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Clearing blocked drains

Cost: Difficulty:


The householder is responsible for the whole drainage system up to where it joins the sewer, where a house is drained individually.

The local council is responsible for cleansing any part of a communal system, if they were constructed prior to 1937. However the council can reclaim the cost of repair from the householders. Systems constructed after 1937 are the sole responsibility of the householders. They must collectively share the cost of repairing and cleansing the drain up to the sewer.

If unsure, contact the Technical Services Department of your local council to determine responsibility for the drainage system.

Determining the type of drainage system

You must ascertain the type of drainage system you have before attempting to unblock a main drain.

The system most commonly used is the single stack system. The wastewater and soil pipes are all connected to the same single stack.

Although drains run underground, they nearly always run in straight lines between the inspection chambers, gullies, fittings and stacks. Inspection chambers or manholes exist at every point where pipes join and where the direction or gradient of the drain changes.

In older properties the inspection chambers can be brick built rectangular structures with cast iron or galvanised steel covers. In newer houses plastic chambers can often be found with iron or steel covers. In older houses built before the Second World War the waste pipes are often divided into two separate pipe systems.

Waste from the WC is fed directly into a pipe of a larger diameter - soil stack, leading to the underground drains. The drain gases are discharged at a safe height into the open air above the house guttering.

The waste pipes from your plumbing fittings leading from upstairs baths and washbasins reach the waste stack directly, via a hopper funnelling the water into another vertical pipe.

An unpleasant smell from the inspection chamber is usually the first sign of a blocked drain. Sometimes if the blockage is severe, sewage can overflow from a gully or from under the cover of an inspection chamber.

Removing the manhole cover

Lift the manhole cover nearest to the house. It is not easy as cast iron covers are very heavy.

If it is rusted in around the edge, use a screwdriver to scrape around the edge, then tap round the edge with a hammer.

If it is still resisting movement, tap the cover gently with some wood and the vibration should be sufficient to release the cover from its seating. Insert a spade under one edge to raise the cover, place some wood under the edge to support the weight and lift it aside.

Special keys are available for help in raising the lid, but if a key is unavailable, a strong hook or piece of bent steel will suffice. If the covers have small handles in the form of small bars across the indents. Here loops of string or wire through the bars attached to a large piece of timber can be lifted by two people supporting the timber.

If the cover is secured by screw bolts, it is wise to soak them in oil before attempting to undo them with a wrench or spanner, then continue in the same way as above.

Finding and clearing the blockage

Once removed, if the manhole is clear, the blockage is between the manhole and the house. The only effective tools for this job are a set of drain rods because they are flexible, unlike a stick.

Sometimes a small blockage could be cleared by plunging with a mop or use of a stick, but the outlets and inlets are hidden by effluent and flexible rods are usually required.

Screw two rods together and slide them into the chamber in the direction of the suspected blockage. Screw on another rod and continue pushing, whilst manoeuvring the rods vigorously back and forth. Always turn the rods in a clockwise direction as you push them, this will stop the rod loosening or a joint undoing whilst underground. Continue until you feel the blockage clear.

The chamber should then be flushed thoroughly to remove all traces of debris. Sometimes the blockage is caused by a build up of fat in the chamber. Flush hot soapy water through the chamber to help remove any remaining debris.

If the blockage cannot be located, it must be further down the run. Lift the next manhole cover and repeat the process.

If the lowest manhole is blocked, then the blockage must be between that manhole and the sewer. If it is an interceptor trap, the blockages are nearly always in the trap.

The trap is usually full of effluent, some stoppers are on a chain otherwise feel for the handle with a stick and try to hook it out of its socket. Try not to let the stopper fall and block the trap. Use the rods through to the sewer and then flush out the channel with running water to clear debris.

Replace the stopper and before replacing the cover smear a little grease around the frame to stop any rust in the future.

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