penetrating damp problem/possible salt in plaster.

[atalossjoss]

Please Please can anyone help. I have a rental end terrace property. There is a arched alley between next door (bedrooms above), giving access to the back. The property was built around 1900, double brick with very small cavity contruction. DPC had been carried out some time ago. We had noticed as we were doing the property up that there appeared to be two areas of damp. Both starting about 5 foot high on facing walls. One patch (few patches together covering about 4ft wide x 2ft high) backs onto the arched area (in effect internal), with no water pipes on this side of the house. The other patch opposite (about 18inches along most of the wall) is the external wall, which had been badly rendered. We took as much render off as possible and partly repointed where needed, then put 3 coates of breathable sealer on (said it was used for this). We had a couple of roofers (who gave quotes for repointing chimneys etc - which have been done now) to look at it at the time, they had no idea where the moisture could be coming in under the arch. Damp contractor said it may be salt coming through. Another just scratched his head! Damp contractor gave us acid to put on, but this did not work. Tenant has phoned to say its all showing through now it has rained heavily again. Can water penetrate when there is a small cavity gap? and can anyone suggest what to do next. Will the plaster all have to be removed and re-done internally, if so using what medium? and will the walls have to be treated 1st with something under the plaster. Strangely on another wall in the same room there was a patch which looked as if it was a damp single brick (as if one brick were wet), again with absolutely no pipes etc within 10 foot of it! So much conflicting advise. Please can anyone help. Thanks