Damp proofing doorways in concrete floor

[IJWS15]

The house is 13 years old and has a concrete floor.

As part of a kitchen renew have recently lifted ceramic tiles and in one of the doorways (2 external doors) in line with the cavity/inner skin of the wall the concrete under the tiles was damp (Looked dark and colour lightened over the next day - August), the rest of the floor was fine.

My assumption ( I have not started digging concrete out and would prefer not to) is that the builders have not got the damp proof membrane in the right place leading to the damp that was unable to evaporate through the tiles.

We are laying a laminate floor so I need to deal with the damp and intend to treat both in the same way although only one showed the problem.

Will a simple paint on membrane work (e.g. http://www.wickes.co.uk/Wickes-Bitumen-Based-Damp-Proof-Membrame-Liquid-5L/p/241217) or does it need something else? Am assuming that I will need to find the edge of the existing membrane and seal to it.

Comments?

 

[dazlight]

Is there bricks in the floor there?

 

[IJWS15]

One has concrete and I can't see a DPC although there is a big crack in line with the inner face of the wall that has been filled with some mortar,

the other looks like bricks and I can see the edge of a black platic DPC/membrane which is vertical in line with the inner face of the wall between the slab and the bricks which I guess indicates that the bricks are outside the DPC. This is the one that looked damp when the tiles were lifted.

The house was built like this so can't see why they would be different unless the builders made a mistake (would not surprise me).

I taped plastic over part of each doorway last night - will see what appears.

 

[ree]

Why not post internal and external pics?

 

[IJWS15]

Finally managed to work out the new system. First is doorway into the conservatory and looks OK. Was an external door before we fitted the consevatory. It is a UPVC door/Frame.

Second is the side door - straight outside. Nothing under the polythene after a couple of days and it looks as if the issue is damp from under the door. The only area that looks damp is the strip of mortar the door frame is stood on. It is a timber frame and steel sheet on timber door.