Sewage smell from under floor

[osh]

Please bear with me during this long post but I'm trying to supply as much detail as possible to get an accurate diagnosis.

My property is a three storey 1930 terraced house. I have a conservatory extension at the back of the house and there is a tiled manhole cover to allow access to the rear manhole below it.

A few years ago we began to experience a smell of sewage in the ground floor. It was intermittent but by process of elimination we worked out that it only came about when the upstairs toilets were flushed, usually after periods of inactivity in the drainage system. We thought it may be a crack in the main soil pipe but a CCTV inspection proved that the pipe was intact.

We raised the rear manhole and discovered that the previous owners had plumbed an extra drainage pipe from the kitchen into the rear chamber by knocking out a brick or two and running the drain pipe through the gap. They had not bothered to seal off the gap around this pipe and when a toilet was flushed, all the stagnant gas and air was being forced through this gap and entering the kitchen through the floorboards. There are large gaps in the floor under the kitchen floor units that allow a lot of draught from beneath the house to come up.

We remedied this by filling the gap with expanding foam. This sorted the problem out immediately and smell was no longer.

A few weeks ago the smell came back. It was only when my kitchen drains started popping that I realised I had a blockage. It turned out the rear chamber was completely blocked. A power flush cleared this easily but during the flush a large ball of expanding foam came out. The drain guy thought this had caused the blockage but I'm assuming that it came loose either from being wet or blasted away by the power flush. In any event, the sewage smell did not go away after the drains were cleared.

We lifted the rear manhole again, and there was still a fair amount of expanding foam in place. The gap was properly closed off with cement, the bottom manhole cover sealed and replaced. However there is still a constant terrible smell of sewage. There is no pattern to it, it can always be smelled. It is clearly coming into the kitchen from under the cabinets - no doubt through the gaps in the floorboards.

I have tried to consider all options as to where it originates from and nothing seems to add up. It is not just a whiff, but a strong and constant smell. I would appreciate any educated ideas.

 

[Ch4 plumbing]

Unblocking with the jet hose could have separated a waste fitting kitchen side, do you know if the pipework is accessible?

 

[oz-plumber]

You will have to have all the drains gone through with a camera again.

If the drains are broken somewhere under you floor, you will want this fixed asap.
Once you get raw sewerige spilling under your floor you are going to find it extremely unhealthy and unpleasant.

The smell will probably linger for months even after the repairs have been done.

 

[osh]

The kitchen waste runs diagonally under the cabinets and a tiled floor and the only part that is accessible is from the sink to the point where it goes under the floorboards behind the cabinets. I find it unlikely that a waste fitting would become separated from the jetting as the waste pipe is not really susceptible to much movement.

I think asking my household insurers for a CCTV survey is the logical next step - I don't really want to be ripping up the kitchen on a whim.

Is there any other way to test for a separated waste pipe without opening anything up e.g. smoke test?

 

[oz-plumber]

Yes ...there is.

Get someone to run a camera down each drain.
If possible, get them to run the camera under the floor, along the drains, to see if there is anywhere where there spillage.

One thing for certain - the problem is not going to fix itself!

 

[osh]

CCTV survey has been and gone. The main soil pipe is cast iron and intact. They mentioned (which I forgot to include in my OP) that there is an old disused appendix that curves off the rear manhole, from the old external pre-rear extension drain, that may have filled up with waste matter during our blockage, and that is letting out gasses when the drains are used. They said that when the 2nd floor toilets were flushed, they could see on the CCTV that water was splashing back and up this gulley from the downforce. Because this appendix is not directly opposite the main soil pipe, they are unable to get their camera up it.

However, if this is correct, I'm still confused as to how the gasses are escaping up and into the house. The gentleman that did our powerflushing is of the opinion that the old ground drain that drained into the appendix was never capped of properly when the rear extension was built over it, and this is now airing out under the house and escaping through the floorboards.

I'm kicking myself for not blocking this gulley off when I had the rear manhole cover up, a few weeks ago, but now I need to know if there's another solution as I really do not want to lift it again.