External Concrete Plinth - Should I Remove?

[dontbelieveawordofit]

I re-iterate, a photo of the inside of the bay window would be helpful.

 

[HandSlander]

Here it is. And the wall thickness is approx 270mm at the bricks (excl any sills, cement ledge etc)

 

[dontbelieveawordofit]

is there a vent behind the radiator?

 

[HandSlander]

No

 

[HandSlander]

• Leaky water main feed?
  
• Leaking sub-ground drainage pipes?

these two first. Because they can be sources of vast amounts of water, continually.

If you have a young person with good hearing, you can stand them in the kitchen, late at night when all is quiet, while you turn the stopcock under the pavement on an off. The noise of an underground leak is very slight, just white noise. But you notice when it stops and restarts. I'd consider a leaking lead pipe to be normal in a house of your age.

Well, last night I had a 4am bathroom visit and left the bathroom light switch off to save my night vision... the extractor fan is on the same switch so stayed silent. In the dead of night I noticed for the first time what sounded like white noise... not clearly directional but appeared to be coming from below the floor in the bathroom.

Bearing in mind this advice, I went straight for the ground floor stopcock access hatch located also in the bathroom, and after a few seconds of being closed off, the white noise stopped. Stopcock back on, white noise back.

So it appears there is some kind of sub-floor leak in or around the bathroom, on the tap side of the stopcock, which also happens to share a wall with the bay-windowed bedroom posted in this thread... dreading thinking of how I'm going to get access to the sub-floor given the bathroom is marble-tiled floor to ceiling.

 

[JohnD]

Thanks, very helpful response.

As I said it was a previous owner that did the 2010 damp proofing... here is all I have, hopefully all should be clear from my previous photos.

Looking at that plan, I see a bathroom next to the humid bedroom. That could be very relevant. What is the humidity in that bathroom? how is it ventilated? Does water leak anywhere, or reach the walls around the bath?

Edit
I wrote that before reading about your "white noise" leak.

 

[JohnD]

Bearing in mind this advice, I went straight for the ground floor stopcock access hatch located also in the bathroom, and after a few seconds of being closed off, the white noise stopped. Stopcock back on, white noise back.

So it appears there is some kind of sub-floor leak in or around the bathroom, on the tap side of the stopcock,

Great news!

In an old house, when you replace a leaking pipe, the new one does not have to follow the same route as the old. It can for example go under the hall floor, or past the outside of the house and enter at a convenient point.

Fit a larger size of plastic pipe than the old one you replace, you will be surprised what an improvement this makes to flow.

By replacing the whole thing, you don't need to find and repair the leak.

 

[HandSlander]

Thanks @JohnD ... I have a separate water hammer issue noticeable in the wall between the bathroom and bedroom which happens whenever the upstairs flat(s) turn taps on/off. Not very loud but enough to be annoying at night if sleeping in that bedroom. My point being it appears that cavity wall is not just servicing the bathroom facilities but also the main cold water feed for the upstairs flats. However, there is no evidence of dampness on either side of that wall whatsoever so the leak must be below the ground floor... Also while the building is not metered, the upstairs flats have their own stopcocks so the ground floor one just services the ground floor flat.

I've put together the below diagram to try and problem solve this... which took me longer than I care to admit... any suggestions on how to attack? I'm also planning on doing major landscaping soon so am happy to dig and do some serious work to make sure leak and drainage are brought up to a modern standard as much as possible.

 

[JohnD]

if it was me, I would start by assuming that the old clay drains are cracked, broken, blocked and leaking, and dig them all up and replace with new. That's a job for some sunny days. You usually find, when you dig one up, that the one it connects to is also broken. And so on.

And I would assume (unless proved otherwise) that the water supply pipes are old lead or iron, and also leaking. So I would run new. With water pipes, you can install new without removing old, so you only have a brief loss of supply while changing over. But, before starting work, ask your water co to test the drinking water for lead content. It is very damaging to the brain and nervous system, especially in babies and children, and there may be a lead replacement or leak repair subsidy, or, at the least, free reconnection of new pipes.

