Cannot find the source if the leak

so OP. with what i detail above about a customer’s roof that a roofer (and i think they had an engineer look at it) could not seem to fix because he did not understand that he had to look at the sides and undersides of the roof as well as the top, the facia board that i told the customer was needed: it solved the problem. no more leak. took me 5 minutes to see that from standing on the ground, i am not lying.

i did not just tell you that so you go look at your facia boards (yes, look at them). i am telling you what i just told you to advise you that if you are determined and you begin looking, you have as good a chance of finding this leak as trained “experts” who have been doing these things for years. in truth, you have a better chance than they do because you will be there when it rains. you will learn a lot in the process. trace this back once and find it and you will never have to come on here again and ask about locating leaks.

If you suspect a leak, you need to determine the most vulnerable parts such as
threaded connections
contact zones with gasket or sealant layer
welding seams
and rectilinear loaded pipe sections

you can fire more questions at me op or tell me more. or start doing this and were it goes and ask. i would get these inspection holes open before the next rain if you really thing rain and wind have something to do with it.

the wind issue sounds suspicious and the skylight is suspect. the last issue i found for a family member on their roof was around a skylight. wind can keep water from flowing blow it back against a skylight well and perhaps force it in someplace. also we have an old skylight in this house and it leaks but i know what it is doing and where it is doing it so i am not worried. if you really had an experienced roofer look at this (and i am not a roofer, so on my own house i may miss something like this while looking at the envelope of my house) i would proceed this way:

i know you have an access panel and i know this is not what a homeowner wants to hear when tracing something like this, but i do it in my own house and when working in buildings we always did it if not because we have the skills to fix them as though we are making a sandwich for lunch but because we don’t know where a leakk originates and we have to look; we cut exploratory holes (unless we KNOW with 100% cetainty what is causing the leak; a toilet wax seal or tub drain). in ceilings we often bre ak open access points not only to relieve the build up of water so it does not destroy an entire ceiling but so we can see. we do all of these without even thinking. keep in mind, in larger buildings and larger brownstones, a leak can develop 40 feet or more from where you see the result (I think you realize this).

i would look up as you have done with that leak and using a drywall saw (like a sawzall but a hand tool that looks like a steak knife – the old ones had wood handles, the new ones are folding – i think lenox makes them, great to have in an emergency – every homeowner should have one and the battlefield medics in the ukraine should carry them). cut an hole 8×10. if you are careful and remove the cutout like you want to replace it later with just drywall compound, you can set it and not have to tape it. it is called a “blowout” patch if you wish to read.

you are going to have to keep looking this way, tracing it with this in mind:

wind can push water into a bad mortar joint or other opening in the envelop and it may appear someplace else. we had a wind driven leak on buildings i managed (hands on superintendent, first job out of college) on the coast of north carolina but only the building with its front side facing directly into the wind coming off the ocean had a leak that occurred in the wind driven rain. we called roofers and other experts and had them redo the lower sloping roof (these were not flat roofs like brooklyn, but slope and ending on a porch) and we did not find that leak in my time there. it is the only leak i never found (today i would find it because i know you have to break sheetrock to locate these things; i did not do that there; if experts cannot find this, you have to do it).

where is your roof drain? at the back of the house i hope. no internal roof drains? what about the gutter around that roof drain and what about the gutter period? i do not climb on brownstone roofs but i was responsible for inspecting large commercial roofs and i maintain my roof here, myself. i learned fast that internal roof drain pipes leak as much as anything else does if not more because people forget about them. we have opened walls on 100 year old buildings and some of the roof drain pipes look like swiss cheese. also the gutter: that is the problem in this house and it was a very simple fix here: the water would flood the gutter and climb it and go between the roof membrane and brick and end up in my kitchen. a roofer who needs work would tell you that the roof needs to be replaced (they did it to a neighbor of mine). i took karnack 19 and mesh drywall tape and ran it along the area where the water runs off the roof into the gutter. problem solved.

