As the owner of Rocky Mountain Bathrooms I've diagnosed a lot of leaky showers and bathtubs over the years. Lots of relationships with customers have started with a phone call from an exasperated customer who has been suffering water damage from a shower or bathtub for years and is ready to fix it once and for all.
I'm going to show you how we go about diagnosing a leak. Leaks let you know they exist by water appearing and pooling, soaking or staining something. It could be a ceiling below, the floor next to the bathtub or shower or even in a hallway floor outside the bathroom. While the proof does let you know that you have a leak, don't read to much into the location. I've seen ceilings get water damaged not from the bathroom directly above them, but from a bathroom above them and 50 feet on the other side of the house. Water has a funny way of running down hill and finding the path of least resistance, it can leak from one area and run all the way to another place before it shows itself.
The first rule is not to jump to conclusions. Think logically of course. Bathroom directly above water damage, could be that bathroom is the culprit, but keep an open mind don't get to narrowly focused yet you could be on a wild goose chase and you don't ever want to start tearing open walls and ceilings until you are reasonably positive you are in the right place.
Rule #2 is the mind will play tricks on you. It's very easy to associate circumstances and confirm circumstances in your mind. I can recall one particular problematic leak that the homeowner swore was related to running the shower. The ceiling was damaged directly under the shower upstairs and they connected in their minds that every time their son who took a shower after work in the afternoon it would leak, but it wouldn't leak any other time. If someone took a shower in the morning it wouldn't leak. Well it turned out it was a plumbing vent pipe unrelated to the shower that went right up next to the shower and through the roof of the house. The roof around the vent pipe was damaged but the roof was configured in a way that rain wouldn't cause much water to get into the hole, but that winter we had a big snow storm and every day after it got warm the sun in the afternoon would melt the snow that had accumulated around that vent pipe and it would leak down and run all the way down that pipe and pool in the ceiling below the bathroom. It would take it 3-4 hours and sure enough it would coincide with about an hour after their son took his afternoon shower. So be careful what you assume and what you associate, the mind will try to find ways to confirm your suspicions.
The final rule is to confirm your suspicions. You must always test your suspicions. And think about how you are testing it, make sure the way you are doing it isolates any other possibility. This is the only way to truly find the leak. You must do isolated tests of each suspicious item one at a time.
So lets take a typical shower or bathtub with a tiled surround and either a bathtub or a fiberglass shower pan. You've got a leak of some sort showing up either outside the tub/shower and you've observed and eliminated that it isn't the kids splashing water over the side at bath time or water leaking out the shower door.1.The tile surround, which could be the grout, the corners or the connection where the tile meets the tub or the shower pan.
The culprits are:
2.The drain, which would be the drain itself or the overflow in the bathtub
3.The valve in the wall.
4.The plumbing connections in the walls such as the hot and cold feeds to the valve and then the connection of the valve to either the shower head or the tub filler.
So we have only 9 individual things that it could be. So that at least makes it easy. 9 Is a lot but if we are systematic we now have a list and a process to find out which one it is.
Lets start with the drain.
Start with examining the drain itself and the area around the drain, are their any cracks is the drain loose or nice and firmly seated? Get in there and step around it, put some weight on it see if a crack shows up that you didn't see until you put some weight on it. If you see anything like that you might have your culprit right there. But like I said don't jump to conclusions we must test, test, test!
Let's isolate the plumbing that is attached to the drain. We need a rubber hose running to a water source not in the shower or bathtub (remember we need to always isolate our tests, you can't run the shower or tub valve to test the drain without running the risk of the problem is really the valve right?) Most home improvement centers will sell different types of rubber hosing by the foot. You can purchase a length of it that will be big enough to go over your vanity faucet aerator. So get your rubber hose and stick it into the drain and run the water for 15 minutes. Come back and see if you see your leak appear where ever it has always shown up. If it usually doesn't show up for an hour, wait an hour. If you see your leak, it's the plumbing attached to the drain and you are done testing. No leak, now it's time to test the drain itself, take your rubber hose out and run the water into the shower pan or tub and do your wait and check. If you get your leak it's something to do with the drain itself, or that crack you found. See how easy this is.
Now lets say no leak yet. Well, now we know that it's not the drain or the drain plumbing let's now move onto the valve and the plumbing in the walls. If you have a leak before the valve you would have a leak all the time. So obviously we don't have leak in the hot and cold water lines attached to the valve in the wall or you would know it by the constant leak. If it's a tub, lets test the tub filler, since we know the drain is not leaking, just turn on the tub filler and fill the tub. Let it run for 15 minutes and check. If you see your leak you've got a leak between the valve and the tub filler, there is probably a copper elbow or a joint that is broken. If you didn't find your leak let's check the tub overflow while we are at it. Close the drain and fill the tub up to the overflow and let it overflow into it for 15 minutes. Find your leak? It's the overflow connection where it is attached to the tub.
Okay let's check the shower head plumbing. The shower head is connected to the valve by a copper pipe all we need to do is plug the shower outlet and turn on the valve, this will pressurize that pipe in the wall with water. To do this you will need to take off the shower head and cap the shower pipe stem with a threaded cap you can buy. Or remove the entire shower head and stem and insert a threaded pipe with a cap into the elbow in the wall. However you do it, just plug it and turn it on for 15 minutes, the pipe will be filled with water under pressure and now if you find your leak it's the connection of the valve to the shower head.
Okay if you still haven't reproduced your leak it's pretty easy from here, you've eliminated everything else but the tile surround which by the way is usually the number 1 or number 2 most likely cause of leaks. So from here all you need to do is turn on the shower head and aim it at the walls, now this is not scientific and will not usually result in a very accurate locater of the exact spot. A better way is to take your rubber hose, hook it up to the shower head stem and hold that hose over each wall for 10-15 minutes apiece until you find your leak. This way you will know for certain exactly where it is. Examine the tile, look specifically at the grout, do you see any hairline cracking or missing grout in the area? That's usually all it takes. Press firmly is it spongy?
The final thing to do is after you think you found your leak, is to wait a day and try it again. Double check it and make it leak again. Stop now and think about it, make sure you have it positively isolated. If you do all that and you are systematic you should now have found your leak!