External Concrete Plinth - Should I Remove?

[JohnD]

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.

You say the paving has "natural drainage only" but does rainwater run towards the house? Or puddle there? If the ground beside the wall is wet, it may be useful to add a french drain.

Can we see this drain hole?

 

[JohnD]

ah, I can make out the drain cover now, though it's dark. It is just next to the bay window.

I'll wager a pound that there is a glazed clay gully there, and that it is broken at the bend, and that water is leaking from it.

It has probably been broken and leaking since 1940, if you are in a town or near industry, or docks, or an airfield.

 

[HandSlander]

Image dump

Photo A - the full end terrace, new extension & guttering on right, original Victorian finishing at bay window on left

Photo B - the property's main external drainage area. Main gutter down pipes on right, Cast Iron SVP in middle, and Plastic down pipe on left

Photo C - the downpipe outlet of the main roof and bay window guttering on cement area with plinth visible

Photo D - manhole. Top of the image is towards the building, bottom is away.

 

[HandSlander]

the hole under the plinth does look very wet. Is it anywhere near the drain you mentioned?

Yes and the soil was very moist... it is located some distance from the drain in the middle of Photo A above, just to the left of the new PVC door of the new extension under the white cabin hook.

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.

No worms. There is not much use in it drying out as I'd imagine it would be similarly damp all the way along that bay and this was only a very small sample section. Normally water doesn't pool next to the building in heavy rain but a few metres away in the garden. Does it look like a slate DPC to you between the two bricks below the plinth? I'm not sure what it would normally look like?

You say the paving has "natural drainage only" but does rainwater run towards the house? Or puddle there? If the ground beside the wall is wet, it may be useful to add a french drain.

Water doesn't typically pool against the house. I'm sure a French Drain would help and I've been thinking about installing one, however I wonder whether I should look to eliminate other sources of moisture such as a drainage/mains leak first (as you suggest RE clay pipe).

Can we see this drain hole?

Photos C/D above

 

[JohnD]

Aha!

A hundred-year old drain, where the concrete around it has recently been repaired or renewed! Possibly twice!

Pull off that iron grille, and look at the gully beneath.

I'll increase my wager to £5.

 

[JohnD]

In your "hole" pic, I can't see a DPC, but the bricks are very muddy. Hose them off. you're looking for a mortar joint that is thicker than the others, with two (usually) horizontal black lines which should be the edge of the slates.

In your hole I think I can see bricks of non-matching colours. If so, the builder will have used them below ground level where he didn't think anyone would see. If it was built with lime mortar, you may find it has been washed away, and replaced by mud, in places where there has been long-term leakage.

 

[HandSlander]

Pull off that iron grille, and look at the gully beneath.

I'll increase my wager to £5.

Looks intact, standing water about 50cm below ground level.

 

[JohnD]

if you put your hand in, or bale it out, can you find a crack?

How far below ground level is the clay square? The iron grill should be on top of it.

I think the clay gulley has sunk into the ground, where the soil or clay has been washed away by the old leak.

 

[JohnD]

here's one we did earlier. Look familiar?

https://www.diynot.com/diy/threads/clay-gully-and-hopper-repair.438674/

and another
https://www.diynot.com/diy/threads/old-house-rainwater-drain-in-pieces.511774/#post-4248480

Shall we say £10?

JohnD2 says he once found one that wasn't cracked and leaking. I never have.

And another

https://www.diynot.com/diy/threads/still-leaking-help-please.505478/page-2

https://www.diynot.com/diy/threads/...-issue-repair-or-replace.506292/#post-4187394

 

[HandSlander]

Well I don't doubt your bet is looking good and intend to see this thing through to fruition... I'll have a read of all of those links later.

Fortunately I have a Wet & Dry Vac so was able to remove all standing water from both drains - Photo E below shows the one on the far right of Photo A for the new extension and Photo F is the main one with the iron grill.

Photo E - New Extension drain - new plastic hopper not blocked but definitely some mud & sludge in here. The top is towards the extension.

Photo F - Main gutter downpipe drain - no visible cracks, the top is towards the house

Photo G - cleaner picture of sub-ground brickwork below plinth.

 

[JohnD]

I think your house is built in London Yellow Stocks, so the red brick must be below ground level. I should think the DPC will be inside that plinth. There is a section of wall where the plinth is half-height, is any DPC visible there?

If you can cure the excess water problem, there is a good chance that your airbricks will ventilate the subfloor void enough, provided there are airbricks on two or more sides to allow airflow. It is always interesting to take up some boards and have a look. You might find an area of damp.

 

[HandSlander]

I think your house is built in London Yellow Stocks, so the red brick must be below ground level. I should think the DPC will be inside that plinth. There is a section of wall where the plinth is half-height, is any DPC visible there?

Not sure where you mean, if you're referring to around the air bricks/grates then the cement always is sealed to the air brick/grate so no chance to see the slate DPC. Would my SDS Hammer Drill work to take off a section?

If you can cure the excess water problem, there is a good chance that your airbricks will ventilate the subfloor void enough, provided there are airbricks on two or more sides to allow airflow. It is always interesting to take up some boards and have a look. You might find an area of damp.

I've got plenty of air bricks/grates on all 3 sides of the end of terrace... taking up boards is not really an option as my floor boards are very tightly sanded and varnished. However when my builder installed the new kitchen 1yr ago which included sub-floor plumbing a boiler relocation I asked him to inspect the sub-floor and he said he didn't find any obvious damp. However no boards were lifted in the bay window bedroom...

So what's your hypothesis @JohnD?

• Leaky water main feed?
• Leaking sub-ground drainage pipes?
• Poor surface drainage?
• Cement plinth covering DPC?
• Insufficient ventilation?

All of the above?

 

[JohnD]

• 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. You can ask the water co to test your drinking water for lead content, they may have a lead replacement subsidy or a leak repair subsidy. Laying a new, larger, plastic water pipe is not complicated and you will be amazed at the improvement in flow and apparent pressure.

The cement plinth is usually very hard, and bricks will be damaged if you try to take it off.

 

[dontbelieveawordofit]

as i see it your new build dpc is at paving level outside . assuming they’ve followed the floor heights through internally and followed the height of the dpc , then none of your suspended floors are ventilated. those installed air bricks/vents are venting rooms not under floors. can you confirm?

 

[HandSlander]

as i see it your new build dpc is at paving level outside.

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

assuming they’ve followed the floor heights through internally and followed the height of the dpc , then none of your suspended floors are ventilated.

New build extension is slab base and floor is about 150mm above ground level as pictured, ground level bricks (that sit on DPC) are typically very dry just wet in pic due to hose usage for hole pictured on left.

those installed air bricks/vents are venting rooms not under floors. can you confirm?

The original building's suspended timber floors are only very slightly lower than the slab level and are at least 100mm above external ground level. When shining a torch in the air bricks you cannot see straight into the sub-floor (it is solid) but looks like it may be similar to this;