Suddenly the ensuite bathroom in our house has started to smell like sewage. Neither the toilet or shower is blocked, so wondering where the smell could be coming from? Is there any way the toilet could allow smells from the septic into the bathroom, like some one way valve has failed?

There "should" be a vent to the outside at the top of the soil pipe, ours is just an open pipe in the wall.

If the local wildlife has made its home in there it could stop the gases from the tank venting out and instead coming into the room.

Of your outside tank is fuller (also could be something to do with the wet season), and you don't have any P-traps, when you turn on the bathroom exaust fan you can suck the smell up thru the drains.

You can buy sort-of P traps for the drains they have a small water cup, stopping smell.

7 hours ago, giddyup said:

Suddenly the ensuite bathroom in our house has started to smell like sewage. Neither the toilet or shower is blocked, so wondering where the smell could be coming from? Is there any way the toilet could allow smells from the septic into the bathroom, like some one way valve has failed?

Depending on the septic system you have, you may want to top up your Effective Microorganisms (EM-1) which stabilizes the biological balance in your septic tank and reduces foul odors by promoting the growth of beneficial microbes. However, in real life, because Thai plumbers generally have zero training, and I'm guessing that you don't have a p-traps, vent pipes or proper seals installed around your pipes, welcome to the never-ending stink. So, depending how the wind blows, the stench comes back through the pipe as there's no way to stop it. Also, even a modern septic tank needs to be emptied every now and again. Thailand has indeed a plumbing standard which has adopted pretty much the International Plumbing Standard - but no 'plumber' here knows anything about that. I'm currently renovating my third house in Thailand and I had to show the builder and his plumber multiple images of a p-traps and vent pipes and explained to them multiple times what they do. I even had to act and do pantomime to explain what happens if pipes don't have an adequate fall and that vent pipes help push the load along. P-traps in the sink and shower are required to stop the stink coming back up. Although I knew that the plumber wasn't qualified, when they asked whether I also wanted them to install a p-trap on the toilet bowl, I explained to them that every toilet already has a p-trap built-in. 😒

I had a similar problem earlier this year.

I had similar thoughts to earlier posters; had the systems (inside and outside) checked, and had Effective Microorganisms (EM-1) applied, inside and out. Problem persisted.

Further checks. Problem was, and this was a new one for me, apparently there was a eroded/damaged seal between the toilet (under the toilet base) and the pipe to the external sewerage system. I was skeptical when told this; however, I was wrong, seal replace bad smell gone.

This maybe your problem also. Not my area of expertise.

Good luck.

Check if there is Tim Hortons around

It is possible that the "emergency" floor waste, not the shower one, has run dry and is letting the smell come back from the tank. Pour water into it to see if it fixes the problem.

10 minutes ago, GreasyFingers said:

It is possible that the "emergency" floor waste, not the shower one, has run dry and is letting the smell come back from the tank. Pour water into it to see if it fixes the problem.

Don't think our house has those.

11 hours ago, ZigM said:

Also, even a modern septic tank needs to be emptied every now and again.

Not sure how you classify 'modern'?

Curious though as I have seen many comments on here over the years about people never having emptied their septic tank. The house I am in was built 18-20 years ago, and certainly in the last 16 years, the septic has never been emptied.

So is this abnormal?

3 hours ago, topt said:

Not sure how you classify 'modern'?

Curious though as I have seen many comments on here over the years about people never having emptied their septic tank. The house I am in was built 18-20 years ago, and certainly in the last 16 years, the septic has never been emptied.

So is this abnormal?

We empty it every year before the rainy season, mostly because of me. I don’t want it to overflow during heavy rainfall.

Do you have a photo or drawing of your toilet setup with the vents?

If you can't find the cause, treat the symptoms. Check the toilet seat seal and replace it if it's damaged. If you flush and hear just the sound on the open end, then seal is OK, but if you hear a second "splunk" sound a moment later, it's coming through the crack/hole in the seal, and it should be replaced. To do that, seat is lifted, old rubber seal removed, possibly plastic casing over the drain hole, and new one installed, then seat refitted.

Then there are drains on the floor and in shower. There are silicone seals you can put inside the drains which allow water to drain down but then they close so the smell and any insects can't get up into the bathroom.

