1. Do we need something between concrete floor and tile?

Yes! It's essential to place a layer of uncoupling membrane over any type of concrete substrate before laying down your adhesive. The membrane will prevent any flexing from affecting your adhesion while also providing insulation.

2. Can I use regular thinset mortar under large format tiles?

Although many would say that it’s possible to set large-format with standard-thinset mortar; using a specific thick bed/mud-set method ensures better grip due to its stiffer consistency, which normally prevents slippage when setting larger-sized stones/porcelain/vitrified ceramic types effectively.

3. How do I waterproof my bathroom floor before installing porcelain/ceramic tiles?

The best way is to install an appropriate decoupled/waterproofing system under your flooring material. Installing Schluter Kerdi-Board Shower Panels or Ditra Matting Decoupling System helps keep moisture out by redirecting water run-off around edges & corners where potential leakage may occur through conventional methods used by inferior tilers too inexperienced/don't know how

4. Does grout work well regardless of surface level differences in shouldered walls/floors?

Nope! Depending on size difference there could be up-to 5mm shadow lines (grout line variations) due uneven surfaces.If you're planning on applying grouting after the initial leveling job has been finished then choosing unsanded good quality epoxy-based grout means no compression later risk -thus preventing more future cracking.

5. How can I prevent tile from looking uneven without using a self-leveling compound?

If you’re dealing with dips and high spots, then it's usually better to address the subfloor first since self-leveling compounds are very hard to level perfectly. A good crack suppression membrane may be another option but your concern is that once replaced & tiled over again each cracked section along where membrane was installed in affected area will form like…