Looking for moisture sensor for behind shower wall

Had a leak in the corner of a tile shower and didn't discover it until it had been happening for some time and water dripped out of the ceiling in the basement (finished ceiling). There are membranes that can be put under the tile during construction - wish that was done but it wasn't...

No major damage though. I plan to seal the corners with flexible caulk instead of brittle grout. This is a common problem on tile corners it seems. But I would also like to put in an "early warning" sensor (or several). The shower is adjacent to a tub so I have some access under the tub and from the basement to put some sensors there and run the cables through the ceiling to the wiring closet close by.

Looking to determine a good sensor. I saw some posts on a "leaf wetness" sensor that is a pattern on a PC board but I would like something that presses up against the wood - I don't think I can position the PCB to reliably catch surface moisture. Maybe some sort of sponge with nails in it that would absorb surface water/moisture. I know that moisture level in wood can be determined by resistance between 2 nails an inch or two apart - that's basically how wood moisture meters work. Since it shouldn't normally be wet I don't have much concern about corrosion but I could use stainless hardware.

Any ideas or links?

I'd look at something like the Ademco 470-12, which is a water detector.
 
http://www.security.honeywell.com/hsc/products/intruder-detection-systems/sensor/special-application/21672.html
 
I'm sure the various sensor companies have comparable items, such as: http://www.grisk.com/specialty/2600water_sensor.html

About 6 years ago, the company I worked for used these spill detection mats to detect leaks in a piece of equipment.   The mats can be cut to any shape and change resistance when they become wet.  
 
There is nothing magic about the sensor unit. You can connect the mats to anything that can detect the change in resistance.  We used the A/D inputs of a microprocessor to do the sensing by connecting the mats in a voltage divider network.
 
Not sure if Warner still makes the mats, though.  But it's probably worth a call to them to ask.

I've seen similiar with some neighbors here occurring.  Mostly though its been relating to toilet backups flooding 2nd floor toilets damaging first floor ceilings.  Some sort of sensor would may have prevented the damage from occurring.
 
Here I went the long way sort of.  My wife didn't like the color of the floor tile in the laundry post construction.  It was a PITA to do but I removed all of the tile glued down to the wood floor.  I then put in that cement water proof board (very thin stuff) and sealed it before putting in the tile. The contractor had just put the tile on the plywood floor.  Concurrently LV "wired" the room for automation cabling it it for her television (well and ran additional cat5e) and sensors in the room.  Redid the shelving and storage sections.  The room was out of commission for a period of time (very low on the WAF to not have a washer/dryer to use). 
 
This past summer on a Sunday morning the washer did break (geez less than 10 years old) and leaked all of the tub water to the floor of the laundry room.  I looked in the ceiling of the unfinished basement and no water leaked through.  Its been now over 10 years that I redid the floor and tile to this room.
 
Today though I do utilize water sensors near the ejection / sump pump wells in the basement.  They are hard wired to the alarm panel.

Over the years, I have had several showers with this problem-- all relating to improper installation of the shower pan.   You should not have this problem is it is installed right.   If you go the caulk route to keep the water in, plan to dig out the caulk and re-caulk every couple of years. 
 
In the house I am currently building, the builder is installing a proper shower pan.   However, I have installed the GRI 2600T water detectors just in case.

The shower pan is not the issue. Originally the plumber installed the rubber sheet and stapled it to the floor and sealed the staples with some fairly thin brush on coating. It was obvious that the sheet would stretch when the floor was filled in and open up leaks. Very poor and surprising since the plumber was experienced and did good work elsewhere. I made him come back and replace the sheet, tacking it only to the walls near the top. The problem was a vertical corner. Even though the crack was very small, some water would leak into the wall, run down inside the wall, and miss the pan. Have talked to others in construction and corner cracking of grout is pretty much unavoidable from what I have been told, at least with lumber framing. Often the corners are redone with flexible caulking which allows slight movement. Several companies make membrane materials that go under the tile - think they are sheet material, brush on, or combination but I am not that familiar with them. My understanding is this prevents leakage even with minor movement. Wish we had done that in initial construction.

Curious about the shower pan itself.  Is it all tile? 
 
In the 1980's we had a new bathroom installed in a very old home in a refinished attic section of the house.  I don't remember much other than the floor was tiled and the shower was all tile with glass panels.  The shower pan was stone and it went up to the tile.  Never saw a leak but only lived in the house some 4 years.  The bathroom though was built in sections by different trades folks.  IE: Plumber, electrician, carpenter and tile guy all were involved.

