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Drains Explained

Drains Explained

For those on main drainage

Your Responsibility

The owner is responsible for all foul and storm drains up to the boundary of the property, from there it is the responsibilty of the local water authority. A damaged drain can cause many problems – the most serious being subsidence.


Before buying or selling make arrangements for a survey of the drain lines both foul and storm

Have a Drain Pressure Test

Most of the drainage systems currently in use within the local area are as old as the property itself, most are made of salt-glazed pipes and fittings.

These pipes are roughly 3’ long and were fitted together with a lime/cement-based mortar. Not only are the pipes fragile, a number of common problems occur with age, the joints can break, the pipe work can sink and crack, tree roots can ingress into the drains – this will lead to the system eventually becoming unusable.

The drain pressure test basically involves plugging the drains at the beginning of the system (or nearest point to the boundary of the property – or where the drains enter the boundary) then filling the entire system with water (or in certain cases – with air).

The system is then left on test for a specific period of time, and the level of the water is measured at the highest point of the system. If this level drops more than the allowable tolerance, the system fails the test (in other word the drains are leaking).

Why does it matter if the drains fail the test?

If your drains are leaking this can cause a number of factors to occur:

  • Subsidence of nearby buildings – which can be extremely expensive to rectify.
  • The drains will sink – this will cause a ‘bellying’ effect and the drains will silt and then block.
  • The contents passing through the system will be leaking into the sub-soil – this will pollute any nearby watercourses or underground springs. This will then involve the Environmental Health Department and in many cases will result in the system not being allowed to be used until the problems are rectified.
  • The longer they leak the more difficult they are to repair.
  • Vermin may enter the drains.

Why do I need to have a camera put down the drain?

If the system has failed the pressure test then it is a good idea to have a camera survey. The reasons are as follows:

  • The problems can be pinpointed – the alternative would be a full replacement of the system.
  • The problem can be identified – building insurance may cover some causes, and in which case they will require video evidence in order to validate any claim.
  • Lime scale, grease and fat deposits will be detected – this will eventually block the drains, caught early this will be less expensive to clear out.
  • The drain condition can be seen down to the connection to the main sewer. Remember it is the local water authority who is responsible for the drain line from your porperty to the main sewer and should any problem exist in this area they should be informed. 

O.K. the drains have failed and the camera survey has located the problem.  What happens now?

Depending on the cause of the damage, there are normally three common options:-

  • Patch-lining – once the drain has been cleaned with high-pressure water jetting a new section of liner can be inserted into the damaged section of drain.
  • Re-lining – again once the damaged sections are clean, new epoxy resin impregnated drain liners can be inserted into the drainage system. However where there are connections (laterals to the drain i.e. toilets, gullies or vent pipes) they are usually excavated and new plastic joints inserted.
  • Excavation – the damaged sections are replaced with plastic pipe and fittings. In the worst case scenario, the full system will be replaced. Should the latter be the case, it will be inspected by the local authorities building control officer – a certificate will be issued upon completion of the works to prove that they conform to current regulations.

For those with no mains connection

In all cases you must hold a Certificate of Exemption from the Environmental Agency to discharge liquid into the ground, ditch or water course.


These are not from the 21st century they are just large storage tanks that have to be emptied by tanker when full. Usually every 2-3 months.

Septic Tanks

People often make the mistake when lifting the lid, and finding the level nearly full, assume that they have a problem. This is not the case as the level will be near the top at the height of the outlet pipe. In other words, this is the level, which will be skimmed from the top into the soak away. A septic tank has two compartments, the first collects the solids and the water overflows to the second compartment. From here the liquid flows through the overflow to the soak away, usually a herring boned system covered with shingle, which filters the effluent, which is then dispersed through the ground. Natural bacteria should break down the solids in the first compartment but with modern household detergents these kill the natural bacteria and in some cases chemicals like enzymes have to be used to induce bacteria. A septic tank should be emptied by tanker annually (depending on usage). The following are the most common cause of failure:

  • Ground water leaking into the tank causing flooding of the tank and soak away.
  • Tank walls collapsed – tank requires replacement.
  • Soak ways silted and clogged  – requires replacement (average life around 25 years)
  • Soak away installed in clay ground  – this will not work.

Treatment Plants

These are new technology, which replace septic tanks. They have a pump and media bales, which cleans effluent to environmental agency standard to allow it to be dispersed into ditches or watercourses. These are ideal where the ground is clay and there is a nearby ditch or water course. They should be de-sludged annually and serviced.

Should you choose Keens for any required remedial work, what happens if something goes wrong in the future?

  • All the works that have been carried out by Keens are covered by our ten-year guarantee (subject to normal designed use) for parts and labour.
  • We will resolve all problems as quickly as possible.

Keen on Price — Keen on Service

Drains throughout Dorset, Hampshire and Wiltshire ~ Blocked Drains ~ Treatment Plants ~ CCTV Survey ~ Relining ~  Roots

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Blocked drain clearance, C.C.T.V surveys, treatment plant installations, soakaways, septic tanks, throughout Dorset, Hampshire, and Wiltshire