Glossary of Drainage Terms
Need expert help from our London drainage company?
On this page we have created a glossary of drainage terms to help our clients and customers better understand the work we do as a London drainage company.
One of the reasons we are chosen to perform popular drainage services such as CCTV drain surveys, drain lining, and blocked drain clearance is because our engineers take the time to ensure our customers feel apprised of the work being undertaken. We regularly check in with our customers, letting them know what's going on in language they can understand.
Our expert team of drainage engineers have a huge wealth of experience working on the drains of properties across London and South East England. We passionately believe in using this to help our clients with their drainage issues.
Explore the glossary lower down this page or use the contact form to get in touch right away and we will be in touch as soon as possible.
Drainage Terms & Definitions
These drains are channels which have a grating over the top and are designed to drain surface water from a hardstanding such as a driveway along the entire length instead of from one point like a yard gully. We advise regular maintenance is carried out to these as leaves and other debris can build up which can cause the water to drain away slowly and possibly overflow.
This one-way air vent is installed on above ground vertical waste pipes such as a stub stack so that when it is drained (e.g. a toilet being flushed), clean air can enter but foul smells and gasses will not escape and is designed to prevent gurgling, etc.
Anti-flood valves are installed in manholes to prevent a back surge from the sewers flooding the property as the flap which allows water running downstream to flow away will not open against pressure from back flow. Annual maintenance is advised to ensure it remains in a satisfactory working condition. These are common in basement drainage, particularly in cities (e.g. London) where colossal flooding can occur.
Like soakaways, attenuation tanks are installed to hold rainwater however cover a much larger area. They are designed to hold a high volume of water during very heavy rainfall which in a normal system would overload the main sewer have a flow control chamber which releases the stored water before running into the drainage system.
Although this looks like a standard gully at ground level, this design has below ground connections and therefore the connecting pipework does not discharge onto the grating.
During the installation of some systems, the pipework has not been laid to a good falling gradient, sometimes flat or with the downstream section higher than the direction the water is coming from and where the water is not flowing away it is pooling which causes the accelerated accumulation of debris deposits. These problems are usually impossible or too expensive to remedy and therefore the only option is likely to be regular maintenance. The ideal fall is 1 in 40 for 100mm pipework and 1 in 60 for 150mm.
Branch (or lateral) connections are drains that are not the main section of pipework running upstream and downstream but a drain that connects to the side of either a manhole chamber or underground section of pipework. Most manholes have connections joining in and this is usually adequate access to carry out a CCTV camera inspection however this is not possible if the connection is underground as the camera unit is unable to navigate bends such as these. Most connections are made with the correct junctions either during the time of installation or whilst carrying out alterations however it is common to see "DIY" jobs where it has been made by simply breaking open the existing pipework and adjoining the connecting drain by means of a cement joint. It is most likely that these type are not watertight.
Similar to an interceptor trap and inline trap, Buchan traps were installed in Victorian times to prevent the escape of foul smells and vermin from the communal sewers however as they are not accessible from a manhole and only have a rodding access point at ground level, there are usually difficulties clearing blockages which are known to build up in the u-bend of traps. When a Buchan trap is found to be damaged and needs to be replaced, this is usually an expensive job as typically they are between 2.0-3.0m deep and are located in narrow front gardens.
Build Over Agreements are required when building within 3.0m of a Local Water Authority's asset. Although we do not submit the applications, we work for a number of architects on a repeat basis for this purpose and the information we provide on our CCTV camera inspection report and computerised drawing showing the configuration of the drainage system is sufficient for the application.
A way to prevent internal flooding during a blockage was the installation of a Burns chamber which is where a metal plate is secured within the manhole chamber under the cover to stop water rising and overflowing. Burns chambers are only incorporated as part of a cast iron drainage system and are sealed with metal bolts which can be opened when access is needed for cleaning, investigation or repair works.
A catchment pit is an empty chamber installed in the surface water drainage system for intercepting silt and debris deposits being carried along the pipework. This works by having a high level inlet pipe which allows the water to rise to discharge through the outlet pipe also positioned at a high level, leaving sediment sitting at the bottom which will require regular cleaning to avoid blockages.
