Tub-to-Shower Conversion: Drain, Pan, and Supply

Must change:

• The 1-1/2 in. drain and trap. A shower requires a 2 in. drain. This is not a judgment call - the fixture changed, so the fixture drain, trap, and trap arm all go to 2 in.
• The tub waste/overflow cannot be reused. A shower needs a clamping-ring or bonded-flange shower drain with weep holes, set with the flange at the finished tile plane.

Can stay:

• The trap-arm length. At 2 in., the limit is 8 ft and the existing run is 4 ft. Vent distance is fine.
• The vent stack size should be checked against the fixture-unit load of the new shower plus anything else on the stack, but the 4 ft arm is not the problem.

Coordinate with the framer before the plumbing is moved:

• A curbless shower needs the drain assembly and trap below the finished floor. A 2 in. trap plus the drain body needs roughly 6-8 in. of depth from the tile to the bottom of the trap arm - more than a 2x10 joist bay gives you once you add the preslope.
• Recessing or dropping the joists is a structural change; get it engineered or use a linear drain at the perimeter wall where the trap can sit in a dropped bay.

Trap detail: the trap must be directly below the drain outlet, and the trap weir must sit at least 2 in. below the drain.

What is wrong:

• Thin-set is not a slope material. It cannot be built reliably to a consistent fall and it will crack.
• Roll-on membrane directly on plywood over a framed floor depends entirely on the substrate being dimensionally stable; over 16 in. on center joists with deflection, that is a failure waiting to happen unless the manufacturer's substrate requirements are met.
• A roll-on membrane needs a drain with a bonding flange. A clamping-ring drain with a vinyl liner is a different system. Mixing the two leaks.

Correct assembly (one listed system, start to finish):

1. Substrate per the membrane manufacturer - typically a cement backer or a mortar bed over a slip sheet, not bare plywood.
2. Preslope to the drain at 1/4 in. per ft. For a drain near center of a 30 in. wide shower, the far corner is about 2.5 ft away: 2.5 x 1/4 = roughly 5/8 in. minimum fall.
3. Bonded-flange drain set to the preslope, flange flush with the membrane; weep holes protected with gravel or a listed weep protector.
4. Liquid membrane applied at the manufacturer's specified thickness, with reinforcing fabric at corners and the curb, and up the walls above the shower head elevation where required.
5. Curb or curbless transition built with the same system; the finished shower floor must not hold standing water anywhere.

Test: flood the pan for the required period before tile and check the ceiling below.

Flow and pressure check:

• 25 ft of 1/2 in. PEX from a 3/4 in. trunk is a long, restrictive branch for 2.5 gpm. Run the manufacturer's friction chart for the actual gpm and length, add the fittings, and subtract from the 55 psi static.
• Remember static is not what the shower sees: subtract elevation (0.433 psi per ft of rise to the shower head) and subtract the loss through the valve, the rain head arm, and the handheld diverter.
• Two outlets open at once (rain head plus handheld) can easily double the demand. Size the branch and valve for the combined flow, or specify a diverter that does not allow both at full flow.
• If the residual pressure at the valve falls short, re-pipe the branch in 3/4 in. from the trunk or use a pressure-balanced valve body rated for the flow, not a bigger trim.

Valve requirements:

• A pressure-balancing or thermostatic mixing valve is required at a shower; a two-handle or manually mixed valve is not acceptable on a remodel.
• Limit the maximum delivered hot water to the shower - commonly 120 F by code and always by the anti-scald standard on the valve. Set and verify the rotational limit stop with a thermometer at the shower head.
• Provide access to the valve through a removable panel or from the adjacent closet.
• The handheld slide bar needs blocking behind the tile and a drop-ear elbow or a listed wall supply - plan it before the board goes up.