Mystery concrete plinth, and damp problems

[Galen]

To try and cut a long story short, I've got some penetrating damp problems with a ground floor flat in a converted Victorian mill building. I posted about this before, so there is some additional information here:

De-tanking a converted Victorian mill building?

I purchased a ground floor flat in a converted Victorian era mill building about 1.5 years ago. Unfortunately since then a penetrating damp problem in a corner of the building has come to light (amongst a number of other problems). Since buying the flat I've learnt a lot about solid wall...

www.periodproperty.co.uk

I'm happy to provide more details if anyone is interested in helping me try and figure it all out.

The penetrating damp is a problem both at the rear and side elevation. At the side elevation you can see in the below picture that the ground level increases by 4-5 brick courses from the right of the picture to the left of the picture (the ground level on the left hand side is higher than the interior floor level):



The company that the management company has contracted to fix this had previously proposed a french drain and an injected DPC to fix the problem (I'm not really that keen on the injected DPC, but I feel there's only so much I can push back here. I do think a french drain would probably be helpful though). The problem is that they're somewhat flummoxed by a concrete plinth that is off to the left hand side of this wall:



And a closer view:



The main problem here being that a french drain can't run through this plinth. And we're not sure how deep it runs. They want to dig trial pits to try and figure out what this plinth is doing (is it part of the foundations?), how deep it runs, etc. But they're taking a long time to do this, so in the meantime I've been doing some research to try and find out what it might have been used for in the historical context of the mill, and what the ground level etc. looked like previously, to try and help figure out if there is a simpler solution here (maybe just lowering the ground level? Although it is complicated by this area being a driveway owned by another adjacent property).

Unfortunately the only decent old photo I've been able to find from this angle is the following one:



It's a rather poor quality photo, but interestingly it looks like there is perhaps a black base to this wall in the old photo (perhaps a brick base wall plinth painted black? Or a concrete foundation/footing painted black? Or something else?) which isn't visible in the present day. I do think the ground level all around the front and side of the building has crept up since Victorian times (I have some evidence that it may have crept up for 2-3 brick courses at the front elevation) so perhaps this is now buried underground and should ideally be exposed again.

Unfortunately I can't find any information about this plinth, or any other photos of this side of the building (I've searched local archives and the Mills Archive in Reading, plus extensive Google searching) so I'm really at a loss as to why this might be there.

Does anyone have any ideas? Or is it really just a case of hassling the management company to get some trial pits done (which I plan on doing regardless)?

 

[elizabeth]

Hello Galen, I can see your bricks at ground level are very wet and green.I could be mistaken, but are those drill holes in the second course of bricks? Also what is the object sticking out of the middle of the wall a couple of meters up? There seems to be a drip line from it.

As you say, I think it would be a good idea to see how deep the concrete skirt goes. Lowering ground levels is the ideal. A search on this site for the ‘Zebcolm” trench maybe of help but you may find just lowering ground levels does the job. Wait for the walls to take a long time to dry out though.

Don’t allow any chemical injections damp proofing. They create issues which are hard to reverse.

We had similar issues with our wall being the boundary of our neighbours land. You are allowed to maintain your property but it is always best to get your neighbours support.

 

[aockenden]

Is the ground floor a suspended timber construction? Can you access / view the under-floor void? Uninformed, drooling, bodge-it builders have been known to fill up entire sub-floor voids with pumped cement screed as a “fix” for damp voids (which 9 times out of 10 just needed external groundwork or improved ventilation), typically making the problems way worse and often rotting the joists.

I’d be trying to figure out what’s going on down there, and assuming the void is still unfilled, I would try to peer over to the area near where the external cement “plinth” is located and see if you can see any of it coming through to the interior of the void.

 

[aockenden]

Also like the prior poster said, a French drain may not be needed if you can properly lower the ground levels and grade the ground at an angle away from the house.

