What's happening in my friend's wall?

[ShoulderThingThatGoesUp]

A friend asked me to come by this weekend to look at a moldy wall that suddenly appeared in his house. I'm wondering what I should look for.

Details I know:
-Interior wall between a narrow second-story "bedroom" and a full bathroom
-Only moldy on the bedroom side
-Bedroom is too narrow to really use as a bedroom, he has a desk and storage in there
-No plumbing in the wall
-Plaster walls on wood lathe
-Built in 1900
-Slate roof
-Asbestos siding
-Gutter near this room is bent
-Homeowner keeps this room closed and runs a window unit in another bedroom on that floor
-There is an attic which he says is dry

He thinks it's his roof or the gutter, but I'm not sure why those wouldn't be beating up his exterior wall.

[CowboyAndIndian]

A friend asked me to come by this weekend to look at a moldy wall that suddenly appeared in his house. I'm wondering what I should look for.

Details I know:
-Interior wall between a narrow second-story "bedroom" and a full bathroom
-Only moldy on the bedroom side
-Bedroom is too narrow to really use as a bedroom, he has a desk and storage in there
-No plumbing in the wall
-Plaster walls on wood lathe
-Built in 1900
-Slate roof
-Asbestos siding
-Gutter near this room is bent
-Homeowner keeps this room closed and runs a window unit in another bedroom on that floor
-There is an attic which he says is dry

He thinks it's his roof or the gutter, but I'm not sure why those wouldn't be beating up his exterior wall.

How sure is he that there is no plumbing in the wall. This is my first suspect.

Second, does he have access to the top of this wall (maybe attic above 2nd floor)? Does he see any leak from roof in the attic?

Does attic have an A/C whose condensate is overflowing into the wall?

[PeachFuzzStacher]

Look for air leaks which could bring in moist air that could get condensation.

[zinethstache]

If you can get on the roof with the owner in the attic above that wall you can carefully look at the roof in that area for any issues. Often you will see watermarks (likely so given the age of the house) If anything looks interesting you can try to patch. (we are DIYers and have patched many a roof in that manner. Alas, lathe and plaster is a PITA but that is the next task. its a small room, so do you re-plaster (or hire that out) or sheetrock over perhaps?. It feels like a leak that is directed straight down that wall. Stranger things have happened.  Good luck!

[ShoulderThingThatGoesUp]

No way in hell I'm going on a 100-year-old slate roof. I think his attic should be above that wall, and he says it's dry. We'll have to measure to see, I guess, because I agree that a roof leak makes the most sense. The attic is plastered too, so there may be some effective soffit that is above that wall that isn't visible.

The plumbing thing I'm pretty sure on, but could be wrong.

He claims you can't breathe in the room, so I will (1) stop at HD and bring some masks and (2) expect that sheetrocking the room will be necessary.

No AC in the attic, the only AC is window units.

[srob]

they could have water leaking into the wall from a shower if shower is on the other side of that wall. I have had water leak from a bathroom vent pipe through a ceiling before (the fan was not working at the time) so that may be an option. Good luck!

[ShoulderThingThatGoesUp]

Shower is on the other side of that wall. It's an old tub with a curtain all the way around, so I would think the bathroom side would be getting the mold first. I'll make sure to tell the forum if I figure it out.

[waffle]

He claims you can't breathe in the room, so I will (1) stop at HD and bring some masks and (2) expect that sheetrocking the room will be necessary.

If the room is so bad that he cant breath in it don't just get masks. You need an actual respirator with a filter. If you are not properly equipped to safely deal with that level of mold then don't attempt it. If there is mold showing through on the room side of the wall imagine what it looks like behind the drywall.

He is either wrong about there being no pipes in the wall or there was a pretty good leak in the attic and it just hasn't rained lately so the attic has dried out after the damage to the wall was already done. I'm betting on some pipes in the wall though. If it developed slow enough that the drywall isn't/wasn't ever soaked through visibly then its got to be a slow and steady leak. If the attic is dry then its got to be coming from inside the wall. The only other thing I can think of is a bathroom vent fan with faulty ducting that is at least partially venting into the wall.

[worms]

I appreciate that the situation is in a different climate from anything I am used to, but two things jump out to me: room is kept closed, next room through the mouldy wall is storage. 

Without airflow walls (particularly lath & plaster) can be just damp enough to support mould growth - just look behind the furniture in any old house that is otherwise fine! 

Also if the storage room is unheated, that wall could be colder than the rest of the room allowing just enough condensation to support the mould growth. 

Before doing anything more dramatic/expensive, I'd open the doors and windows and give it a couple of weeks to air properly!

[Greg]

You need to water test (hose on the roof) to see if you can spot any roof leaks.  Slate roofs can have small imperfections like misaligned tiles, cracks, incorrect flashing around pipes etc. that can cause leaks and mold just like any other roof.

There could be a vent pipe that goes through the roof above the moldy wall, or the mold could be from moisture that migrates from another area uphill on the roof.  Water moves downhill but can also move sideways through wicking and capillary action.

Use some vinegar or bleach solution to kill any surface mold, he may need to wipe down the desk and other things as well.  Mold can travel once established, so I wouldn't just "air it out" without attacking any surface mold. 

A moisture tester could help you figure out where moisture is coming from if it is coming from outside.

[ShoulderThingThatGoesUp]

It was the exterior wall, we'd miscommunicated. The answer was that after we took down the moldy parts of the ceiling, we could see fallen slates and a bit of gutter. So we decided that it is a job for a professional.