CCTV camera inspections allow information to be gained regarding the underground system including the pipework diameter and if there are any branch connections, all of which is pertinent when deciding the best drain repair technique. There are a number of reasons a drain survey may be carried out and you can see these here. Find out more about CCTV Drain Surveys in London.
A cess pit is a sealed underground tank with manhole access that wastewater and sewage discharges into instead of connecting to the main sewers. These require regular emptying and the frequency depends on the size of the tank, the size of the property and the number of occupants.
Many drainage systems are of earthenware or clay construction and although strong, it is a material known to fracture. A circumferential fracture runs around the circumference of the pipe and will allow the loss of water into the surrounding ground area, possibly leading to subsidence. These fractures are usually easy to repair by drain lining/sleeving if there is adequate access but they can deteriorate over time if left and could result in a collapse where an excavation will be required to carry out the repair.
Since the "Transfer of Sewers" Act in 2011, the Local Water Authorities including Thames Water are responsible for the majority of the sewer network. This act superseded the previous 1937 legislation, increasing the level of drainage the Local Water Authority is responsible for. They took ownership of all shared drains even within private property boundary lines with the exception of most PVC drains as well as any that are in public areas such as those that serve just one property but have crossed the boundary and are under the pavement. Private contractors are not permitted to work on these drains and should you be planning to build within 3.0m of these assets, a Build Over Agreement will be required.
Although manholes are for providing access to the drainage system, some properties have built over them and therefore rendered the chamber useless. This is not advisable as in the event of an emergency blockage, engineers would have difficulty clearing it without full access. These also prevent inspections and maintenance being carried out to any connecting pipework. In numerous cases, expensive kitchen floors have to be broken up to access these chambers.
This is a piece of machinery predominately used to reopen branch connections with a robotic arm after a structural soft felt liner has been installed. You can read more about this here.
As foul smells and sewage from a blockage can escape through the edges of a manhole cover, the standard design is not permitted to be used internally and Building Regulations state a bolt down, double sealed cover and frame needs to be used as it is airtight.
CCTV camera inspections allow information to be gained regarding the underground system including the pipework diameter and if there are any branch connections, all of which is pertinent when deciding the best drain repair technique. There are a number of reasons a drain survey may be carried out and you can see these here.
This one-way air vent is installed on above ground vertical waste pipes such as a stub stack so that when it is drained (e.g. a toilet being flushed), clean air can enter but foul smells and gasses will not escape.
If the configuration of the drainage prevents passage of the camera, fluorescein dye is dropped into the system to confirm a connection. You can read more about this method here.
Whilst CCTV camera inspections allow for us to see what is happening underground, it is not always possible to determine where the pipework is. Tracers are inserted into the pipework and tracked above ground and are used to pinpoint certain areas such as a concealed manhole or a collapsed section. You can read more about this service here.
Foul water is waste water serving a bathroom, kitchen or utility room and is discharged into communal sewers, septic tanks or cess pits. New drainage systems are separated into foul water and surface water where possible to help with the level of waste that needs to be treated before being released back into the environment and therefore combined systems which take both are mainly found in older properties. Foul water cannot discharge to a surface water system or soakaway due to its contaminated waste and the Environmental Agency and Local Water Authorities can issue correction notices and fines if these illegal, unhygienic misconnections causing pollution are found. So called "grey water" which only serves sink wastes, etc. is also classed as foul water and cannot be discharged into a separate surface water system or soakaway.
A gap between sections is where the original male and female ends of an earthenware pipe connect at a joint but have not been fully adjoined, leaving a space between the two sections. It is not possible to tell from a CCTV camera drain survey if these joints are leaking as the cement fillet of the joint may still be in place.
Gullies are often found at the base of above ground sections of pipework such as a kitchen sink waste pipe or rainwater pipe connecting to the underground system with a trap to prevent the escape of foul smells and rats. They are also used in large external spaces to drain surface water from the ground into the drainage. Newer gullies are designed with a rodding access which allows for cleaning and investigation works to be carried out beyond the trap should the need arise. Gullies are usually identified by the small circular or square grid at ground level.