 

[aockenden]

Oh I just read the other thread you linked and saw that you have concrete floors internally with wooden flooring glued down onto it…

So here’s one theory: the building originally had ventilated, suspended timber floors. Then some silly builder came in, found an underfloor vent brick or semi-underground window well, stuck a flexible screed-pouring pipe in there and pumped the subfloor cavity full of cement screed. He fills the cavity up until the window well itself begins to back up and flood with liquidy cement. He then shuts the pump machine off, and removes his flexible pipe and the pool of cement filling the old window well hardens into that externally visible “plinth” you’re seeing. Meaning it’s part of the same contiguous mass of cement as the internal floor under the wooden glue-down flooring. Does that sound possible? Plausible?

 

[Galen]

Hi both,

Thank you for your replies and sorry for not acknowledging them sooner. I've had a busy couple of days and was expecting to receive email notifications if anyone replied to the post!

To answer some of your questions above:

Elizabeth - the bricks at ground level are very wet yes (some seem to even have moss/algae growing on them). The drill holes you can see are a previous injected DPC - there are actually 2 on this wall, one in the bricks (which to my understanding is the worst way to do it), and another injected into the lime mortar joints. Alongside this there is also a physical DPC that my survey labels as original (although what it consists of I'm not sure - perhaps bitumen?).

The pipe you notice is some sort of overflow pipe from one of the flats above. I'm not entirely sure if it's in active use (I've never seen it in use) but you can see staining beneath it. Perhaps that is historic I'm not sure.

I'm very keen for them not to have another injected DPC on this wall (as the previous 2 don't seem to have helped much - although they are essentially at or below ground level on the left hand side of the picture, which isn't helpful). Are you aware of any authoritative material on injected DPCs and alternatives from the likes of historic England or any other prestigious body that might hold weight in my discussions with the management company and damp company?

I will also look into the Zebcolm trench, thank you.

aockenden - Thank you for your theories on what this plinth could be. You may be right that this was done during the conversion to flats in the late 1980's. Unfortunately I haven't been able to find any definitive information about this in the planning documents (which I have found online) or any other historic information that I've been able to get my hands on. I am planning to try and locate the original builder(s) that worked on this, as they may well still be living in the local area. I guess one thing that goes against this theory is that there is a brick course that seems to sort of surround the plinth - I'm not sure why this would have been done if a builder had used it to fill in the previous floor with screed?

The only clue I have as to the historic flooring is this diagram from a technical journal entry in the 1880's. This isn't for my section of the mill, but the one adjacent to it (my section was added as an extension at some point in the following decades, although presumably it would have been of very similar construction to the section shown in the diagram).

The diagram seems to indicate suspended flooring, but I guess the caveats to this are:

1. I can't see any evidence of bricked up air bricks or ventilation either on the building itself, or in any historic photos that I've been able to locate
2. The journal was concerned with the new equipment fitted to the mill, and doesn't mention the flooring on the ground floor at all (presumably they weren't really interested in the foundations or flooring - the article was mostly about the new steam and turbine machinery that had been fitted to the mill)
3. Would a suspended timber floor have been sufficient to support the significant machinery located on the ground floor and above? Would a concrete floor not have been superior in this regard?



At the bottom right of the diagram you can see 2 "little giant" water turbines which would have powered the mill alongside the steam engine. These turbines are close to where my flat is located, and the river would actually have run beneath the building at this point. I can't seem to find any information about how the river was blocked off here and what impact this could have had on the foundations.

In terms of lowering the ground levels and grading the ground at an angle away from the building, this is unfortunately complicated by the way the mill pond behind the building was filled in. I haven't posted any pictures of this but essentially the mill pond was filled up above the existing ground level, so the ground actually slopes towards the building all along the rear. From memory the side elevation has a similar issue but it's much less pronounced.

In terms of the concrete floor potentially having been installed incorrectly, is there anything that can be done to rectify this? Presumably re-instating the suspended floor would be preferable but I don't see any way in which that is even remotely feasible unfortunately.

 

[aockenden]

Hi both,

Thank you for your replies and sorry for not acknowledging them sooner. I've had a busy couple of days and was expecting to receive email notifications if anyone replied to the post!

To answer some of your questions above:

Elizabeth - the bricks at ground level are very wet yes (some seem to even have moss/algae growing on them). The drill holes you can see are a previous injected DPC - there are actually 2 on this wall, one in the bricks (which to my understanding is the worst way to do it), and another injected into the lime mortar joints. Alongside this there is also a physical DPC that my survey labels as original (although what it consists of I'm not sure - perhaps bitumen?).

