This condition is called ammonium sulphate contamination . It has nothing to do with rain or roofing . It is hygroscopic salts in the wall where the chimney was . It is very easy to fix but messy ! The damp area or the whole area where the chimney was should be stripped back to the brickwork + 500mm to each side . The stripped wall area is to be waterproof rendered with Sika-1 and Leighton buzzard sand (Travis Perkins) . This will cure the problem , it's common as is the lack of knowlageble about this condition .
If you have to repair the roof was a wet section of the wall (not the side wall of the house , if I understand correctly ), then it will be renovated for a while - a few months now because the wet season. You have repaired the roof, stop flow of water flows , as previously water from your roof throughout the inner portion of your wall , and took a sponge soaked your entire wall with moisture. So on top of moisture is not, on the part of your bedroom redecorating done , everything is beautiful, and the moisture inside the walls remained , and the new spot of dampness may appear in any other place. You can do minor repairs but it will not solve the problem, I think a waste of your money for repairs. Of course , there are some solutions to this problem , but the process is time-consuming and expensive. All can be repaired , ready to pay for it ...