Shower pan
Also called: shower base, shower floor pan, mortar pan, tile-ready pan, pre-sloped pan
The waterproof floor of a shower under the tile: either a factory-made acrylic or fiberglass base, or a site-built pan — a sloped mortar bed over a liner or a bonded membrane, draining to a matched drain. The tile on top is decoration; the pan keeps water out of the floor.
Every shower floor has two jobs: hold water on its surface long enough to reach the drain, and stop the water that gets through the surface from reaching the subfloor. Tile and grout do the first and fail the second — grout is porous and tile joints are not sealed — so under every tiled shower floor is a pan that does the waterproofing.
Prefab bases — acrylic, fiberglass, or solid-surface — are the pan and the finished floor in one piece, set in a mortar bed for support, with an integral slope and a drain that seals to the base. They’re the choice for kit installs and most tub-to-shower conversions, and the reason those are DIY-friendly.
Site-built pans are for tile. The traditional method is a mortar pre-slope on the subfloor, a PVC or CPE liner folded up the walls and clamped into a two-part drain, then a second mortar bed sloped 1/4 in per foot over the liner, then tile. The modern method is a foam or factory pre-sloped tray, or a mortar slope, covered with a bonded waterproofing membrane that connects to the membrane’s own drain, with tile set directly on it. Both work when done exactly to spec; neither forgives a skipped step.
The flood test is the check: plug the drain, fill the pan with two inches of water, mark the level, and leave it 24 hours. No drop, no leak. It’s required by many inspectors, and it’s the one thing to insist on before the first tile — a pan that fails after tiling is a shower rebuilt from the floor.