The pipe you notice is some sort of overflow pipe from one of the flats above. I'm not entirely sure if it's in active use (I've never seen it in use) but you can see staining beneath it. Perhaps that is historic I'm not sure.

I'm very keen for them not to have another injected DPC on this wall (as the previous 2 don't seem to have helped much - although they are essentially at or below ground level on the left hand side of the picture, which isn't helpful). Are you aware of any authoritative material on injected DPCs and alternatives from the likes of historic England or any other prestigious body that might hold weight in my discussions with the management company and damp company?

I will also look into the Zebcolm trench, thank you.

aockenden - Thank you for your theories on what this plinth could be. You may be right that this was done during the conversion to flats in the late 1980's. Unfortunately I haven't been able to find any definitive information about this in the planning documents (which I have found online) or any other historic information that I've been able to get my hands on. I am planning to try and locate the original builder(s) that worked on this, as they may well still be living in the local area. I guess one thing that goes against this theory is that there is a brick course that seems to sort of surround the plinth - I'm not sure why this would have been done if a builder had used it to fill in the previous floor with screed?

The only clue I have as to the historic flooring is this diagram from a technical journal entry in the 1880's. This isn't for my section of the mill, but the one adjacent to it (my section was added as an extension at some point in the following decades, although presumably it would have been of very similar construction to the section shown in the diagram).

The diagram seems to indicate suspended flooring, but I guess the caveats to this are:

1. I can't see any evidence of bricked up air bricks or ventilation either on the building itself, or in any historic photos that I've been able to locate
2. The journal was concerned with the new equipment fitted to the mill, and doesn't mention the flooring on the ground floor at all (presumably they weren't really interested in the foundations or flooring - the article was mostly about the new steam and turbine machinery that had been fitted to the mill)
3. Would a suspended timber floor have been sufficient to support the significant machinery located on the ground floor and above? Would a concrete floor not have been superior in this regard?

View attachment 15584

At the bottom right of the diagram you can see 2 "little giant" water turbines which would have powered the mill alongside the steam engine. These turbines are close to where my flat is located, and the river would actually have run beneath the building at this point. I can't seem to find any information about how the river was blocked off here and what impact this could have had on the foundations.

In terms of lowering the ground levels and grading the ground at an angle away from the building, this is unfortunately complicated by the way the mill pond behind the building was filled in. I haven't posted any pictures of this but essentially the mill pond was filled up above the existing ground level, so the ground actually slopes towards the building all along the rear. From memory the side elevation has a similar issue but it's much less pronounced.

In terms of the concrete floor potentially having been installed incorrectly, is there anything that can be done to rectify this? Presumably re-instating the suspended floor would be preferable but I don't see any way in which that is even remotely feasible unfortunately.

The diagram you posted definitely makes it look like a suspended floor with brick Pylons supporting it along the length. So I’d proceed under the assumption it was originally a suspended floor which was likely pumped full of cement screed at a later date.

If the ground levels have been significantly raised over the years then all the ventilation bricks or well-windows might be underground which is why you can’t see them?

As for the brick course surrounding the plinth, that’s exactly what made me think this was some original well-window or semi-submerged ventilation or access structure which was used as an inlet for filling the suspended floor cavity with screed, which then backed up and pooled into the brick-enclosed, partially underground enclosure, and then dried in place.

One easy thing to do to gain more info is to digs small hole right next to your wall outside and see how far down you have to go before the brick courses start to “flair” out at the base as you hit the top of the brick foundation shown in your diagram. That’ll give you some sanity check on where the original ground levels were.

As for ground levels, even if the current lay-of-the-land has the ground levels sloping into the building, this can still be rectified by lowering the ground levels, cutting the ground back to slope away from the building again, and then implementing a retaining wall to hold the rest of the topography in-place unchanged (albeit now a good meter or two away from the building wall). We’ve had to do this at our house due to high, unchangeable ground levels of roads and driveways on two sides of the house. This approach with a retaining wall is similar in some ways to the zebcolm trench idea.