A few people asked me about fixing leaks under the tiled shower pans.Detecting such leaks is a task in itself, because they are very hard to detect, and there are many reasons why they could leak. When opening the ceiling bellow, you will not see the waterproofing membrane. The waterproofing membrane located between the mud-pan under the tiled floor and the plywood sub-floor.
Leak could be caused by a rubber membrane or in some cases, instead of rubber... Fiberglass coating is used, which cracks over time due to a constant expansion and contraction of the building during seasonal changes. Another reason could be a shower floor has a hole in it someplace, or rubber membrane has separated from the drain, etc. In some instances the tiled pan is old, so wear and tear took its toll and more investigation need to be done.
To pin point the leak, you must open the ceiling bellow and try to pinpoint the location where water penetrating the sub-floor from bellow by running a few tests. When the ceiling is open and you can see a large area of water staining, you could have a serious problem. Best thing to do is to call a licensed plumber or a professional contractor to find the problem.
If you like to attempt finding the leak yourself, the best way to do it is to run a few tests yourself. You will need an inflation device (HRDWR store might have it, if not try a plumbing store) you insert the device in to the drain and fill it with air to block the water from being drained. Fill the pan with water 2” inches below the shower pan-curb and check bellow if you see any signs of water, if not, leave it over night and check in the morning.( make sure you know what you doing, because if the device falls in to the drain, you will have another problem). If you see water, the problem is in the drain, or the tile is lose or cracked along perimeter of the drain or the pan itself, or membrane separated from the pipe on the drain. If there is no water, hold the device by the chain and let the water out.
Next test is to run the water on the side walls of the shower but not the wall where the diverter is or shower fixtures, and check for water. If there is no signs of water, run the water on the shower wall where diverter is and check again. Many leaks occur due to the fixture covers become lose or have not been caulked properly or maintained and they allow the water enter the wall cavity, causing the leak. Other place to look for damage is along the threshold itself. With constant activity, there could be bad door trim or deteriorated tile grout, or caulk which can cause a leak. Check the area real good, because the minute water enters this area of the shower, it will cause the damage real quick.
Other signs to look for are small horizontal cracks or grout breaks/voids in the tile across the wall, the pan, and the corners which are very common and they need to be maintained.
If you cannot find a leak and the problem is not the tiles, call a licensed contractor.