Damp on internal wall at top of stairs?

There could be a cracked tile or slate allowing water to enter the roof. Another alternative is water breaching the felt or membrane under the membrane.

See this article for more details:

http://eugbug.hubpages.com/hub/What-Causes-Damp-in-Houses

Eugbug
December 2012

Brian is on the ball! look for daylight which will be cracked or shifted slate or tile, however low down roof or felted you wont see daylight.

You may have to get out the ladder and look over damp patch site see if any slate/tiles broken or split and most check the last row which sits on half tile/slate this closes the gap between tiles/slates at roof edge. Check gutter and pointing. And if the downpipe is cast iron check it's not rotten at the back.In some cases I've seen rain rolling down under the rafter droping off lower down so also look at rafters inline with the damp patch see if there are any wet stains on it.

BB
December 2010

go into loft in the daytime and see if you can see daylight in the general area of the damp patch if you can the roof slates need attention

brian
December 2010