cameras can identify blockages, cracks or root ingress.

Cover Level

The highest point of a manhole (or other access chamber) is the cover level (CL), which is the height of the cover above a fixed benchmark level. It's normally seen with the Invert Level (IL).


A rainwater downpipe is used to take rainwater from a building, typically from roof guttering to a rainwater gully. Downpipes used to be made of cast iron or lead, but more recently with UPVC.

Drain Testing

After construction, all newly-laid drainage and internal pipework should be tested for water-tightness, normally in the presence of Building Control, before building work can signed off.

Drain Tracing

If you need the exact location and depth of a pipe, we use sonde and trace. A transmitter (sonde) is placed in the drain pipe or sewer and we follow it above ground using a receiver.

Dye Testing

Drain dye is environmentally friendly and comes in a variety of colours. It is used to help detect the source of leaks and to confirm connectivity between different parts of a drainage system.


A fall is the term used to denote the distance that a drain drops vertically relative to its horizontal length - essentially the slope of the pipe. A bigger fall is not necessarily better.


Foul drainage is the pipework that carries waste water away from all of the sinks, toilets, baths, showers, washing machines and dishwashers on your property, everything part from rainwater.


A gully is an outside drain pipe that takes rainwater from your guttering and grey wastewater from your washing machine, sinks, bath, shower and dishwasher. It has a trap to stop odours.

Inspection Chamber

Inspection chambers are access points to underground pipework which give you access to clear blockages. Inspection chambers allows access for cctv cameras, drain rods and jetting units.


A drain interceptor is a series of tanks that collects and holds contaminants such as petrol or diesel, allowing the remaining wastewater to be discharged safely into the main sewer system.

Invert Level

The invert level is defined as the base interior level of a drain, sewer, manhole or inspection chamber. The invert depth can be calculated by subtracting the invert level from the cover level.


High-pressure water jetting (HPWJ) is used for drain unblocking and cleaning. The pressure is up to 4000 psi (far higher than a domestic jetter) to clear blockages, remove roots and debris.


A lateral drain is a pipe which carries wastewater away from your home. It is located off the boundary of your property under the pavement or road before it joins up to the public sewer.


Relining involves installing a resin drain lining that bonds to the inside of the existing pipe, creating a new pipe inside the old one. This is a cheaper, no-dig alternative to excavation.


A Localised Structural Repair (LSR) is the repair of a cracked or disjointed pipe by inserting a patch or a liner into a drain, which is then impregnated with resin, creating a pipe within a pipe.


Manholes are inspection chambers that are big enough for an engineer to climb inside to access the underground pipework to clear blockages, carry out drain repairs and cctv surveys.


Patches are very similar to drain lining, except they allow the engineers to repair localised cracks and disjoints rather than lining the entire pipe, making it even more cost effective.

Percolation Test

A percolation test involves digging holes and filling them with water to determine whether the water absorption rate of the soil at your property is suitable for the installation of a soakaway.


Picotes are specialist heavy duty machines designed for drain cleaning where a high pressure water jetter is not enough. Perfect for scale build-up, large tree roots and concrete deposits.

Pitch Fibre

Pitch fibre was used in the 1950s to 1970s as a cheaper alternative to traditional clay pipes. Unfortunately pitch fibre only has a life span of 40 years, before it becomes misshapen.

Private Drains

Private drains are the drains on your property that are not shared with a neighbouring house. This means they are your responsibility to clear any blockages or to carry out any repair.

Rodding Eye

A rodding eye (alternatively a rodding access or rodding point) is an opening with a removable cover on a drainage pipe that allows access for using drain rods to clear a blocked drain.

Rodent Valve

A rodent valve (also known as a rat blocker or rat stopper) is a valve with a flap that is inserted into your drain that allows waste to leave your property, but stops rat entering from the sewer.

Root Ingress

Root ingress is a common problem with drains that are close to trees. The roots grow through cracks and displaced joints, looking for water. The cracks get bigger and the pipes start leaking.


Limescale builds up in drains due to the excess mineral deposits in London's hard water. This scale decreases the bore of the pipe and slows the rate at which waste water flows through.

Septic Tank

Septic tanks take wastewater from properties that are off-mains. They remove solids and some bacteria to make it clean enough for release into a drainage field, but not a soakaway or stream.

Sewage Treatment Plant

A sewage treatment plant is similar to a septic tank, but it produces a non-polluting effluent which is clean enough to be discharged directly into a stream, ditch or soakaway.


A sewer carries sewage or surface water and serves more than one property, whereas a drain normally only serves a single property. Sewers can either be privately or publicly owned.

Shared Drainage

Any drainage that is shared by more than one property will be the responsibility of the local water authority, so Thames Water in London, to carry out any repairs or to clear blockages.


A soakaway is a hole dug into the ground, filled with coarse stone or plastic crates. It allows water to filter through and soak into the ground rather than overpowering the sewer system.

Soil Vent Pipe

A Soil Vent Pipe (SVP), also known as a Stack Pipe, is a vertical pipe installed inside or outside a property taking waste from toilets, baths and sinks into the underground drainage system.

Sonde & Trace

A transmitter (sonde) is placed in the drain pipe or sewer and we follow it above ground using a receiver to trace the location and depths of the drains. Often used as part of a build over survey.

Surface Water

Surface water drainage is used to carry away rainwater from gutters, patios, driveways, roads and car parks. The rainwater will then be transported to a stream, a river or a soakaway.


A U-bend or trap is designed to hold water to prevent unwanted sewer gases and odours from entering buildings through sinks and toilets, while allowing waste materials to pass through.

Water Authority

Thames Water are the water authority who have responsibilty for the removal of wastewater and sewage from commercial and domestic properties in London and the area within the M25.

Still confused?

If you have a drain related question in the London area, please call 020 8168 2345 or email info@limehousedrains.co.uk or complete the enquiry form below and we will be in touch.

Approved London drainage company

Get a quote

Your name:
Email address:
Telephone no:

What areas of London can we come out for blockages, surveys or repairs?

Our engineers can offer a complete range of drain services across all of South, North, West and East London from Romford in East London to Hounslow in West London, from Lewisham in South London up to Enfield in North London, not forgetting Central London.
If you have are having problems with your drains and are looking for a London drainage company, call Limehouse Drains on 020 8168 2345 today for expert advice and a FREE no-obligation quote!