Drain problems/rats with no consistent advice

13 Aug 2012
Reaction score
United Kingdom
Hi all. Ive got a 1930s house but most of the drains are new due to work we've done over the last few years. We occasionally get drain smells in the utility room and some of the bathrooms. The drains used to block up quite regularly- 2 or 3 times a year. The blockages seem to have stopped now.

About a year ago I had someone out to take a look, wash the drains out and advise on what was causing the smell. He said it was likely the interceptor- the cap is missing so he said it was allowing smells to come up from the sewer into the house; and to replace it with a non return valve.

We are about to have work done on the driveway so it's the perfect time to sort the interceptor. But the plumber who is doing the drainage for the drive said as I have u bends throughout the house, the interceptor isn't the problem and he'd never fit a non return valve as it will cause more trouble than it's worth. He said he can take out the interceptor and replace with a straight run to the main sewer. He said the smell likely means there is a crack somewhere; and/or when we flush an upstairs loo it pushes air down the soil stack and that "sucks" water out of some of the traps allowing smells into the house. He told me to get a CCTv survey done to find the crack.

So I did. And there is nothing wrong with the drains- no breaks, they are working fine. But we found a rat. The CCTV guy said that the rat was the problem and causing smells. His advice flip flopped between replacing the cap on the interceptor, to installing a non-return valve, to doing nothing (but to check all of the plumbing inside the house and go have it cleaned out)

So I'm confused, and only have a week to work out what to do as the work on the drive starts next week.

Any guidance/education would be appreciated:

- I don't like the idea of a rat in my drain, but is it actually going to cause a smell? Not really sure how that works. I understand new builds don't have any interceptorso or non return valves - do they therefore just accept that rats will be in their drains?

- i assume that the right thing to do is to stop the rats getting into my drains. What's the best way to do that?

- if the rats aren't causing the smell and the drains are working fine then what do I do? If the plumber is right and water is getting sucked out of traps- what do I do?

Any guidance appreciated.
Sponsored Links
Have a look a document H building regulations section H1 2.22
All covered there
Personally I dislike interceptors nothing but trouble.
If water is getting sucked out of traps that's a venting issue.
Are your drains correctly open vented at highest point of drains on your property?
Thanks. The soil stack is vented through the roof-weve got this thing with fake tiles so it looks like part of the roof.
Sponsored Links
Not sure if this is what you mean but here is a basic diagram of the drainage we have.


There is a soil stack inside the house - inaccessible as it's boxed in behind false walls in an upstairs bathroom and the utility room. It vents via the roof. Connected to it are two bathrooms upstairs; and a downstairs WC. There is a sweeping bend at the bottom of the soil stack to join the foul drain, which runs under our side extension and out to the main manhole on the drive; there is then the interceptor before it joins the main sewer in the street. This is shown in brown.

Separately there was some new drainage put in down the side of the house to take the waste water from the kitchen and utility room (the blue line on the diagram). The waste from those rooms goes out through a black pipe into a drain like the picture below. This joins the main foul drain under the driveway.


There are two of these- one for the kitchen, another for the utility room sink.

The WM is where we generally get the smell, although we do get it in one of the upstairs bathrooms too. The WM waste is actually connected to the kitchen sink waste - I wish it wasn't, it's quite noisy. The WM waste is the flexible grey tube on the right:

Interceptors are common, yes they can and do cause issues with blockages, but usual practice if they are damaged/broken/causing issues, is to smash them out and replace with a straight section of pipe. One of the few times you would look at fitting a Non Return Valve is if the main sewer was subject to surcharging during heavy rain for example and the property was at risk of flooding from backflow. Otherwise, your Plumber is spot on, they can cause more hassle than they're worth and would be very unlikely to offer any effect on your odour issue.

Previous Guy sounds like Cowboy, (there is unfortunately plenty of them in the Drainage game), who was looking for any excuse to clean out your wallet as well as the drains. CCTV Guy is no better, Rats are often found in drains, and unless dead, dont cause smells.

Did the blockages you experienced stop after you had the drains cleaned out last year? Possible some kind of obstruction was dislodged during the process, and has solved that issue at least. Golden rule with Drainage is 'Air behind water', if you've a vented stack then the system is able to breathe correctly, and there shouldn't be any issue with traps being sucked out if the system is installed correctly.

If the CCTV survey hasn't shown up any issues, system is vented and working correctly, (i.e. everything flows away), then there isn't much else you can do. A Smoke test as a last resort may show any slight leaks in the system which could be allowing odours into the building, but that could also be intermittent depending on weather conditions.

