I think this is salt on the brickwork. The bricks have changed colour in the two areas above where the salt is. Does this indicate a significant problem with water ingress or something? I have read about this happening to new walls, but this is 120 years old and I only noticed it a few months ago (thought it might have been happening for longer as I only bought the house two years ago):
It does look damp, and the salts would indicate water migrating through the brickwork and drying at the surface.
Is it damp on the inside? Is there any plumbing on the inside against that wall which might be leaking?
When it's raining, go outside and look carefully at the roof and guttering, and make sure the water is running away they way it's meant to, and not runnung down the front wall.
That type of roof has no soffit, so the wall gets less protection than it otherwise might. Is the wall bowing outwards at the point, which would make it more vulnerable to water penetration?
That pic does seem to show a large damp patch centred on the mullion between the door and window...
Have you got central heating with a header tank?
Yes, and there's a rad under that window. However I haven't taken the floor boards up to check because I had assumed any leak would become evident on the downstairs ceiling before the external brickwork - am I wrong and is it worth having a look? What about the area lower down around the drainpipe?
Inside looks fine
There isn't really a bulge there or anything. However just at the top of the damp patch between door and window are where all the estate agents have nailed their boards, leaving quite a few holes and nails in the mortar. I don't know if these might have let some rain water in that has then leached out further down?
I would say its certainly worth looking - a slow leak in a pipe that is pressed against the wall could slowly soak it.
Watch the pipe in the rain, it should be easy enough to see if water is running down the outside of it in any quantity.
Seems unlikely unless they installed a funnel at the same time!
Efflorescence means water's migrating through the brickwork. Your challenge is to find the source of the water. It does look like a damp area, so I'd take the time to look in those awkward places if observation during rain doesnt yield anything.
NT
I looked under the radiator inside and luckily it's totally dry (after I'd figured out how to get under the 15-foot long wall to wall Vicotrian floor boards).
I guess it must be drips from the guttering/roof landing on that bit of wall? I will have to wait until we get some decent rain and then have another look.
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