If the water in your toilet bowl bubbles, rises slightly or gurgles when the bath, shower or basin empties, air is being pushed through the toilet’s water seal. That usually means water is struggling to get away down the shared pipe.
The two usual causes
A partial blockage downstream
The toilet and bath often drain into the same soil pipe or underground drain. When that pipe is partly blocked, water from the bath pushes air ahead of it, and the easiest escape is through the toilet. You may also notice the toilet flushing more slowly than usual.
This is the more common cause, and it tends to get worse until the drain blocks fully.
A venting problem
Drainage systems need air to flow properly. The soil vent pipe on the outside wall or through the roof, or an air admittance valve (a small cap on the pipework, often in the loft or boxing), lets air in and out. If the vent is blocked, for example by leaves or a bird’s nest, or the valve sticks, draining water pulls and pushes air through the toilet instead.
How to tell which
- Fittings are also draining slowly, or the outside chamber is filling: likely a partial blockage.
- Everything drains at normal speed but the toilet bubbles: more likely venting.
- There are smells as well: traps may be being emptied by the air movement, which can happen with either cause.
What you can do
- Check the inspection chamber outside. Standing water means a blockage. See how to find your drains.
- Look at the top of the soil vent pipe, if you can see it safely from the ground, for obvious blockages.
- Do not climb onto the roof.
When to get help
If the bubbling comes with slow drainage, rising water or smells, have the drain checked before it blocks. Clearing the drain usually stops the bubbling. If it returns, a CCTV drain survey shows whether there is a defect such as a dip or roots.
See also why are my drains gurgling? and blocked toilets.