Restarted on the build - any views on how to waterproof the cavity/retaining wall like a basement?

[AdTee]

Hi all, 

Some of you may remember my posts from late last year where I had to pause my extension to get an SE to detail how to prevent my neighbour's conservatory from sliding into my extension... https://forum.buildhub.org.uk/topic/40498-i-am-soooooo-stuck/

It's been a tough 8 months with 2 sewage leaks and a water leak all beneath the neighbour's conservatory pouring 'fluids' all over my foundations (not helped by their lack of build-over agreement with the water co.).Since then, I've been researching how to waterproof the lower part of the wall (blockwork cavity wall with concrete infill), and think I'll definitely be using a bitumen self-adheseive layer over a primer. BCO suggested Bituthene, but Permaseal looks good too and is slightly cheaper.

Question is, as it's only a 285mm gap between the two buildings, direct rainfall should be minimal, so do I need to worry about hydrostatic pressure, or even their vent pipe leaking sewage (as it's the head of a 5-cottage sewer run)? I've considered if there's any way to fit a dimpled drainage sheet between the waterproof layer and the concrete, but that would need something to drain into, and a perf pipe underneath all that concrete would be pointless (unless perhaps I put a layer of EPS between the permaseal and the Geodrain dimpled mat creating a cavity below for the perf pipe?).






Would be great to hear your opinions on this one, please!

Thanks

[Redbeard]

I imagine the lack of replies so far is because people looked at it as I did and got stuck on questions and practicalities. I'll try to start now, and you can tell me if it is of no help!

 

Do you consider that, before you dug out for your extension, your land was giving full, and adequate, support to next-door's conservatory? What it is, or isn't, sat on looks quite sketchy to me (sorry; I have not re-read your whole other thread, though I looked at the time). The question in my mind is whether you have an obligation to provide support or to allow access so that the neighbour can repair/strengthen their footings before you progress your build.

 

You say

On 03/07/2025 at 11:17, AdTee said:

I've considered if there's any way to fit a dimpled drainage sheet between the waterproof layer and the concrete, but that would need something to drain into, and a perf pipe underneath all that concrete would be pointless (unless perhaps I put a layer of EPS between the permaseal and the Geodrain dimpled mat creating a cavity below for the perf pipe?).

 

Why would it be pointless? (I am beginning to remember why I din't get further with my reply last time!!). OK - rephrase that: I take it the main reason for the lump of concrete is to hold up the neighbour's conservatory. Yes? ... and the drainage at the bottom to allow any moisture trapped between the concrete and your membrane to seep down to a perf. pipe? And potentially the perf pipe also receives the neighbour's sh** if it all goes horribly wrong? How would that work? Can you add their soil pipe to your dwg? Surely no 'liquid' could run down 'their' side even if you had a dimpled membrane in (which would face the wrong way, serving 'your side')? And equally surely, a perforated pipe is the wrong 'conduit' for an accidental poop spillage. You would surely have less to do and worry about if your neighbour's 'outfalls' didn't 'fall out'.

 

Lots more info, and details of any further thoughts you have had since posting, please! (Or tell me I have got entirely the wrong end of the stick!)

[AdTee]

Thanks very much for taking the time to reply, Redbeard. 

Yeah, it's a bit of a messy situation. The conservatory needed planning (outside of permitted dev due to length) and building regs (opened into main house and kitchen inside it) which it doesn't have, neither does it have a build-over agreement with the water co. The neighbour has inspected from my side and isn't concerned about any of it, including their footings. The retaining concrete fill will be more of a rough triangular shape from the bottom corner my side, to around 100-150 mm from the top corner their side, blanketing the spoil. 

Regarding it being pointless, I was also concerned that it would weaken the concrete retainer by having the perf pipe and loose fill at the bottom of it.  You're so right about the pipe being the wrong conduit for any 'fall out'. I could add it to my drainage system but it feels like I could end up making the whole situation worse in the event of any problems. The BCO pointed out to me last week, that there were other options available for their vent, but they chose to stick with that one, so any future leaks would need to be dealt with via the conservatory floor. 

I've ordered the Permaseal membrane and primer for my blockwork, and will either add some kind of jablite sheets or a bitumenous protection board in there to prevent damage to the membrane. I'm also toying with the idea of blackjacking the surface of the concrete infill but whether that'll hold over time I'm not sure.

 

Edited Tuesday at 11:01 by AdTee
typo

[Iceverge]

Could you perhaps label the materials in the drawing please and also draw the neighbours conservatory and pipes. 

 

In my experience nothing deals with water long term better than gravity. This would always be my first port of call.  Establishing a low drainage plane for any liquid to flow harmlessly away. 

 

Failing that mass concrete is an option.

 

I'd be tempted to pour a 200mm reinforced retaining wall at the boundary line of your property and forget all about the neighbours pipes. 

 

Maybe something like ICF could actual work with the concrete core acting as the support for the outer leaf of your extension. 

 

 

[AdTee]

25 minutes ago, Iceverge said:

Could you perhaps label the materials in the drawing please and also draw the neighbours conservatory and pipes. 

 

Yep, will do. I'll get chance to fiddle with it tomorrow.

Mass-concrete is the solution the SE came up with. He just left it to me to figure out the waterproofing. The BCO suggested bituthene, hence going with the Permaseal.

For now, the green cross-hatching is concrete, the red vertical dashed line is the conservatory (not the boundary as labelled). The two red columns are blockwork, forming the base of a new wall - the cavity, up to the height of the conservatory is also to be filled with concrete. The steel encased in concrete is a ground beam foundation.
The boundary line is approx half way between the conservatory and my new block wall. The pipe in the photos sits against the conservatory, pretty much along the boundary, and possibly a little over it. 

We've now started the blockwork, with the exterior skin up to 600mm. I'm hoping to get the waterproofing in this weekend.