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If needed, use 2x4 lumber to frame in the "box" that will serve as the form for the concrete pour. Check local building codes for the correct concrete depth and the required size and thickness for the rubber membrane. The shower in this project will be flush with the side of the whirlpool bath, so the frame needed to extend under the tub overhang.
Note: Get a professional plumber to rough in the shower drain before starting the concrete installation.
Spread out the rubber membrane in the space and spread it over the bottom of the form so that it overlaps the form. Make sure there's enough material on each side of the form to extend up the wall to the correct height.
Starting in one corner, flatten the membrane against the bottom of the space and push it tight against the bottom of the form board. Attach the membrane to the studs with large-head nails at least 8" from the bottom of the form. Except for the drain hole, the membrane should not be penetrated any lower than 8" up the wall to keep it completely waterproof. (Local building codes may have different requirements.)
Carefully cut out the drain hole with a utility knife, making sure not to cut the hole too large. If any water ever gets through the concrete, the membrane will funnel it down the drain.
Screw in the drain to a height of 1-1/4" to leave room for the concrete. Cover the chrome drain with tape to protect it during the concrete pour.
Pour concrete mix into a bucket until it's about one-third full. Mix in water and more concrete mix, following the manufacturer's instructions, until the result is the consistency of a milkshake. Use a spade-handle drill, which works like a giant hand mixer. Concrete comes in different "recipes." The sandy mix will dry to a very smooth surface.
Pour the concrete and spread it out with a trowel. The height of the concrete should gently slope from 2-1/2" around the outside of the form to 1-1/4" near the drain. When working with the wet concrete, strike it gently with the flat side of the trowel to help bring the moisture to the surface and smooth out the concrete.
Allow the concrete to cure according to the manufacturer's instructions before tiling.
Note: An old or uneven floor or one that needs patching to fix water damage requires leveling before tile work can begin.