I am planning to install river rock pebbles over existing cermic tiles in my shower stall. But I need to extend my drain about 1 1/2", to be flush with the rocks.
Is there any special preparation before applying mastic adhesive to the existing tiles.
And How does one extend their drain.

I don't know about installing rock pebbles in your shower floor, ( that would hurt ny feet) but to raise your drain it would be a hassle. Are you on a slab or is the shower on the upper floor? Regards, Tom

Hello Ferd1: you don't "extend" existing drain. You leave it as it is. Just remove the grid cover...

First, I strongly suggest you use fortified THINSET, not mastic, for your installation...

What you do is you buy Tile Drain Frame (square) with round, removable grid strainer (Home Depot, Lowe's ) in finish of your choice. Usually, this frame is 4 1/4"square. It has no depth to it. It is about 1/4" thick. Install this frame into Thinset , exactly over the existing drain opening, and shim it up to the level of the pebble tile. Proceed with installing pebbles. Once Thinset sets, remove shims. When grouting, fill all spaces around ( and on the bottom ) of the frame with sanded grout. Install round drain cover when dry.

I am enclosing photo for better visual...

I'm impressed! Milo came up with a solution that I didn't even know existed. Made to order! Way ta go Moilo. Tom