Thank you so much for your answer. Some of it is a bit greek to me though I must confess.
The smell has been there for about 2.5 years! I keep thinking we've fixed it and alas we have not. The sink and tub and shower all drain well... no changes... no slow drainage. The toilet does not flush well at all but the plumber says it's because it's a cheap toilet, water saving toilet etc. He says the toilet is not the problem. Where is the vent? Got to like questions like that don't you
I had the plumber check the pipes under the sink as far as he could see... all is well... BUT... things were not sealed and he sealed them as far back as he could see. The plumber asked me to check the attic the next time there was a strong smell to see if it was also in the attic. So far, the only place I can smell anything is under the one cabinet... the smell doesn't even drift into the other cabinet which is amazing to me.
No trees around the roof vent. Roof vent was replaced when we redid the roof. At that time there was a terrible septic smell outside. The roof vent was moved as they told us it was too close to the edge of the roof. The bathroom that smells is the one right under the roof vent. The only thing between the bathroom and the roof is gyproc and insulated air space.
I put my hand all around the pipe under the counter and nothing is wet as far as my hand/arm can reach.
Would putting some javex down the drain help at all? You can see that I am truly a newbie at this.
The upstairs bathroom is linked to the kitchen, 1/2 bath and lower bathroom and none of those smell... it's just the one upstairs.
The plumber says he really doesn't know what to think and of course wants to start taking out gyprock... sure wish I had more confidence in him.
Thanks 21 Boat