Here’s another take on wet rooms when there isn’t enough height for a mud base.

The first step is to position blocking at the lowest point of the room, where you want to locate the drain. Then add 2 Xs, or strapping to establish a lever perimeter around the room. Lay a 2X diagonally from one corner of the room to drain so that one end sits on top of the perimeter strapping, and the other on the drain blocking. Take a scrap of 2X, the same depth as the diagonal and cut it on the same angle as the diagonal. Set the block on top of the joists, against the diagonal and draw a line on both sides of the diagonal to mark the depth of the notches you will cut in the diagonal. Use a straight edge on the sides of the joists to mark the sides of the notches (both sides of the diagonals). Scribe the cut angles on each end of the diagonal. Cut out the notches, and cut the diagonal to length. Slide it down over the joists and screw it down. This will give you the perfect slope from perimeter to drain. Repeat for the other 3 diagonals.

Now cut cant strips to fill in the tops of the joists, between the diagonals. Cut 2Xs to length, set them on top of the joists and using a straight edge to mark the cut lines. Rip them to the correct width/tapper and screw them on. In this project, the long strips between the diagonals are app the same thickness, while the shorter strips at the ends of the room are tapered.

To sheet the floor, cut the ply (3/4″) to length, lay it in position and mark the edges at the center of the diagonal. Cut a kerf about 1/2 the plywood’s depth to allow it to bend over the diagonals. Screw it down. I used t&g so that all edges are supported.

The beauty of this approach was the natural sag of the joists allowed me to construct the wet room so that there is no step up from the doorway into the room. The slope is only about 1/8 per foot, but it is precise, no little rolling speed bumps where water could pool. Because the drain is not centered in the room, the slopes from side to side, front to back, are all different, but this was all determined by the level of the perimeter strapping and drain. It took about 3 days to figure it all out, but now I have the technique, I could do it allot faster.