Similar to an interceptor trap and Buchan trap, inline traps were installed to prevent the escape of foul smells and vermin from the communal sewers however as they are not accessible from a manhole and do not have rodding access, there are usually difficulties clearing blockages which are known to build up in the u-bend of traps.
These are for the same purposes as a manhole (see definition here) but a smaller version, usually circular. A PVC base is usually installed with the branch connections that are not in use being sealed off and PVC risers built up to the required height.
Interceptor traps are usually found in the outlet of the last manhole chamber and this is the favoured design to prevent the escape of foul smells and passage of vermin from the communal sewers as there is incorporated rodding access. This is beneficial as it is quite common for the traps to block up but these are usually cleared with ease.
This drain repair method uses a different technique to standard structural soft felt lining where instead of the sleeve being positioned in place and inflated, it is prepared "inside-out" and blown through the pipe which seals any defects to a watertight condition. This is typically used in situations where there are numerous bends in the drainage system or limited access.
Lateral (or branch) connections are drains that are not the main section of pipework running upstream and downstream but a drain that connects to the side of either a manhole chamber or underground section of pipework. Most manholes have connections joining in and this is usually adequate access to carry out a CCTV camera inspection however this is not possible if the connection is underground as the camera unit is unable to navigate bends such as these. Most connections are made with the correct junctions either during the time of installation or whilst carrying out alterations however it is common to see "DIY" jobs where it has been made by simply breaking open the existing pipework and adjoining the connecting drain by means of a cement joint. It is most likely that these type are not watertight.
These drains are channels which have a grating over the top and are designed to drain surface water from a hardstanding such as a driveway along the entire length instead of from one point like a yard gully. We advise regular maintenance is carried out to these as leaves and other debris can build up which was cause the water to drain away slowly and possibly overflow.
Many drainage systems are of earthenware or clay construction and although strong, it is a material known to fracture. A longitudinal fracture runs down the length of the pipe usually from a joint and will allow the loss of water into the surrounding ground area, possibly leading to subsidence. These fractures are usually easy to repair by drain lining/sleeving if there is adequate access but they can deteriorate over time if left and could result in a collapse where an excavation will be required to carry out the repair.
The main purpose of a macerator such as a Saniflo is to break up solid waste with blades, making it easier to pump the liquid sewage uphill or vertically. They are most commonly used in basement toilets where it is not possible to install a gravity system. In our opinion, although they have improved in recent years, they are not always reliable and need replacing every couple of years.
A manhole is an access point into the drainage system and is designed for someone to be able to fit into, the standard size being 600mm by 450mm. The cover should be left accessible and the correct strength used, e.g. pedestrian or vehicular. The depth of the manhole is dependent on each individual system and the walls are usually of either pre-made concrete manhole sections or brick construction. There will be a channel running through the base connecting the upstream and downstream runs and any branch connections discharge through the side.
A 'Multi-Kwik' is a plumbing connection for a WC pan and although it contravenes Building Regulations, we sometimes find these underground. One reason these are not allowed is that is reduces the bore of the pipework.
Many drainage systems are of earthenware or clay construction and although strong, it is a material known to fracture. Multiple fractures are likely to have developed over a long period of time and will allow the loss of water into the surrounding ground area, possibly leading to subsidence. These fractures are usually easy to repair by drain lining/sleeving if there is adequate access but this is likely to deteriorate further if left and could result in a collapse where an excavation will be required to carry out the repair.
Anti-flood valves are installed in manholes to prevent a back surge from the sewers flooding the property as the flap which allows water running downstream to flow away will not open against pressure from back flow. Annual maintenance is advised to ensure it remains in a satisfactory working condition. These are common in basement drainage, particularly in cities (e.g. London) where colossal flooding can occur.
Obstructions in the drainage system restrict the free flow of waste and the camera unit will be unable to pass it when carrying out a drain survey. Water usually holds behind an obstruction which prevents a clear view of the cause however it can be anything from a build up of non-disposable wipes and nappies, a roots mass or collapsed section of pipework. You can see what else causes blockages here.
An offset joint occurs when two sections of earthenware pipework are connected but were not aligned properly during installation. An offset joint does not necessarily leak as the cement fillet of the joint may be intact.
