I don't know what kind of brick walls you have, but the middle layer of my exterior walls are like a concrete block made of brick, and they seem to be hollow in the middle. If water infiltrated to the middle of this type of block it could run for quite a distance before coming out through surface cracks. Its not unusual for a water leak to show up 10-15 feet away from the source if it finds a path to run.

My best guess would be water infiltration through a void in the brick; and water always takes strange paths. I would start out looking at flashing and trim at the top of the wall, looking several feet in each direction. I would also look at plumbing fixtures and drains that are anywhere in the vicinity.

Bruce

i'd be looking at the masonry. do you have any water running down your exterior walls? how are your downspouts and gutters?

Eventually you will have to chisel that out and replaster, but

Plaster can take several weeks to show any damage from a leak - it slowly changes from the water and then suddenly blooms out with those lumpy things.

Start with the roof, the gutters, the downspouts and any windows above (even if not directly above) the damaged area. Check for leaks, loose flashing, missing caulk ... etc.

Thank you so much for your help! In case it didn't come through VERY clearly, this isn't just my first old house, it's my first house, period. *nerves*

I didn't realize it could take so long for it to show up, or that water moves in weird ways.

We had epic downpours here in Philadelphia over the summer so perhaps it was fate.

So, I will: check around the plumbing (although it's on the other side of the house), and the inside of the windows and inside of the roof.

Below is an exterior photo of the wall with the damage.

There don't APPEAR to be leaks in the stucco. You can see there are some grey streaks where water has run down the wall. There's no gutter on this side of the house, presumably because it's the tippy top of the roof line?

How bad of a thing is that?

If it's not a window or the plumbing, there's no way for me to check the outside roof/flashing myself. :(

Would you call a roofer, or a mason first, to investigate?

Thanks again for your help!

Do you think it'd be helpful to remove the damaged plaster right away⦠would that help trace the problem at all?

I figure it'll be hard to get a plasterer to fix it so I'm going to have to DIY anyway!

i'm really wondering about the interface between the lower roof and wall in the front right of that picture. i think there is a good chance that is where your water is coming from.

i don't know enough about your roof or building system to say for sure, but if the water was coming down all the way from there, you would have signs on the floor above first.

oh, and don't worry. the building looks nice enough on the outside. this kind of stuff happens and it isn't the end of the world. just make sure you figure out the source of the water intrusion before fixing the plaster.

My immediate suspect would be the chimney or whatever is projecting from the wall ... make sure its flashing is OK and the jpoints in the brick are tiught.

Also check the joints in whatever that siding is.

From your picture, it is obvious that the roof run off is going right down that wall. Proof of this the dark streaks. There may very well be a fault in the parging (concrete) , a crack or small hole that enlarges with freeze thaw cycles allowing water in and not necessarily in your suspected area. It could be several feet above , left or right, water does funny things. First order , a gutter and down spouts , have the exterior wall thoroughly inspected for any faults and repaired