External Concrete Plinth - Should I Remove?

[JohnD]

Correct DPC is at ground level between brand new engineered bricks and the London yellow stocks.

Well spotted!

Do you think the builders set their new DPC to match the old one they found during building work?

The DPC should be two bricks above ground level

So it looks like you need either to trench round the wall (you can fill the trench with cobbles or large pebbles), or to lower the paving height.

If that works, it will prevent watersplash during heavy rain from wetting the wall above the DPC.

A modern concrete floor should be on a DPM joined to the DPC in the wall, so it should be dry. If you have an old concrete floor, it will draw damp from the ground, and pass it to any wall it touches, so reducing wet ground by fixing your leaks will help a lot.

 

[HandSlander]

Do you think the builders set their new DPC to match the old one they found during building work?

The DPC should be two bricks above ground level

I had wondered this, the new build extension as you see it is actually a full replacement (all walls demolished) and extension of a smaller 20th century extension that was there previously (which was on its own, smaller concrete floor, so probably late 20th century). The builders are imo fairly reputable guys, FMB-accredited (if that means anything), and are great at coming back and fixing any snagging issues, even almost a year after the build so I wouldn't have thought that the low DPC would be a bodge, but never asked them.

So it looks like you need either to trench round the wall (you can fill the trench with cobbles or large pebbles), or to lower the paving height.

If that works, it will prevent watersplash during heavy rain from wetting the wall above the DPC.

Yep already planned to trench as part of landscaping, which I intend to do after I fix the potential leaking gully/drain/pipe issue given it may require digging anyway.

A modern concrete floor should be on a DPM joined to the DPC in the wall, so it should be dry. If you have an old concrete floor, it will draw damp from the ground, and pass it to any wall it touches, so reducing wet ground by fixing your leaks will help a lot.

No damp issues in the new extension.

Here a couple of photos of the build for reference

Part demolish and excavation

Old slab and service pipes

Foundations Trench

Brick Laying & DPC visible

Where the extension meets original Victorian

 

[catlad]

I can't see any of your images unfortunately but gable ends were regularly slated to prevent damp and it is breathable.

 

[JohnD]

Foundations Trench

I can see something under the yellow bricks, but surely not a DPC? Too low.

Can you see anything like a black line half-way up the plinth?

 

[HandSlander]

Well that was the old double-brick extension, that wall was subsequently demolished, which was not part of the original Victorian so not sure it would be the same level or type of DPC as the main building.

 

[Harry Bloomfield]

the hole under the plinth does look very wet. Is it anywhere near the drain you mentioned? Did you see any red worms? If you leave it open for a few days, it might dry out. If it fills with water after rain (or a bath) that will give clues. Is there any sign of a DPC? I would have expected slate.

Curious - What is the relevance of the red worms John?

 

[JohnD]

Curious - What is the relevance of the red worms John?

They like to eat the detritus from leaking soil pipes

 

[Harry Bloomfield]

They like to eat the detritus from leaking soil pipes

Thanks John.

 

[HandSlander]

Well the plot thickens, or rather the water deepens.

After having removed all Standing water from the main rainwater gulley drain yesterday afternoon, this morning the drain had about 12cm of water in it. I don’t think it rained overnight here, and my other rainwater gulley drain didn’t have much water in it, although Admittedly that’s a smaller roof section. I’ve wet vac’d it empty again and will see if it fills - this is how much came out overnight

 

[JohnD]

water may be gettin IN through the crack today.

 

[HandSlander]

OK @JohnD, my theory right now is looking like there is a leak or leaks somewhere that have been going on for over a decade. This caused damp issue pre-2010 whereby in 2010 the previous owner got Kenwood In to do a bunch of, presumably expensive, damp proofing works including the cement plinth, cement pad around the main rainwater gulley and manhole, air bricks, tanking the works. This then stopped the damp entering the building but did nothing to address the source of the moisture which continues to this day, manifesting in damp earth along the boundary and very high persistent humidity levels in the building.

I have some drain rods en route and I also have a 5m endoscope already in my possession. Next step is DIY drain inspection for cracks or leaks.

 

[JohnD]

quite likely

except I think the cement plinth is much older

 

[dontbelieveawordofit]

your next step should be trying to find out where the water is coming from.
you wont necessarily have a crack or break in the drain.
a large leak from a feed /main will find it’s way into the drain regardless.

 

[bobasd]

OP,
your original complaint was roughly about humidity its causes and consequences.
You mentioned previous work by a D&T company, and the possible humidity consequences of the sand & cement plinth.


To first get the plinth issue out of the way: the plinth is not a cause of your humidity. Cut the plinth off by 40mm to 50mm from ground contact
and then leave it alone.

The D&T claim of installing a "full length vertical DPC" means what?
You have solid walls - so was a horizontal chemical DPC line installed anywhere on that elevation?
What kind of tanking was installed?
Are the installed air bricks telescopic?
Site inspection might be needed to determine if your re-pointing is adequate? Its, perhaps, sand & cement over an original sand & lime beds and perps? It should be raked out to a min of 20mm.

Condensation creates mould and mould is bad for health esp a child's health. Humidity can damage interior fabric.
You should have a permanent air brick high level vent in the bed room.
Trickle heat & trickle venting often help with condensation.
Timber frames provide better leakage ventilation than PVC windows.

AAMOI: wood frames & chimney stacks (period details) often add value to a property.

A site call: could be its time to remove the tanking back to brick, and then render with a sand & lime render?
Any joist tails landing on that wall are at risk of fungal damage, & need inspecting - maybe use your Endoscope?

 

[dontbelieveawordofit]

one timber sash window in your property while everything else is pvc won’t add value to your house. in fact it will actively devalue it. particularly as it’s a bay and particularly expensive.
lime render is highly porous in comparison to cement render . given the humidity issue, beware -removing the tanking back to brick and replastering with porous lime render which will give you a soaking wet wall in no time. no doubt back to square one.