To tank or not to tank

Entirely uneccessary.

I have done exactly the same as you. Sealed tray to wall with flexible silicone, tiled over raw plasterboard and used a decent grout.

Provided it is all rigid and you dont allow pools of water to accumulate all fine after 16 years

I have also done similar, and always had good results doing this. In reality if you are properly sealing all the gaps and board joints etc before tiling, as well as taping the board joins, then that *is* getting pretty close to tanking anyway.

Having said that I have usually used waterproof backings to tile onto. In the past I have rendered the walls adjacent to showers (with SBR admix). More recently I used aquapanel, which seemed to work well.

Its usually a process of sealing all the gaps, taping all the joins, and then painting with a water proofing finish that remains slight flexible, prior to tiling.

Companies like Mapei do complete kits of it with enough content for a typical shower. e.g:

Plaster disintegrates if it gets & stays wet. If the grout between tiles cracks, water gets in. So tanking would help ensure it stays ok, but as you now know is often omitted, just with lesser reliability.

NT

It is worth sealing the plastered wall with waterproof pva before tiling. Also worth being aware that timber in stud walls and joist under floors will shrink and may open up gaps.

TW

Personally, my preference for showers is Mermaid or similar melamine faced plywood bonded with thermosetting resin. I don't question that tiling can last a long time if done really well, and where no cracks open up over time owing to movement.

If the shower tray does not have an upstand that the tiles go over, I would use that L-shaped strip that seals on to the tray so that the tiles do go over an upstand, if you see what I mean. You still have a potential leak site at the bevel, so bed and seal that bit well.

At least with not tiling direct on to plasterboard you don't have the most common cause of tiling failure.

I have always used this stuff. Gives a neat finish too.