Damp ‘external’ wall in outhouse

[RJRock]

Some advice please.

I have a damp/ wet wall which is what would be the outside wall of a cavity wall 60’s house. The damp / wet bit is and always has been covered / inside an ‘outhouse.’ The problem has been gradually getting worse over the last few years. There is no issue inside the house/ the inside wall of the cavity. There is no sign of dampness on any ceiling or loft space wall either. The roof is a flat roof above the problem area. The roof is in ok condition and I removed two sections of plaster below the flat roof to check the wood below and join of the timber to the wall and it’s dry.


I have also removed some of the plaster, as it’s shot, to try and understand what is going on better.


As soon as it rains there is very soon a problem with moisture visible running down the concrete render not a lot but enough to be obvious .


I’m thinking the outside wall may just need a new coat of paint as it south westerly facing and gets a beating every winter. I’m thinking this, due to the gradual worsening of the problem from nothing to the present over many years.


Is there anything obvious I’m missing?

And advice welcome. Pictures included

 

[Notch7]

A cavity tray would sort it out.

Water is getting into the cavity from outside and running down the wall.

A cavity tray closes off the cavity above the flat roof and weep holes allow the water to escape.


Given that it's a rendered wall, in theory it should be waterproof....so you could have a look at possible entry points....my money would be on the upstairs window where the external cills meet the reveals.