I have to wonder if the smell around the Washing Machine is actually due to a build up of crud inside the machine. These modern low temperature detergents are all well and good, but the machine may never get hot enough to keep the filth inside at bay. I would also lift the drain hoses up slightly, so they rise up to almost touch the underside of the sink before dropping to join the trap.

As an aside, I would keep an eye on the water connection to the plastic pipework, appliances shut the water supply off almost instantly after filling, if it is constantly jerking the Tee every time it cuts off, you may run the risk of something failing at some point in the future. If it's not found quickly, there will be some water spilled....
Really helpful- thanks.

With the rats: feel like I should be stopping them getting into the drains on my land (I thought I had some legal obligation to do so too) but I was wondering what harm they could actually do, and how they might actually cause a smell. Good to hear that they are not causing the smell issue - even if them having access is a separate problem in its own right. (The basin upstairs empties slowly too)

I think you are suggesting just getting rid of the interceptor and replacing with a straight run, and accepting that rats will get in. Which is at least consistent with the plumbers recommended.

Blockages: the reasons given have been down to stones and other stuff in the drains. We've not had a blockage for over a year tho so hopefully that's all sorted (and the CCTV guy yesterday said it was all ok).

Wm: I've cleaned it I don't know how many times, I'm sure it's the cleanest machine on the street! That, plus the fact we get smells in the upstairs bathroom, makes me suspect there is a problem somewhere.
Looking at your layout I would get rid of interceptor causes problems and been know for some time rats can swim through them anyway.
I would check that your vent pipe is clear and has a vent pipe guard on top I have seen them blocked in past.
If your getting smells particularly after wc has been flushed may be momentum sucking a trap out .
Consider anti vac trap on basin.
Hi, Had a smelly washing machine at work, when the sink was being emptied, waste water was being back siphoned into the WM. Also saw the tub filling with 'water' when not in use. Just a thought.
No legal obligation to stop rats, although given that they are disease ridden, chew everything in sight, and multiply like there's no tomorrow, they're not most Peoples idea of House Guests. Provided they are not getting to the surface, then they shouldn't cause too much of an issue, but they are opportunists and will seize any chance. The flexible WC Pan connectors on Ground floor WC's are a good example. Being translucent, the rat sees (literally) some light at the end of the tunnel, and finds said Connector very easy to chew through, so soon works out an escape route. If the drains are in good condition, and no breaks, then Ratty and friends may well stay put.

They're usually in a section of the system for a reason, (suitable spot to build a nest, broken pipework giving them easy access to the surface, and/or a regular nearby food source is available. You can get 'Rat Blockers' but these have to be inserted into the pipework at a Manhole, and I have to question their effectiveness, as whilst they may well stop rats, they could also stop the solid matter in the drains, and cause a blockage....

Only need to remove/replace the Interceptor if it's damaged/broken, or causing frequent issues. Many thousands still in the ground up and down the country not causing an issue. If there was an accumulation of stones, and these have since been removed, could well explain the blockage issue previously!

If you're getting smells upstairs, it is worth checking underfloor for any dead rodents, but failing that, make sure the vent is clear on the stack, no-one is 'missing' the Pan when taking a #1, and all traps are full of water, otherwise afraid I really am struggling to think of any more possibilities on the information you've given.
Really helpful, thanks. I was going to ask if there is a way to keep rats out of the drains because it seems I have two problems that are almost certainly independent:

1) rats
2) smell in WM and upstairs Bathroom

I was advised to get a non return valve to deal with the smell. But nobody seems to think it would help with the smell. A non return valve might help with the rats, but most people seem to think fitting one is asking for trouble. The interceptor is supposed to help with the rats, but it doesn't. I was going to ask if there is anything else I could fit to keep them out, but your message suggests anything designed to keep rats out would likely result in blockages. So I am sort-of resigned to having rats in the drains. I did wonder about putting in a non return valve and then trying to fit some sort of early warning system for blockages as an alternative.

I'm having a plumber in to look at the smell (again). I'll ask him to look at the plumbing for the WM and the upstairs bathroom. As regards checking the stack is venting - I'll ask him to do that too but as it vents out of the roof tiles, I've no idea how hard that might be.
It would seem that the air intake through your stack is the cause if I understand the following is true..?
You get traps/u-bends in the house being sucked/gurgling. It smells in the same place, at the same time.

I.e., In the moment this happens, due to your stack not supplying the system with air, a volume-for-volume exchange is occuring between air in the pipes (sent indoors) with water in your traps/u-bends (sent outdoors).

I am wrong every day. Good luck.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links