Last, in case this only happens when water is running, if you are using any cotton filters on the shower head, or if you have had a shower hose a long time, especially if it's a budget model, it may be possible that there's bacteria or fungus growing inside of them. Replace the filter / shower hose, possibly shower head.

Most of the local builders don't understand the function or P-traps and DWV systems that are a standard plumbing code in the Western World. Thus, few new construction units don't have them. The condo I reside is an example. The sewage gases are a regular problem here.

On 7/15/2026 at 7:57 AM, JimHuaHin said:

I had a similar problem earlier this year.

I had similar thoughts to earlier posters; had the systems (inside and outside) checked, and had Effective Microorganisms (EM-1) applied, inside and out. Problem persisted.

Further checks. Problem was, and this was a new one for me, apparently there was a eroded/damaged seal between the toilet (under the toilet base) and the pipe to the external sewerage system. I was skeptical when told this; however, I was wrong, seal replace bad smell gone.

This maybe your problem also. Not my area of expertise.

Good luck.

When I renovated a house and had new toilets fitted I found the new toilets had not been sited propertly over the existing hole/pipe and just because the cistern didn't quite meet the wall they had botched it .Not only that they didn't fit a proper seal and simply put silicone round the toilet base !! This seems to be quite a common "botch job" by thai workers so it pays to watch every little thing when having any work done ! "S" bends and "P" traps are another area that many don't understand !!!

Shower drains and other gray water drains often don't have P Traps in Thailand...

Diluted bleach can help they also make a rubber condom like thing that opens at the the bottom when water flows in and closes when water passes through blocking smells coming up from below.Can be retrofitted in any drain that doesn't have a P Trap

On 7/15/2026 at 7:57 AM, JimHuaHin said:

I had a similar problem earlier this year.

I had similar thoughts to earlier posters; had the systems (inside and outside) checked, and had Effective Microorganisms (EM-1) applied, inside and out. Problem persisted.

Further checks. Problem was, and this was a new one for me, apparently there was a eroded/damaged seal between the toilet (under the toilet base) and the pipe to the external sewerage system. I was skeptical when told this; however, I was wrong, seal replace bad smell gone.

This maybe your problem also. Not my area of expertise.

Good luck.

We had a broken floor seal in a rented house. EM1 helped but the smell kept returning. Ours was very difficult to diagnose until the toilet was lifted.

Edited July 16Jul 16 by IsaanT

Could be the joining gasket between the pan and floor pipe. Often in Thailand not done properly. The toilet pan has it own s or p trap to stop smells.

I don't know the "standard" for Thailand, if there is one but the house I rent has 2 systems. The septic system into which both toilets flow and the other is the grey waste water system where the 2 shower drains, 3 sink drains and the washing machine drains all flow into a grease trap. In my case the grease trap flows into a drain that runs behind then down the side of the house into the buried main drain in the street. Part way along the side the septic outflow joins that drain line.. I was told by a couple people here that the septic tank never needs to be pumped out but after watching a few videos on the subject I had it pumped. A plastic tank and while empty i looked in side and could see have it was arranged and I definitely DID need it pumped out. One shower has bad smells sometimes and I just put it down to the pipes and grease trap smell coming back. septic system in my case could not.

On 7/14/2026 at 7:59 PM, giddyup said:

Suddenly the ensuite bathroom in our house has started to smell like sewage. Neither the toilet or shower is blocked, so wondering where the smell could be coming from? Is there any way the toilet could allow smells from the septic into the bathroom, like some one way valve has failed?

It’s been about half a year since I decided to buy a new toilet bowl; the flush mechanism on the old one was causing too many problems, even after it had been replaced.

I bought a new one from the Cotto brand, and that’s when the trouble with the smell began. To cut a long story short, the siphon on the new bowl is too low, with the result that whenever a toilet elsewhere on the same sewer pipe is flushed, a brief vacuum is created, causing the water in the new bowl to be sucked down and the siphon to no longer seal properly as a result, there’s a regular sewer smell in the bathroom. It’s extremely annoying, and the only solution is to replace the toilet bowl yet again.

12 hours ago, Hummin said:

We empty it every year before the rainy season, mostly because of me. I don’t want it to overflow during heavy rainfall.

Do you have a photo or drawing of your toilet setup with the vents?

The septic tank sits at the back of the house under a covered area so the only chance of flooding it is when I clean the tiles around it.

