Damp coming through floor where extension meets

[Staxi]

Hi,
We have an Edwardian semi. The downstairs bathroom leading off the kitchen is an extension, possibly built around 25 years ago.
We had someone knock down the wall between the two (the old external wall) and put in a steel. My concern is that from inspection the DPC on the extension seems only to be in the brickwork and not underneath the whole floor, and therefore might not meet up with any DPC in the original kitchen.
There seems to be a really small amount of damp on the floor where the original wall used to be. Can someone suggest what I should do about it?
Thank you in advance.

 

[pinenot]

If you can, dig a small hole/trench (which can be covered with weight bearing plate or something) about two & a half feet deep and where you suspect damp most, monitor this when the surrounding ground is wet (after heavy rain) you will see if the ground under the floor is actually affected by the water table in your area. Armed with this information you can make/ask an informed decision as to the best approach to this. Could save you a lot of unnecessary work...pinenot

 

[Staxi]

Thanks for the advice. I'll make the small hole today in readiness for when it next rains.
Out of curiosity, if it does get wet when it rains does this mean a huge problem?
The bricks that the original part of the house are made with are those soft bricks (I believe London Reds) which absorb water easily, therefore I'm concerned that if I end up having to damp proof the floor (I was thinking something like a bitumen paint) that it would simply push any damp into the walls, which in turn would start to decay.
Thanks again

 

[pinenot]

Damp/water filling the hole will indicate the level of the watertable, you could actually do this outside and find the level in the same way. Once you establish this level it will indicate how much pressure of moisture your floor is likely to come under, no significant water no need to worry beyond a liquid dpm or the suchlike. Significant water would indicate something more drastic.
What is the construction of the floor/floors, both concrete by the sound of your description. One fully covered with a dpm draped onto to the dpc or suchlike and the other suspect?...pinenot

 

[Staxi]

The original part of the house (so the kitchen floor) has a slate type DPC. We could see this from the outside of the house.
With regard to the extension, there was a slow leak from the boiler before we moved in which corroded some of the concrete floor next to the wall so I was able to dig some of this up to check the DPC. I can actually feel with my fingers the edge of the membrane, i.e. that it just runs in the brickwork and stops either side.
This indicates to me (happy to be corrected) that there isn't any real effective DPC in the extension and therefore possibly why I can detect a small amount of damp where the old wall used to be.
To investigate further we took up a tile at the other end of the kitchen and dug out the concrete - so a few inches down (we didn't go as far as the DPC in case we damaged it). Everything was bone dry.
My thoughts on this now are that we will need to put down a liquid DPC in the extension but dig a little 'trench' where the new build meets the old, down to the original house DPC and damp proof that trench to make sure the two DPC's match up (if that makes any sense!). Would you agree?
Of course we will do the hole regarding the water table first though, just to double check.

 

[pinenot]

If the water table is below the two & half feet i.e. the test hole proves dry, dressing the liquid dpm onto the brickwork behind the skirtings would be sufficient I would think, however that's up to you. personally I would dry the damp (dehumidify) repair the concrete and be done with it unless the damp recurred, in that case I would put the liquid dpm down as I described...pinenot

 

[Staxi]

Thanks for all your help. We'll wait until it rains and take it from there.
Thanks again.