I want to install wood stair treads over a concrete and tile flight of stairs. The tile is porcelain tile.
I couldn't find any tutorial that addresses this specific scenario. Do I have to remove the top tile or have to drill holes to secure the treads (would rather not, but will if I have to)? Is construction adhesive sufficient? Tile is nice, flat and even.
May be obvious but I'm not in professional construction business. Just a DIY'er.
IRC R311.7.5.1 specifies that the difference between the largest and smallest riser in a flight of stairs shall be 3/8" (9.5mm) or less. If your applicable code isn't based on that, it still very probably has very similar language regarding stair riser heights.
Unless you engage in an awkward (usually) overlay of the landings at top and bottom, which then end up having a transition to the rest of the floor (what makes them awkward) that's very difficult to manage with any overlay of non-trivial thickness.
As such, I expect you'll need to remove the tile to have any hope of making a wood overlay work, and it will be a fairly thin wood overlay at that. One of the faux-wood porcelain tile products might get close to the look you want with more likelihood of success.
Heavy duty construction adhesive will do the job, assuming that the tile is all well bonded. Use a 3/8" (8mm) bead in a zigzag or grid pattern and hit the perimeter well.
I'd be concerned about moisture, though. Wood over concrete is always a bit sketchy in that regard. You may see cupping as the bottom side expands from dampness propagating through the concrete, which will have enough force to tear the adhesive loose.