Shower pan is tile. I think construction of that is pretty standard. The drain was fitted which included a flange for the rubber membrane. The rubber membrane was installed and folded at the walls (no cuts) and stapled near the top. It went up the walls about 6". The height of the drain grate was adjustable and was set. Then some sort of grout/concrete was poured in and sloped slightly toward the drain. Then concrete board was installed on the walls (overlapped at the bottom onto the rubber and screws were kept a couple inches above the lower edge). Then the walls and floor were tiled. As I say, I think this is pretty standard,

Yes, that does sound standard.   When I had mine replaced, the tile guy told me that they will not leak if installed properly.   He told me that he would tear out the pan and repair it for free if it ever leaks.   It never did.  
 
Apparently there is a high degree of skill required in the install.  

@Jim,
 
Now you have me looking at the kitchen ceiling as it sits under the main master bedroom bathroom.  I always have heard water; never paid attention to it though.  Its been years and the ceiling looks OK.

The "pan" is typically concrete. Usually a membrane is placed on the wood flooring. Then a sloped first pan layer is poured. Now the main rubber membrane is placed on that with a pressure seal squeeze from the drain assembly. Now the second layer of the concrete pan is poured on top of the rubber membrane. Tile is placed on top for aesthetics and feel. The outer walls also should have a membrane under the concrete wall board shingled to the floor membrane to avoid your situation.
 
With all this outer sealing for moisture do NOT use grout sealants. Moisture will seep through into the grouting and concrete under-structure and needs to evaporate out somewhere otherwise you will develop a mold problem. The mold may even be underneath your tile and then a bigger problem develops. Ask a tile installer. They are never around years later when these problems develop. THINK.

JimS said:

Had a leak in the corner of a tile shower and didn't discover it until it had been happening for some time and water dripped out of the ceiling in the basement (finished ceiling). There are membranes that can be put under the tile during construction - wish that was done but it wasn't...

No major damage though. I plan to seal the corners with flexible caulk instead of brittle grout. This is a common problem on tile corners it seems. But I would also like to put in an "early warning" sensor (or several). The shower is adjacent to a tub so I have some access under the tub and from the basement to put some sensors there and run the cables through the ceiling to the wiring closet close by.

Looking to determine a good sensor. I saw some posts on a "leaf wetness" sensor that is a pattern on a PC board but I would like something that presses up against the wood - I don't think I can position the PCB to reliably catch surface moisture. Maybe some sort of sponge with nails in it that would absorb surface water/moisture. I know that moisture level in wood can be determined by resistance between 2 nails an inch or two apart - that's basically how wood moisture meters work. Since it shouldn't normally be wet I don't have much concern about corrosion but I could use stainless hardware.

Any ideas or links?

Click to expand...

How about a relative humidity sensor?  The wall cavity is sealed, so, depending on the sensitivity, there'd be a spike in the readings if enough water got through.  So, as a test, you might want to run the shower extra long, so that if there is a leak it would register.
 
Ordinarily, before installing the tile, they're supposed to do a shower pan leak test, where they block the drain with a pressure bulb and fill up the pan with water and see if it loses any height after 24 hours.  After installing your moisture sensor, maybe you could do a similar test, but after first saturating the tile, grout, and whatever thinset or thickset there is, so that you know any lost test water wasn't simply absorbed by those.  It's all but certain that in your case you'll get noticeable dripping with such a test, even after the caulking you propose, and so you  may not even need a sensor at all to prove to doubters (yourself or others) that you have a leak that mere caulking won't solve.  

I know this thread is some months old but I hadn't seen it before.  As it happens, I'm now trying to track down a water problem in our basement shower.  Just wanted to say thanks to the several posters who added detailed descriptions of construction methods and issues to look for.  You've given me some leads!
 
Thanks,
 
Craig

Humidity sensor is a good idea. 
 
There is no indication the pan leaked.  The water entered the wall above the pan and ran down the wall studs outside the pan.  There are products that provide a water barrier behind the tile above the pan but those are fairly new and it wasn't used here.  If I was to do a shower today I would make sure something like that was used.

When we built the new house I made sure everything that could be done was completed properly for any place with water draining.  Had the showers all done with the foam board and I checked all the work before any tile went up.  The washing machine has an electronic valve with both a moisture and a current sensor.  It won't open the valve unless the washer is on (pulling power) and there's no leak detected at the sensor on the floor.  Still didn't catch the half-assed job the "plumber" did with the steam shower drain pan (idiot didn't tighten the connection to the pan, and had the drain line tilted wrong anyway.  Thankfully nothing got wrecked.
 
For good measure I've placed a couple of inexpensive LeakFrog battery operated moisture detectors under the sinks and near the sump pumps.  One caught a problem just the other day, the outflow for a sump pump had frozen and the pump couldn't eject the water.  Took a sawzall to the section that got frozen and it immediately cleared the pit.  Wouldn't have known without that sensor beeping like mad.