Hi all -

I've had an issue in my ensuite for about 4 or 5 months now - there's a pretty offensive smell in there but it only happens at night.

A bit about the house and bathroom. It's a two story house with the master bedroom and ensuite sitting on the second story above the garage. The ensuite is in the corner of the house (meaning it has two exterior walls) and the bedroom is the same just the next corner around. The bathroom has a sink, shower and toilet.

Since this began happening I've tried and done a lot of different things, tonnes of Google and YouTube searches but nothing's ringing out out me about what's going on. At first I thought it was a dead rodent, like a rat or mouse, in the walls as the smell kind of smells like rotting flesh. This drove me to get up into the ceiling and have a look around the bathroom area but nothing was there that I could see. Google's told me though that the smell from a dead rat should only last 3 ~ 4 weeks for the flesh to rot away and then a couple of weeks after that, so we're long past that point now. Also it doesn't make any sense that the smell only happens at night, if it was that it should be worse during the day.

I then read / saw on YouTube that the drains could be smelly, so I've done a couple of batches of vinegar, baking soda in both the shower and sink, as well as now three batches of draino and that hasn't done anything either. I should also point out I stupidly broke the drain for the shower and caused a leak into the garage below a couple of months ago and had to get a plumber around. Since then I can 100% say it's not the shower or sink drains causing the problem ...

I've also checked outside the bathroom window to see whether there's anything on the roof below it (which covers the rest of the garage below) but there's nothing on it that I can see.

So this leaves me with the toilet. A number of forums and other Google searches point to a S / P trap, wax seal or venting problem. Both while I was in the ceiling and last weekend I put my phone on an 8m extension pole and checked the roof for a venting pipe but I cannot see one anywhere. Then on level.org.nz I see that some toilets can have an air admittance valve. All of this is making me wonder whether I have something like this and this is the problem? Surely if I'm using the shower, sink and toilet everyday I can't have a dry S or P trap right?

I cannot 100% reproduce the issue either - some nights it's not as bad or simply doesn't happen at all. I've also been away quite a bit recently - weeks at a time, and when I return the bathroom doesn't smell at all. So it's like something different is happening at night and I think it's to do with the sewer system and sewer gas is getting into the bathroom somehow. Even though the bedroom is connected to the bath, if I close the bathroom door I cannot smell anything in the bedroom at all - it's completely contained to the bathroom. Even though I said I cannot 100% reproduce the issue, I'm about 70% sure it has something to do with flushing the toilet but then again I could be imagining it.

So hopefully this is a plumbing question that has a plumbing answer and someone out there is thinking "I know exactly what your problem is!".

Cheers - going slightly nuts over this!!!