Design the Assembly, Not Just the Tile
A below-grade bath lives or dies by its assembly. Start with a continuous vapor/waterproofing membrane (sheet or liquid-applied) tied into a factory-formed drain with clamping flange. On slabs with dips, self-level first, then slope the shower area at 1/4″ per foot toward the drain. Use a bonded flange linear drain at the doorway for curbless transitions and set the entire wet zone on a decoupling membrane to prevent hairline cracks from telegraphing through tile.
Substrate, Grout, and Movement Joints
Prefer cement backer board over moisture-resistant drywall; wrap niches and benches fully with the same waterproof layer. Choose epoxy grout for stain and mildew resistance, and honor soft movement joints every 8–12 feet and at plane changes. In flood-prone areas, specify large-format porcelain and a solid-surface shower pan for fewer seams.
Plumbing & Odor Control
Use trap primers for floor drains, and vent the bathroom per code to avoid siphoning. Route condensate from ERVs/dehumidifiers into a hub drain with a check valve and cleanout. Seal the toilet flange to the membrane and use waxless seals for easier future service.
Ventilation & Dry-Down
Install a continuous-rated, low-sone bath fan (or tie the space to a ducted ERV) with a 20–30 minute post-shower timer. Maintain 45–50% RH via ducted dehumidification so finishes dry between uses.
For a step-by-step spec that coordinates drains, membranes, and ventilation, see our NYC basement remodeling service — waterproofing & moisture control.