As a starting point:

www.which.co.uk/reviews/damp/article/dealing-with-...e

To start with you can ventilate the room where the dampness is occuring. Windows open, doors open etc to allow the air changes necessary to achieve relative humidity. Next control the amount of water vapour in the room by maybe not boiling kettles, cooking or drying clothes in there. The dampness should dry out if it is condensation. If it remains wet then the water is getting in through the brickwork and pointing. Check the pointing on the outside wall to see if it is still sound. For upstairs rooms check the roof and guttering to see if they are leaking and wetting the wall, for downstairs check for water splashing from leaking gutters, check the DPC (damp proof course) usually a black bituminous membrane about 150mm above ground level. This may be breached and allowing rising damp. Lastly if this persists then maybe the cavity (if there is one) is bridged and allowing moisture to travel from the outer wall to the inner. Pretty simple checks for a builder to make if you have a friend in the trade. One point to note, if dampness mainly occurs after rainfall then that is a big clue to the cause. If it damp all the time then you have either a rising damp issue or a water leak from pipework. Hope it helps.

Cheers Concrete

If it is a solid wall and on the upper floor chances are the water is penetrating the wall after leaking from the first courses of roof slates/tiles or guttering. Should be simple to spot, although a decent ladder and safety ties to secure it will be needed. Should also be a simple remedy too. Once the leak is found and fixed then examine the pointing to see if it sound. Even a well pointed wall with good bricks will allow water to penetrate to the inner skin if the water is coming down from the roof or gutters. You would be surprise how much water falls onto your roof even during a small shower. Good luck.

Concrete

All sensible advice so far scot. If, as we all suspect, there is water ingress from the roof or guttering, once the problem is remedied then any moisture in the surrounding timbers and brickwork will evoporate over time. Pity the damp did not show earlier. A fix in the spring would have allowed all summer for moisture to evaporate. Timber and brickwork are very hardy materials and take years to break down, so maintainance is key in keeping them in good condition. Rendering is definitely a last resort. It will only protect the wall from driving rain, not rising or falling damp. Most renders are applied by plasterers. No disrespect to plasterers but exterior rendering is a completely different animal to internal finishes, and really needs an experienced hand in preparation and application. try the obvious first, then move on if that is not the problem.

Cheers Concrete