Why does my bathroom smell like sewage?

[JMVIllot]

We’ve got a bathroom in a 3 year old loft conversion. Pretty much the first time warmer weather rolled around a few months after it was done, I started to notice a bad smell in there – somewhat sewage-like, but different from ‘recently used toilet’ smell. This cycle has repeated each year: cold weather fine, warm weather smelly.

I was always pretty sure that while the smell was coming from ‘the toilet corner’ that it wasn’t coming from the toilet itself. The smell is more noticeable slightly off to the side than directly over it.

Eventually I got down on my knees and followed my nose to small gaps that exist between the base of the toilet and the wall.

Could anyone please advise on the likely cause and what we will need to do to fix it?

(Photos below – including view of ‘toilet corner’ from the outside).

 

[oz-plumber]

I hope they haven't left a vent disconnected in the wall.

If the pan collar was leaking, there would be evidence by now of water pooling on the floor.

 

[Basher]

As above. If the toilet is mounted on a stud wall you might expect to find in the void behind an Air Admittance Valve installed in a vertical extension of the soil pipe around cistern height. They don't often fail but if hasn't been fitted properly, or is missing, you would have this problem.

Do you have an open vented soil stack elsewhere on the house? I wondered if it has been closed off at some stage?
AAV's are all very well to allow air in to follow a flush, but they don't let positive pressure in the drain system out. As I understand it, Building Regs require an open vent on the foul drains somewhere, even if on an adjacent house, which may be adequate.
We had a house with a severe attack of the problem you have, due to a sewage pumping station nearby periodically putting a positive pressure on the drainage system. The only venting we had was an AAV in a stud wall, which turned out to be defective. Once replaced with an external vent to atmosphere, problem solved.

Alternatively you might have a faulty pipe connection. I agree that would most likely result in a visible leak. Pressure testing could perhaps check that situation?

 

[SimonG]

Depends how stack is vented, could be blocked air admittance valve if not venting to atmosphere. Correct depth traps to basin and shower? Could be pulling water out the seal.