This machine is for a method of drain repair that seals leaking pipework to a watertight condition. The flexible liners are cut to length and are used to cover numerous defects in one section with bends that would prevent a standard structural soft felt liner being installed. You can read more about this technique here.
Pitch fibre was a material used predominately in the 1960's and was subsequently found to be inherently defective, becoming deformed and blistered over a period of years. Advanced stages of the "squashing" cause the pipework to block completely, requiring excavation and renewal. You can read more and see photographs of deformed sections here.
Property owners are only responsible for the drains within their boundary lines that serve only their property. Once these sections of drainage cross the boundary line either under the pavement or into a neighbouring garden, etc., they fall under the ownership of the Local Water Authority.
Many drainage systems are of earthenware or clay construction and although strong, it is a material known to fracture. A radial fracture runs partly around the pipe and will allow the loss of water into the surrounding ground area, possibly leading to subsidence. These fractures are usually easy to repair by drain lining/sleeving if there is adequate access but they can deteriorate over time if left and could result in a collapse where an excavation will be required to carry out the repair.
Marine grade steel rat blockers are usually installed in place of interceptor traps to prevent the passage of vermin from the communal sewers into the private drainage system. These do require regular maintenance by way of an annual service to ensure that the hinged flap continues to operate satisfactorily. These are usually used as an economic alternative to replacing the defective interceptor trap.
Standard manhole covers are not aesthetically pleasing and therefore it may be decided that a recessed cover is used as they allow for bricks or slabs to be installed to match the surrounding area, making it blend in to the floor but still allowing full access.
During house renovations, it is possible that some drains no longer have a use and we often find that these have just been left open or poorly capped at the end of the run with a builder's plastic rubble sack. We advise these runs are sealed at the manhole chamber as rats prefer to nest in these sections and can escape inside houses.
Pitch fibre pipework is prone to "squashing" and therefore rerounding is a technique used to return the drain to its original circular shape. You can read more about the use of pitch fibre pipework here.
Vertical downpipes that do not discharge to a gully are connected to the underground system with a rest bend. This 90° bend has a long sweeping angle to allow the waste to flow smoothly and usually has a footing to help absorb the force of the flushed water as it hits the base.
During the installation of some systems, the pipework has not been laid to a good falling gradient, sometimes flat or with the downstream section higher than the direction the water is coming from and where the water is not flowing away it is pooling which causes the accelerated accumulation of debris deposits. These problems are usually impossible or too expensive to remedy and therefore the only option is likely to be regular maintenance. The ideal fall is 1 in 40 for 100mm pipework and 1 in 60 for 150mm.
Rodding accesses are found on interceptor traps, new style gullies and some sections of above ground pipework to allow entry to the drainage system, mainly to attempt to clear a blockage however CCTV camera inspections can also be carried out from these points. They are sealed with caps to prevent the escape of foul smells and vermin and these should be able to be removed when needed and not be cemented into place. Whilst they do provide access, this is usually limited and we do advise there is further access in the form of a manhole.
Similar to an interceptor trap and Buchan trap, running traps were installed to prevent the escape of foul smells and vermin from the communal sewers however as they are not accessible from a manhole and do not have rodding access, there are usually difficulties clearing blockages which are known to build up in the u-bend of traps.
The main purpose of a macerator such as a Saniflo is to break up solid waste with blades, making it easier to pump the liquid sewage uphill or vertically. They are most commonly used in basement toilets where it is not possible to install a gravity system. In our opinion, although they have improved in recent years, they are not always reliable and need replacing every couple of years.
To ease the burden on sewers, rainwater can discharge to soakaways which is simply pipework connecting to a hole in the ground (at least 5.0m away from the building) filled with crates wrapped in non-woven Geotextile membrane and holds water, allowing it to disperse back into the earth slowly. This is a modern alternative as previously the hole used to be filled with rubble. This prevents water logging in the garden, stops damp seeping into the property and can save you money on your water bills. Because of the limited access, soakaways are difficult to repair and may need to be replaced if damaged.
As drainage systems need some ventilation to allow the escape of gasses, most properties have a soil and vent pipe where the top of the pipe is open to let air circulate. This usually serves upstairs bathrooms and Building Regulations state that the pipe must extend up to at least 1.0m above eaves level.
