There has been smell of sewerage emanating from my downstairs loo for the last year. My plumber can’t fix it.

February 2014


I am at a loss to know who I should contact to try and solve my problem which is as follows: I have a smell of sewerage emanating from the vicinity of my downstairs loo. The problem started about a year ago. This is more or less continuous with some days worse than others.

My plumber has made valiant attempts to locate the source including removing the floor boards to locate the pipes, etc, but without success. It seems to pervade the house at times. I am at a loss to know who to go to next. Would a civil engineer be the right person to contact or an architect?

The house is 35 years old and on its own grounds. There is no smell from the area outdoors and drains outdoors are kept clear.

I would appreciate any direction you may be able to provide.


The most common cause is a loss of water in the trap below the appliance allowing foul air to enter the house. This is associated with unvented modern plastic pipe systems. Dwellings will generally have an external soil vent pipe which receives waste water from toilets, showers, hand basins etc. Some appliances at ground floor route direct to outer drains. Internal pipes are normally 100mm diameter for toilets and 32mm waste pipes from general appliances.

Building Regulations state that an unvented 100mm soil pipe should not exceed 6 metres in length from WC to the outlet. Similarly hand basin waste pipes should not exceed 1.7 metres for 32mm pipes and 3 metres for 40mm pipes. If this criterion is exceeded, then there is a risk for foul smells to enter.

The smell you refer to may be for the aforementioned reason, however smells from drains can be deceptive. Dishwashers and washing machine plumbing can be unsealed and cause smells to enter the room.

Added shower rooms and toilets are prone to this kind of problem as they can be installed in an ad hoc manner. Even soaps and detergents can give off pungent odours.

Other issues could relate to lack of venting in the WC compartment with no window and/or mechanical extraction. Blocked and cracked drains can also present this problem. A CCTV specialist drain survey may even be necessary.

Your first step is to check the unvented issue. To assist, look at the Department of Environment, Community & Local Government website, (see Building Standards — Technical Guidance Documents — Part H Drainage and Waste Disposal — Section Diagram 3). This will help to determine whether there is risk of a loss of water in the trap.

If so, the installation of a 32mm vent pipe at the appliance vented to the outside, or an internal air admittance valve should solve your problem.

Failing that, I would seek the advice of your local chartered building surveyor to investigate.

Jim Drew is a member of the Western Region Branch of the Society of Chartered Surveyors Ireland.