I gather the old house has been divided into independent flats, so you will have to get some kind of agreement with the other residents. They might have an obligation to share the cost.

 

[dontbelieveawordofit]

if it was me, I would start by assuming that the old clay drains are cracked, broken, blocked and leaking, and dig them all up and replace with new. That's a job for some sunny days. You usually find, when you dig one up, that the one it connects to is also broken. And so on.

And I would assume (unless proved otherwise) that the water supply pipes are old lead or iron, and also leaking. So I would run new. With water pipes, you can install new without removing old, so you only have a brief loss of supply while changing over. But, before starting work, ask your water co to test the drinking water for lead content. It is very damaging to the brain and nervous system, especially in babies and children, and there may be a lead replacement or leak repair subsidy, or, at the least, free reconnection of new pipes.

I gather the old house has been divided into independent flats, so you will have to get some kind of agreement with the other residents. They might have an obligation to share the cost.

when i rang to query our supply my local water authority reckoned there’s no proof of any adverse problems caused by lead pipes, which is apparently they stopped the grants in our area .

 

[JohnD]

but, as far as I can make out, several of them will reconnect your new pipe free and/or replace any lead pipe between the main and your property, e.g. under the pavement. This is helpful as it means they will dig the hole and organise the connection, saving you the difficulty of connecting new plastic to an old lead stub or obsolete size of stopcock.

 

[HandSlander]

Well today my 3-piece Milwaukee SDS Plus Shank Chisel Set arrived from Screwfix. I took to the cement plinth at the rear of the property (adjoining the new build extension) and I was very pleased at the performance of the chisels at removing the plinth. Although the cement was in reasonably good condition, and fairly dry probably by virtue of being near impermeable to moisture, the vibration of the SDS Hammer action helped to easily separate the plinth from the brickwork in large pieces as you can see below. It appears the mortar was raked out above the 1st brick upon installation of the plinth, such that the cement was recessed into the joint - this is probably why the plinth has adhered so well over the years.

Unsurprisingly, the bricks behind are damp. I'm not sure whether the thicker mortar line above the 2nd brick may contain a Slate DPC, further investigation required - this was the top of the plinth.

However, I was surprised that after taking off the plinth that it appears a thin cement bed had been laid right up to the brickwork, under the plinth. My assumption was that this would come off with the plinth, but it's a separate piece that has been left to dry before the plinth was made. As such, zero water is being allowed to evaporate from the sub-surface bricks as it is all effectively cemented into the wall. I broke up a section and the soil is damp underneath and appears to contain broken up pieces of brick as well... I wonder how old.

 

[HandSlander]

more cement plinth removed, the air grille was not sitting on a full brick like I thought but just cemented together pieces of brick and is now coming apart... slightly problematic if it rains as the water will now run straight into the sub-floor. I'll be breaking up the concrete ground level layer tomorrow so that shouldn't be a problem until replaced, I'm thinking a telescopic air brick.

 

[HandSlander]

I finally got some more 18V Lithium batteries for my SDS drill (supplier was out of stock) which was absolutely chewing through the single puny battery I had... so went to town today. The horizontally laid concrete running up to the brickwork on the bay was pretty tough... about 70mm thick and absolutely filled with gnarly stuff like bricks, stones, pebbles etc. which meant it only broke off in tiny pieces and not large easy chunks like the cement plinth did.

However, my persistence paid off and I finally found the slate DPC which you can see in the image was actually BELOW where the concrete was laid and with the plinth on top all kinds of damp bridging action was happening, complete with small plant roots growing in the damp. Madness. @JohnD was right it appears the new build DPC which is at ground level was laid at the same level as the original Victorian.

Main question now is what to do about (a) DPC below/at ground level (French drain?) (b) damaged bricks and pointing (c) loose air grille - all coming from both an aesthetic and performance perspective. Any ideas on best practice?

 

[JohnD]

Dig out the ground beside the wall, a few courses below the DPC. If you can permanently lower the ground/path level against the wall that would be best, if not, refill the trench with cobbles or large pebbles (no small material) which will prevent capillarity.

Repoint the brickwork after hosing out any mud or dirt.

Lots of airbricks to ventilate the subfloor void.