Tide marks

[nickkellymetal]

Hi all, posted this before, but have a question if anyone could shed some light, tide marks after rain, internal party wall, no chimney on this side, plaster doesn't go below skirting.

I've noticed that if I lightly sand the plaster during a "wet" episode, the plaster seems to be completely dry underneath? Does this suggest it's surface moisture?, salts?

 

[Cubist]

Several possibilities occur and it would be wise to rule some out if at all possible. The blackened areas in your photos just above the skirting imply that this problem has been going on for some little while. Also, you mention nothing about the floor of this room and that too may have a bearing on the source and nature of the damp you're seeing. Before going too much further though it would, I think, be wise to rule out the simplest explanation of moisture condensing out onto the wall. This could be caused by cold air rising from behind the skirting and the ground below being met by warm air generated by the heating system. Theres a very simple test that can be used to rule this In or Out of the consideration that consists of taping a sheet of kitchen foil or clingfilm directly over the surface where damp appears and leave it for about 24 hours. If the side of the foil/film facing into the room becomes wet then the problem is atmospheric condensation. If, on the other hand this outer face is dry and the inner is wet that confirms penetrating damp from some other source.

 

[nickkellymetal]

Thank you very much for the reply. The floors are suspended timber, with a very deep void, about 4ft. Guess what I really need to do is get under the floor. Thanks for the advice.