1930s Concrete Slab / Internal Wall Damp Help

[j_s]

Hello and first off thanks to everyone contributing to this resource. A real font of knowledge.

Having gleaned as much as I can I've got a more specific issue I wonder if anyone can shed some light on.

A specific damp patch that appears only where an internal wall once stood - runs perpendicular to the rear external wall and has been flattened off with cement. The floor is original 30s concrete slab, unsure of DPM presence or condition and there is at least one course of bricks still set in from the remains of the wall. The wall removed used to be the bathroom so it's likely the floor has had a bit of a soaking over the years. Moisture meter readings are not great throughout the slab but I'm taking that with a pinch of salt(s) - when covered moisture accumulates only in this one particular area (dark patches not marked in red in the 1st pic are residual adhesive).

Any clues whats going on here? No signs of water ingress outside the property. Ground levels are higher than I'd like but below the DPC and internal ground level. Moisture photos below were covered for 24hrs and had mostly evaporated after 3-4 hours. We've left in uncovered and had an industrial dehumidifier running approx 12hrs a day to try to remove any residual moisture but it seems to be reappearing at a similar rate after a month so I'm wondering what else it could be.

Is it possible the wall runs below the slab and is drawing up moisture from the ground? Or is it just a very slow escape route for any water in the slab?

Many thanks for any insight.

 

[Cubist]

Looks like someone just spalled a coat of mortar/something else over the realms of the wall below floor level. If that's right then it could be damp rising through the brickwork below or tracking in along the top of the walls remains from the external wall. Have you checked outside to see if water is puddling there?

 

[j_s]

Cheers Cubist yeah that's right. The bricks are visible below the mortar - wasn't sure if the brickwork continues below floor level or sits on top.

And no theres no sign of water collecting outside. Walls are cavity if that makes any odds.

If its possible damp is travelling directly up would removing the bricks and filling in with something less permeable be advisable?

Looks like someone just spalled a coat of mortar/something else over the realms of the wall below floor level. If that's right then it could be damp rising through the brickwork below or tracking in along the top of the walls remains from the external wall. Have you checked outside to see if water is puddling there?

l

 

[Cubist]

spalled

Spalled... SPALLED! I was typing SLAPPED!

Please, please, can one of the human intelligences get rid of this electronic 'Dumb AI' driving the autocorrect function before I do something rash like check myself in for a dementia check!

 

[Cubist]

Given where the bricks are and as they are also perhaps in direct contact with the wooden floor theres a more than reasonable chance of damage to the boards and possibly the nearer joists. I would be tempted to excavate and remove the top course from under that patch and then cover the remaining part of the wall section with a DPC type material before closing the cavity with whatever may be appropriate.

 

[j_s]

haha ive got some problems with spalling on the front wall as it goes but thats a whole other story

as you can see by my shoddy capitalisation i have no faith in computers spelling let alone intelligence. i know what you meant anyway!

 

[j_s]

Given where the bricks are and as they are also perhaps in direct contact with the wooden floor theres a more than reasonable chance of damage to the boards and possibly the nearer joists. I would be tempted to excavate and remove the top course from under that patch and then cover the remaining part of the wall section with a DPC type material before closing the cavity with whatever may be appropriate.

thanks yes that seems sensible to me. theres no wooden floor in that room (only ground floor boards are in the living room next door are set directly into the concrete!). but i'd still prefer a lighter touch where possible.

 

[Cubist]

only ground floor boards are in the living room next door are set directly into the concrete!

Whhaaaat? You're kidding I hope.

 

[j_s]

Whhaaaat? You're kidding I hope.

wish i was. boards nailed in before the concrete set. the whole street seems to be like it so i assume its original but it's very strange all the same.

maybe i should bite the bullet and get a proper suspended floor put in!

 

[Cubist]

The black stains on the board ends looks fairly minor. Could be theres some kind of damp protection installed and if that's the case you may not have to bite the new floor bullet. Any evidence near the board ends?

 

[j_s]

No sign of damp or rot on any of the boards so perhaps there was a method to the madness with them.

Really appreciate your input on all this btw it's been very helpful.