Tanking basement

Hi,
This isn't strictly a DIY question, but it is building related and there's a lot of knowledge and experience in this group, so I'm hoping you can help.
My basement was very badly "converted" by a builder who failed to put any waterproofing system in place. As a result it is damp. I've looked in to putting it right, which basically involves stripping it all back to bare walls & floor, putting a waterproofing system in and then refitting everything.
A builder who has quoted for the job has offered to show me another conversion he had done, which I have just checked out. It all looked perfectly good, but I was slightly concerned about one thing. Where the gas meter is sited, the waterproof membrane (a plastic sheet with large dimples) wasn't sealed up - that's to say, two pieces met behind the meter, but you could easily pass your fingers between them and touch the brick wall, in contact with the earth, behind.
AIUI, such systems are dependent on a perfect seal and any breaks in that need repair to prevent the system from failing. When I asked the builder he said that the specific spot wasn't a problem and that moisture could still drip behind and gather in the sill etc. This was at about eye-level, if that makes any difference.
Am I being overly-cautious or should I question this point further?
TIA.
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JustMe wrote:

I suppose it depends on the height of the water table and other local conditions. But yes, I would question further.
Owain
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Owain wrote:

These systems are not really dependent on a perfect seal in most circumstances. You can either run the system sealed whereby all entry points (like the gas meter) are sealed and the top of the membrane links to a membrane over the ceiling and stops water vapour escaping. More reasonable is that the wall membrane links to the floor membrane and is open (i.e. vented) at the top. This means any free water or large amounts of vapour don't come into contact with your inner wall but run down behind the membrane. In general if entry points aren't sealed water will still tend to flow down hill towards the floor.
I'd be more concerned to check that he plans to put in a sump pump and some perimeter or sub-floor drainage. Cavity membrane systems need a drain. I was very glad I ignored the structural engineer who said it wouldn't be necessary and put it in before the floor went down. My pump (actually a bucket at the moment as I haven't had time to install the pump) extracts around 10 gallons a week at the moment. Not a huge amount but enough to cause a problem if not removed. The system works well, I can see water on the inside of the membrane at various places depending on weather, temperature etc. but no problems inside the room.
Fash
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Thanks, that's what the builder said, so it's good to hear some confirmation. It seems to match with the plan, which doesn't include a membrane across the ceiling, so that sounds good.
Yes, he's putting a sill/drain all around the perimeter and he's digging out for a sump too.
Are there any other issues that I should be looking out for?
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JustMe wrote: .

Look at 'Curtain Injection' whereby an Acrylic Gel is injected, under pressure, through the wall at predetermined injection points and a fast acting gel forms a permanent waterproof barrier in the gap that is always present between the back of the brickwork and the subsoil.
This prevents the water getting to the brickwork in the first place. Very sucessfully used on bridges, tunnels etc etc. Google 'Resin Injection' for names of contractors.
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