A leak source


I've got a small upstairs bathroom (tub w/a shower). The bathroom has a bedroom on either side with no access panels to the plumbing. The third wall is to the exterior. It's a newish house (three years old).
Directly below the bathroom, on the first floor, is a dining room. Evidence of a leak is showing on the ceiling, more or less directly below the upstairs bathtub. It strikes me that there are three most likely sources for the leak. (Note: the bathroom serves as a very infrequently-used guest bath. Probably not more than ten or twelve showers have been taken there in the three years it's existed.)
Possible source 1: plumbing, either the drain or the supply.
Possible source 2: leaks in the tile wall or joint between tile wall and tub, or (less likely) joint between tub outside bottom and floor tiles.
Possible source 3: the roof joint where the exterior wall of the second floor joins the top of the extended downstairs roof (the upstairs portion of the house sits atop only half of the downstairs house, and the exterior upstairs wall joins the lower roof almost above the leak spot on the dining room ceiling.)
With no ready access to the underside of the tub without pulling out the drywall, how can I determine which of these is the actual source of the leak? I'm hesitant to just spray water all over to see which possible source ends up on the downstairs ceiling, partly because I'm hesitant to further wet down that ceiling and partly because I don't know how I can be sure that water from testing one area isn't just taking a delayed trip, fooling me into thinking it's water from testing the next area. Am I overthinking this? How do plumbers do it -- just wet things down and see what leaks?
The matter would be a little simpler to solve if it weren't for the fact that the rare use of the tub for showers is pretty well matched by the rare rains. I've never seen the leak when it was actually wet; I only noticed it recently when it hadn't rained in months and no one had showered there in months.
Thanks for tolerating (most of you) my simplistic questions over the years.
Jim Beaver
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Jim Beaver wrote:

A tradesperson won't have the time or patience to do "testing", unless you were willing to pay for it.
Most direct would be to make a small incision in the drywall opposite the front of the tub (bedroom?). Or into the affected ceiling. Might not need to be very big.
Besides the obvious tub leakage places, don't overlook things like the tub overflow plate. Often, the gasket on the back side of the tub doesn't seal well at all and water running down during a shower can produce quite a flood inside the wall.
Less likely, but still possible, is a leak at the tub drain (shoe) gasket underneath.
Can you get into the attic to look for any sign of roof leakage up there?
Jim
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Jim's advice is good. I had a severe leak around the shower valve plate. I would locate the studs in the wall behind the plumbing end of the tub and cut an access door into the wall. If you work carefully, you can reinstall the drywall and make an invisible patch or make a good looking permanent access door. Some local plumbing codes require such access doors. You can look under the tub and up into the wall to detect any leaks in that area.
Don Young
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A tradesperson won't have the time or patience to do

Thanks. There's no attic to look into, unless you mean the second floor attic, to look for a leak there that runs down inside the wall to underneath the tub. Is that what you meant?
Jim Beaver
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One of the last thing Jim wrote was: - I've never seen the leak when it was actually wet; I only noticed it recently when it - hadn't rained in months and no one had showered there in months.
Do I understand this to mean the ceiling is showing wet spot even though no one has used the shower for months *and* it hasn't rained in months either?
Doesn't that just about eliminate the roof joint, the shower walls and the drain?
Doesn't it have to be the incoming plumbing?
Jim Beaver wrote:

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I can see how you got here from what I wrote, but what I meant was that there are DRY signs of leakage on the ceiling. Clearly something leaked there sometime, but it's been long enough that the leak (clearly a small one, from the bubble in the ceiling drywall) has completely dried up. Which actually suggests to me that it's NOT the incoming plumbing, or the spot would still be wet.
I'll probably eventually cut a hole in the wall by the plumbing end of the tub and look, but I guess what I'll do first is try to control the water and do some tests. Perhaps first filling the tub from another water source and seeing if a leak shows up when that water drains. Then pouring water down the overflow valve. Then, once I've eliminated those, I can try pouring water on the tile-wall to tub-top joint. Then I could run a hose up and water down the roof/wall joint above the dining room. If none of that turns up a leak, then it's pretty clear it's the incoming plumbing, I suppose. Lots of tedium ahead, it looks like.
Hey. Is there maybe a practical way of adapting that toilet-leak test (the one where you use food coloring) for this? Probably only if I'm willing to pour ten or twenty gallons of food coloring all over my tub and walls. Maybe not such a good idea.
Jim Beaver
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I really hate breaking open drywall unless I absolutely have to.
I'd monitor the area like a hawk and try to determine if the moisture increases:
1. After rain. 2. After running water into the tub (no splashing on the tile or surround). 3. After taking a shower (with splashed tiles etc).
I might invest in a moisture meter also. That would enable you to detect much smaller peaks in the moisture level than you'd be able to spot by eye or touch.
Finally, I'd inspect the faucets as carefully as possible without removing tile or drywall. Leaks frequently arise where the pipe connects or at the stem. You'll utter some choice language if you tear down drywall only to discover the leak was caused by dried/failed packing in the faucet stem ;-)
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drywall repair is easy and a skill to learn, open the cieling and inspect, since the damaged cieling is easier patched with new drywall than trying to fix the surface, the drywall turns to dust once its wet.
dont overanalyze, open it up find and fix leak then leave it open for awhile so you know you got them all
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