sewer gas smell - downstairs wc

[sandyman44]

Is there a wash basin in the cloakroom?

If there is are you sure the smell is not coming from this? Possibly due to siphoning of trap

Beat me to it @TCCHeating

OP: fit an antisyphon trap to whb if there's one in the room.

Other things to consider are...
Damp in walls or floor from source other than waste.
Does cistern have traditional overflow pipe to outside, through which, odours could be entering cistern and boxing.
Does the room have an extractor? Could one be fitted?
Some 'cloakrooms' are just not good for airflow, are always sealed off from rest of house and eventually pong a bit!
My downstairs shower room with wc and whb is in the middle of the house with no external walls or window... I put a powerful extractor in there and the only time it has a bit of odour seems to be in the midst of very damp weather, when the water table is high and the ground sodden- even then it's a damp smell and not a sewer smell!

room has an outside wall and double glazed window, with a night vent thats always open, so always has some airflow. the smell only comes on flush, its not there under "steady state".
toilet has internal cistern overflow, its modern back-to-wall unit with boxed cistern.

I considered damp but I can't see any evidence, we are in a very well drained soil area, and as the timing of the appearance of the smell is clearly linked to flush, I discounted that as a possible.

not getting the smell at the mo, I'll have a look at the basin trap next time it appears.

 

[Nige F]

Where does the washbasin waste go to ?

 

[sandyman44]

Where does the washbasin waste go to ?

separately down into the floor, its on opposite side of room (approx 1.5 metres away) from the toilet. it doesn't join the toilet waste inside the house. it almost certainly does so underground outside somewhere.

 

[HERTS P&D]

Blocked manhole.


Andy

 

[picasso]

I had a very similar problem but it was a bit easier to diagnose, bloody rats building nests luckily the tenant stopped using the toilet as soon as they noticed the smell.