Patio threshold damp advice please!!

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After 7 weeks of kitchen renovation (cowboy builders and no show plasterers) I was kinda overjoyed to have finally finished all plaster and about to start floor tiling.

Then a damp patch appeared at the threshold of our new patio doors where the void has been cemented Suspect this might be condensation (?) as it retreated when i opened patio door vents. Its a Victorian house however so I also suspect no DPC so now worried it could also be rising damp. Half the plaster was dead when we ripped out so maybe condensation is our culprit..who knows.

My question is could I paint some liquid DPM over areas of existing tile and cement before re-tiling and which product is easiest to use? Help
 

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Couple of weeks after patio, just one week ago?
 
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I only asked in case it had just been done. How high is the ground level outside? 6” below your inside floor level?
 
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About 2 bricks lower. Kitchen is kind of an extension beyond the original house but has no vent bricks which possibly added to previous moisture. I've dug out original timber survey which advised resealing floor to wall joints and tanking walls - although they were quoting the work so not impartial. I've already plastered without tanking but hoping some DPM might remedy any rising damp plus the threshold issue?
 
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Hmmm. I’m making an assumption it’s a solid concrete floor and not timber covered by self levelling compound. That wouldn’t need air bricks unless they are extended to serve the original timber floor.

Hope someone else puts there thoughts in as I’m not sure why you’ve got this issue - looks more than condensation.
 
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Yes it's concrete floor and had DPC injection when I moved in - mortgage condition - couple bricks above floor level so assuming (naively maybe) that at least would prevent damp creeping up newly plastered walls? Ive emailed BRE about floor damp which seem impartial but any further thoughts welcome ‍♀️
 
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If the rest of the floor was dry before the threshold was filled in, then you are better off taking that infill up, and 50mm in to the room then painting the liquid DPM in the recess and filing it back in. But this could rcreate a condensation risk unless you insualte the perimeter under the door

If you leave it and just coat the top with a liquid DPM, then you will need to treat a much wider area as the damp will travel beneath the coating further into the room - and potentially up into the walls and skirtings from the rear - which the surface coating won't prevent.
 
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This makes sense, although what if there was no dpc elsewhere under the floor? Would the problem not spread anyway? The rest of the floor isn't visibly damp though so I would hope there was dpc and your solution would bridge the gap.

I thought I'd also seal the floor to wall joints where possible. Would some of this under door perimeter help?
 
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All original Victorian floors do not tend to have a DPM, but tend to stay dry.

So if by infilling that threshold the builder has allowed moisture across into the otherwise dry floor, he has created a damp problem that would not otherwise exist. The way damp spreads, will mean that it's either a limited area or a wider area depending on the amount and pressure of the moisture. The problem is you wont know which, so it's the higher risk that you need to cater for - the risk of damp spreading to a wider area of the floor. Plus as the floor is in contact with the walls, this new damp problem will potentially spread to the walls and timber skirtings.

Thar's why surface coatings are not always the best option. Easy, but not best.

The best option would be to remove the cause of the new damp problem, and then the floor and walls are not at risk. As you wont know if the damp is coming across from the wall or up from the foundation wall and then into this new concrete patch, then you would deal with both by a liquid DPM in the recess. In addition, to remove the condensation risk at the cold bridge between wall and floor, you would but something like a 10mm strip of foam or insulation.

But having said all that, if you are using certain tiles and a specific water resistant adhesive and grout then they will happliy stay fixed to a damp floor no probelm - just like they did in Victorian times.
 
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Ah I understand so isolate the patio portion and at least this won't transfer beyond that area.

I can accept some moisture will remain but is there still value in sealing floor to wall around the perimeter to protect the new wall plaster and can I install air bricks? The previous survey suggested both but I can't see how you could achieve ventilation through concrete floor .

P.S The external walls were covered in horrendous tyroseal coating, not my choice. This seems to be covering bricks all way down to ground level outside. Could removing this a few courses up help to dissipate moisture from the concrete floor inside?
Thanks so much for the advice already!
 
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