by the way. speaking of gutters and that area by the gutter. i was working on a house before the holidays. the couple began telling me about other issues in the house, issues i wanted nothing to do with except to tell them who to call to fix them. their house had been a sort of flip or owned by someone who took ever shortcut they could and did something to the roof before they moved in and they had something done to the rood by a ROOFER after they moved in. they had a leak, a big one. they had every expert look at the dam roof and the house but this was the problem: the roofer looked at only the roof and the other people looked at what they knew best. i walk out back and look at the rear gutter or the line below the roof. there was a gap between the rood and the mortar. everyone who looked at this house before me should have spotted it. i told them who to call and what to TELL them to do to stop that leak (if i was 30 and did not mind climbing ladders i would have done it for $1800; it needed a facia board). they had someone do it while i was still working there (i will call them now and see how all that has gone since). what i am saying here is, the pros do not always spot things because they are to focused in their area of expertise – leaks cross disciplines. i am also telling you that if you bought your house this way or even if you had it redone, with the way some people work now, there are chances that even “experts” do things everyday might make mistakes that someone like myself, who has no training with roofs or masonry, can spot from the ground in 5 minutes (this really happen; if this worked, i might even have those people comment here).

another note, if you start guessing at things on your roof yourself because pros cannot find this and you do what i did with my gutter, now this: the roofers will tell you that karnack 19 and mesh will not last. what they are not telling you is that they will not last unless you paint that sort of patch with the same paint we paint the roofs with. do that and the patch becomes part of the roof and yes, it lasts. roofers do not want you to know that. if you want to look at things, go ahead. do not start putting karnack on masonry though as you have more of a chance of trapping water and it will freeze in the winter and lead to other problems.

what i am talking about here. i am not a roofer. not a plumber, not a mason (i did used to install windows, pre college, early 80’s). i have done this for years on buildings (because i worked in them and then family members will ask for help and then there is my own house) and found scores of leaks this way. starting when i was just out of college. the only leak that got me was that one i mention on the coast of NC. what i am saying, is YOU can do this yourself. what else i will tell you is, you may be better at it than any one roofer, plumber or mason because you can be in your house when it is happening and you really do not need those skills to find a leak and you will look at it with an open mind that it could be anything.

i am not marketing myself here to do this sort of thing. the people who i mention above with the leak at the back of their house told me i should become a home inspector and if anyone who reads what i write on here regularly wishes to comment to that, i would welcome feed back.

Later tonight i will read all of that and respond. I have had to trace leaks in commercial buildings which are designed, stacked, and built a little different than these houses but i have also had leaks in my own house. I can throw ideas at you, but that is all.

Thanks for your responses, stevecym and ceci . Here are the additional details that we know:
1) Leak is occasional, after periods of moderate to heavy rain or snow, but not consistently- so its rain plus some other factor (maybe wind speed or direction?)
2) Above the parlor bathroom are 2 other full bathrooms (toilet, washbasin, shower, bath x2)
3) Above those bathrooms is a flat roof, also fairly new, (< 5 years) and recently inspected under warranty and roof penetrations re-sealed as a precaution. The leak has repeated after that was completed. One of the bathroom showers also has a full length skylight/doghouse vent directly above the shower 4) there is a stackpipe recessed into the wall (common wall between us and neighbors), and all the outlet/waste plumbing from the above fixtures flow into that stackpipe. The stackpipe has a vented cap, which was added after the leak started and doesnt seem to have much effect. The stackpipe is cast iron and original, i think 5) the stackpipe runs down behind the parlor bathroom wall and the leak shows up there because metal fra ming for the bathroom sheetrock wall/ceiling touches the stackpipe at that point and gives the water a route into the sheetrock. 6) we have a small inspection hatch and its possible to see the water on the outside of the stackpipe for a short distance, but the number of different fixture connections into the stackpipe between the parlor floor ceiling and upper floor make any tracing of water/moisture flow close to impossible 7) there is no evidence of the leak in the upper floor bathrooms, either because the sheetrock framing is slightly different and its running down the outside of the pipe without obstruction, or, it coming in a different access point and finding its way along the upper floor plumbing until reaching the junction with the stackpipe and the parlor bathroom sheetrock framing, all of which is very close together

Can u post photos of where the leak is? U call a plumber. I had a leak from my upstairs bathroom to the living room ceiling below, we removed a section of drywall and pipes were not leaking from tub. It was the caulking around tub that needed recaulking. So, in my case a simple fix.

Is there a bathroom above? What is directly above and is it near a back wall? Is the heat on or off when this happens and what kind of heat do you have? Does it happen after rain and be aware the roof or wall leaks travel and might appear for days after. Parlor bathroom. Is there a riser with pipes going up between that bathroom and the kitchen? Where in the bathroom is the leak appearing? Ceiling or wall?