Not sure what you mean by vents - I have as previously mentioned the

15 hours ago, GreasyFingers said:

"emergency" floor waste,

but I always thought they were to get rid of the water from mopping the rest of the tiled bathroom floors outside the shower areas.........😆

On 7/14/2026 at 7:59 PM, giddyup said:

Suddenly the ensuite bathroom in our house has started to smell like sewage. Neither the toilet or shower is blocked, so wondering where the smell could be coming from? Is there any way the toilet could allow smells from the septic into the bathroom, like some one way valve has failed?

Bathroom

Which ever hole that's coming out of there is no S bend water block to stop it would be my guess

anyone adds bacteria to the tank every month ? I do. And I check all 3 vents every once in a while. (toilet line). And bleach to the shower drain, bathroom sink. But my house is not a rental, I know all details and tricks.

Edited July 16Jul 16 by OneManShow

On 7/15/2026 at 7:57 AM, JimHuaHin said:

I had a similar problem earlier this year.

I had similar thoughts to earlier posters; had the systems (inside and outside) checked, and had Effective Microorganisms (EM-1) applied, inside and out. Problem persisted.

Further checks. Problem was, and this was a new one for me, apparently there was a eroded/damaged seal between the toilet (under the toilet base) and the pipe to the external sewerage system. I was skeptical when told this; however, I was wrong, seal replace bad smell gone.

This maybe your problem also. Not my area of expertise.

Good luck.

I had a similar problem too. I bought a new toilet to replace an older one, but the builder/ handyman didn't install it correctly so we were getting serious sewage smells coming into the bathroom. It was difficult to locate where the smell was coming from until we lifted up the whole toilet to see that the vital seal between the toilet and the sewage pipe hadn't been installed correctly. As soon as the seal was installed correctly our problem was solved. No more smells.

Our toilet would start to smell after heavy rain - I surmised that nearby sewers had flooded and the 'exhaust gas' was being pushed back up some pipe

I had to get traps and vents retrofitted during our new build.......the Thais seemed to think I was very odd wanting them fitted.

On 7/15/2026 at 7:57 AM, JimHuaHin said:

I had a similar problem earlier this year.

I had similar thoughts to earlier posters; had the systems (inside and outside) checked, and had Effective Microorganisms (EM-1) applied, inside and out. Problem persisted.

Further checks. Problem was, and this was a new one for me, apparently there was a eroded/damaged seal between the toilet (under the toilet base) and the pipe to the external sewerage system. I was skeptical when told this; however, I was wrong, seal replace bad smell gone.

This maybe your problem also. Not my area of expertise.

Good luck.

Same issue regarding the seal between the toilet and the sewerage pipe not being properly installed, and I'm living in a condo. I had both toilets taken up and the (usually rubber) seal was not installed when I put the new toilets in. Had them installed and the toilets re-installed, and now no smell.

As @ZigM explained, there are few (if any) properly trained plumbers around. One of our in-house technicians is a "plumber", but he has repeatedly (about 6 times) incorrectly "fixed" the mechanics of the cistern in my guest loo, and it's still not working properly, so I will continue to hunt for someone who can do it.

2 hours ago, topt said:

Not sure what you mean by vents -

This is a plumbing ventilation system.

44 minutes ago, Hummin said:

This is a plumbing ventilation system.

No roof vents on any of the originally built houses in the moo baan I am in.

UK house obviously a different story.

1 minute ago, topt said:

No roof vents on any of the originally built houses in the moo baan I am in.

UK house obviously a different story.

None it my village either. Not a standard thing here. Keeps building costs down and smells up...

On 7/14/2026 at 7:59 PM, giddyup said:

Suddenly the ensuite bathroom in our house has started to smell like sewage. Neither the toilet or shower is blocked, so wondering where the smell could be coming from? Is there any way the toilet could allow smells from the septic into the bathroom, like some one way valve has failed?

Most likely, as most have said, no plumbing proper venting system. Also, had similar issue at my house and come to find out they never (another cheap standard) a toilet flange between the toilet and the submerged drain pipe. All they did was put tons of caulk around the base of the toilet. I guess thinking the water would just stay totally under the non sealed toilet.

I would take the toilet off of the floor and look under it first.

16 minutes ago, topt said:

No roof vents on any of the originally built houses in the moo baan I am in.

UK house obviously a different story.

If you want to get rid of the smell, you need to redesign your plumbing, or live with the smell.