If you live in an older house, particularly a Victorian or Edwardian terrace like many in Southampton and Portsmouth, there may be an interceptor trap in the last inspection chamber before your drain joins the sewer. It is one of the most common places for a blockage in these properties.
What it is
An interceptor trap is a U-shaped bend, usually made of clay, built into the floor of the last chamber on your drain. Like the trap under a sink, it holds water, which was intended to stop sewer gases and rats coming up from the sewer into the house drains.
Above the trap there is a rodding arm, a short pipe that lets the section towards the sewer be rodded. It is sealed with a stopper, often a clay or ceramic plug.
How it causes blockages
The stopper falls out
Over time, stoppers can loosen and drop into the trap, or be knocked out during rodding. Waste then flows into the rodding arm and builds up there, or the stopper itself blocks the trap.
Debris catches in the bend
The tight bend of the trap catches wipes, sanitary products and fat more easily than a straight pipe.
The chamber fills, but the sewer is clear
A typical sign is that your last chamber is full of water, but the sewer beyond is flowing normally. The blockage is in the trap.
Signs of an interceptor trap problem
- The last chamber before the road or boundary is full of water
- Toilets and sinks back up even though neighbours are fine
- Blockages that keep returning at the same point
- A missing or broken stopper visible in the chamber
See why does my outside drain keep blocking?
What can be done
- Clearing the blockage and checking that the stopper is in place.
- Replacing a lost or broken stopper, which can stop the problem returning.
- Replacing or modifying the trap where it is damaged or keeps blocking. There are pros and cons to removing an interceptor trap altogether, as it was originally there to keep sewer gas and rats out. We will talk through the options for your drain.
A CCTV drain survey can check the condition of the trap and the drain on both sides of it.
Who is responsible?
The last chamber and interceptor trap are usually within your property boundary and, if they serve only your property, are usually your responsibility. If the drain also serves neighbours, it may be a shared drain. See who is responsible for a shared drain.
If your last chamber is full, see blocked drains, or read about rats in drains.