Water – How to find the water connection behind a shower wall

I'm very sorry to say this but you're going to have to remove all the tiles and start over. You will never been able to prevent this from leaking fully.

It may also mean removing and replacing the backing as well.

The critical step you missed was the waterproofing membrane that goes under the tiles and in and over the drain pipe. The tiles don't stop the water at all but are for aesthetics only (and to protect the water proofing). What makes the shower water tight is a sturdy backing covered with a water proofing membrane like liquid flash (one I used) when I did my tile shower. http://www.constructionchemicals.com.au/products/waterproofing-membranes/

But there are many different brands. They are a paint on product and very easy to install.

Don't expect your tiles to be waterproof. At best they are only splash resistant. Water goes right through the grout and can also enter around the side of the tile where the grout is. Porcelain tiles aren't quite as porous as ceramic, but most wall tiles are ceramic because they are a lot lighter. These tiles act like a wick and just draw the water through to the back of them. If you don't fix this you could be in for some serious mould issues down the track.

Unfortunately there is no simple fix to this but to start over. I would not recommend just trying to seal it. It's like covering up rotten wood with paint, it won't last and won't fix the problem long term. Although you've got water resistant plasterboard. It won't last long term. These plasterboards are just designed to repel water to keep it out of the gypsum they are made of, not designed to be constantly wet.

Interesting construction that your builder used water treated plasterboard + chipboard for the construction. 6mm or 1/4 in thick concrete based tile & slate underlay board + waterproofing membrane is all you would have needed. I wouldn't recommend tile on plasterboard at all. Even with the layers that he has. It's not as strong.

Since I know how much work goes into these DIY tile showers I will say this. If you feel overwhelmed, call in an expert. They will have the job done in a week and you'll be able to move on with life. But if you're up for the challenge and want to learn to do it right this time, then dive in and go for it!

You should check to see what the water pressure is coming from the water main. A screw-on water pressure gauge can be connected to your garden hose connection to find out what it is. Ideally you would want to see a water pressure between 60 and 80 psi.

If your pressure is significantly lower than 50 psi from the main, then you will have issues with having enough pressure for most shower heads. If you have city water, you may have a water pressure reducing valve. These are typically installed near the water meter. If your house is equipped with one, take a measurement of the water pressure, and then incrementally tighten the screw and take measurements. If the pressure is too high, you can loosen the screw. You want to avoid water pressure over 100 psi. When the pressure is too high, it can cause damage to your plumbing system.

If you have a pressure issue at the main, this may fix your shower issues. If the main is not the problem, or you find the pressure is still too low, then the problem is somewhere in your plumbing system. If the pressure is good in the tub and sink in your bathroom, then the shower head itself might be the culprit. Unscrew the shower head itself and see if the pressure is good coming out of the wall. If that is good, then the shower head is clogged up and should be cleaned out or replaced. Certain shower heads contain a water saving feature that basically causes less volume of water to come out. They work fine in areas that have excellent water pressure such as cities, but not so much when you are running off of a well. In some cases the water saver part is removable. If it can be removed, then this may also fix the problem. Shower heads are pretty easy to replace. There are ones on the market which are specifically designed for low water pressure. Using one of those may help the problem, but if the water pressure is terrible, you may need to have a plumber inspect your entire system to find out where the problem is.