This is a method of drain repair that seals leaking pipework to a watertight condition. The structural patch liners come in pre-cut lengths and are positioned in the drain to cover specific defects. You can read more about this technique here.
This is a method of drain repair that seals leaking pipework to a watertight condition. The structural soft felt liners are cut to length and are used to cover numerous defects in one section. You can read more about this technique here.
These are short sections of above ground pipework that serve waste pipes and toilets (similar to a soil and vent pipe) but do not extend to the top of the property with a vent.
Surface water is clean rainwater which enters the drainage system through guttering and yard gullies, etc. and as it is not contaminated so doesn't require treatment, it can discharge into a soakaway, stream or river. New drainage systems are separated into foul water and surface water where possible to help with the level of waste that needs to be treated before being released back into the environment and therefore combined systems which take both are mainly found in older properties. Foul water cannot discharge to a surface water system due to its contaminated waste and the Environmental Agency and Local Water Authorities can issue correction notices and fines if these illegal, unhygienic misconnections causing pollution are found.
Swan necks are used in vertical downpipes to allow for a difference in the structure of a building such as the gutter on the edge of a roof to the rainwater pipe adjoined to the wall.
Pipework needs to be laid to a good gradient for water to drain away however the fall cannot be too sharp. If the downstream manhole is in a lower ground area, the drain is usually laid to the correct gradient but has a vertical section of pipework with a rodding access within the chamber connecting it to the channel. This is similar to a vertical backdrop with the difference being this vertical section of pipework is inside the chamber walls.
Vent pipes are often combined with soil pipes and Building Regulations state they must be at least 1.0m above the eaves/guttering level of a property. This allows airflow through the system to prevent the build up of gases and keeps the waste and water flowing freely through the drainage.
Pipework needs to be laid to a good gradient for water to drain away however the fall cannot be too sharp. If the downstream manhole is in a lower ground area, the drain is usually laid to the correct gradient but has a vertical section of pipework underground outside the manhole walls connecting it to the channel with a rodding access within the chamber. This is similar to a tumbler with the difference being this vertical section of pipework is outside the chamber walls.
Some redundant drains are left open once the original connection has been removed and if they have not been sealed, the surrounding earth will move over time, leaving spaces underground. This will also occur if there is a large hole in the pipework or a collapsed section where the shingle surround falls into the drain and is washed away. These will lead to movement above ground and potentially to a building's structure.
Drain surveys are able to see if there are fractures in the pipework however they do not tell us how much water is leaving the system. Carrying out a water test by capping off a run at the manhole then filling the drain with water determines the extent of the damage. You can read more about this investigation here.
We hope you found this Glossary of Drainage Terms useful. We appreciate the concerns which can arise when dealing with drainage issues and the mystery surrounding them - not many people want to know much about their drains, just that they are working properly. This is why many of our customers appreciate the feedback from our drainage engineers. The Drainsmart team are always happy to answer any questions or concerns you have about your drainage issue or any of the work being done.
If you choose to have a CCTV drain survey in London conducted by Drainsmart, you will be provided with a report which has the findings. You can have a look at our Sample Report page by clicking the option in the website menu. This report can be invaluable to many of our customers, especially those looking to purchase a new property. If anything on your drain survey report doesn't make sense, please speak to a member of the Drainsmart team - we will be happy to go through it with you and let you know about any of the findings and/or recommendations that emerge from the survey.
One of the reasons we chose to add a glossary of drainage terms to our website was to help all customers, no matter their knowledge levels, learn more about what we do to help them. We work with all types of drainage customers, from individual homeowners to large construction companies, to assist them with their drainage needs. If we feel it is important for you to understand an aspect of your drainage issue, we will ensure to take the time and go through it with you, offering plenty of time for questions. In a lot of cases, most customers just want the problem fixed and to carry on with their day - which we appreciate and will do our best to accommodate.
To learn more about our drainage services, visit our menu page or use the contact form below to ask our friendly team a question. You can also use the contact form to request a drain repair or drain survey, please just let us know the details of the problem you are having and where the property is located. We hope to hear from you soon.