Code requires a waterproof pan or liner under the tile, a floor sloped about 1/4 inch per foot to the drain, the liner turned up the walls at least 3 inches above the curb, a weep-hole drain, and a water test of any new pan before tiling.
The waterproof pan: the part you never see
Under the tile and mortar of every shower sits a hidden waterproof layer. People call it the shower pan, the shower liner, or the receptor. It can be a sheet membrane (a PVC or CPE liner), a liquid-applied coating, a bonded sheet over a foam tray, or a one-piece molded base. Whatever the material, the job is the same: catch every drop that gets through the grout and tile and send it to the drain.
That last point surprises most homeowners. Grout and tile are not waterproof. Water seeps through grout lines, soaks into the mortar bed, and would keep going straight into the subfloor if nothing stopped it. The pan is the real water barrier. The tile is the wearing surface and the looks. So when a pan fails, you usually cannot see it from the bathroom. The tile still looks fine while water quietly rots the framing below.
The 2024 UPC and IPC Section 417 both require shower floors to be built over a smooth, hard, nonabsorbent, waterproof base. The code also sets where the liner has to go and how it has to behave, which the next sections cover. In Phoenix, your bathroom remodel is checked against these rules at inspection, so the pan is not optional or a matter of taste.
A pan also has to be the right size and shape for the drain it sits on. The liner wraps a special clamping-ring drain (also called a shower drain or sub-drain) that bolts the membrane down and seals it to the waste pipe. A regular floor drain will not work, because it has no way to grip the liner or to drain the mortar bed above it.
Slope, curb height, and the 3-inch wall turn-up
Three measurements decide whether a shower base passes inspection, and all three are about keeping water moving and contained.
Floor slope. The shower floor has to fall toward the drain so water actually leaves. Code calls for a slope of about 1/4 inch per foot, and no more than 1/2 inch per foot. Too flat and water pools and sits on the tile; too steep and footing gets unsafe and tile sets poorly. The slope is built into the mortar bed under the tile, not just the tile surface, so the liner under it drains too.
Curb (threshold) height. The raised lip you step over is the curb, dam, or threshold. Code sets the finished curb 2 to 9 inches above the drain. The low end keeps water from running out onto the bathroom floor during a normal shower; the high end keeps the step from becoming a trip hazard. Curbless and roll-in showers are allowed too, but they need a larger sloped area and a trench or linear drain to make up for the missing curb.
Wall turn-up. This is the rule DIY jobs miss most. The liner cannot just lie flat on the floor. It has to turn up the walls at least 3 inches above the finished curb or dam. Picture the pan as a shallow tray: if the sides are too short, a deep puddle or a splashing shower sends water over the edge and behind the wall. The 3-inch minimum above the curb gives the tray walls that beat the height of any water the curb would hold back. The liner also wraps up and over the curb itself.
These numbers are not arbitrary. They are the geometry that makes a tray hold water instead of leaking it.
Weep holes and the water test
Two more requirements catch the things you cannot see.
The weep-hole drain. A clamping-ring shower drain has a row of small holes, called weep holes, around its base, below the strainer. They exist because the mortar bed above the liner stays damp. Water that soaks through the grout reaches the liner and runs along it to the drain, but it sits in the mortar until it can get out. The weep holes are the only exit for that water at the liner level. If they clog with mortar or thinset during the build, the bed stays saturated, the pan holds standing water, and you get smell, efflorescence (white mineral crust on the grout), and eventually a leak. A proper install protects the weep holes with gravel or a drain-protection cap so they stay open.
The water test. Before any tile goes down, the new pan gets a water test, and inspectors in Phoenix expect it. The method is simple: plug the drain, fill the pan with about 2 inches of water, and confirm it holds for around 15 minutes with no drop in level. IPC Section 417 puts it plainly, requiring that "shower receptors shall be tested for water tightness" before they are covered. A pan that loses water during the test has a hole, a bad drain seal, or a torn corner, and that is the moment to fix it. Once tile and mortar are on top, a leak means tearing the whole shower out.
This test is the single best protection against a hidden leak, and it costs nothing but water and time. It is also why a permitted, inspected job is worth far more than a fast cash one.
Why DIY pans fail, and why a leak is so costly
A failed shower pan is one of the most damaging hidden leaks in a home, which is the reason code is so specific about it. Water that escapes the pan goes straight into the subfloor and framing, where it stays dark, warm, and wet. The EPA states that mold needs moisture to grow and that wet materials should be dried within 24 to 48 hours to prevent it. A shower pan leak never dries, so it does the opposite: it feeds rot and mold day after day, often for months before a soft floor or a stain on the ceiling below finally gives it away. (If you are already seeing a stain spreading on a ceiling, see our page on a water leak from the ceiling.)
DIY pans fail inspection and fail in service for a short list of reasons:
- No slope or wrong slope in the mortar bed, so water pools instead of draining.
- Liner turned up less than 3 inches above the curb, or not wrapped over the curb, so water gets behind the wall.
- Weep holes packed with mortar, trapping water in the bed.
- No water test, so a pinhole or a bad corner fold is never caught until the damage is done.
- A regular floor drain instead of a clamping-ring shower drain that seals to the liner.
- Punctured liner from a screw or a nail set through it during the build.
This is why a shower pan is a poor first DIY plumbing project, and why Phoenix requires a permit for the work. The permit triggers an inspection that catches the slope, the turn-up, and the water test before they are buried. (For when a permit is needed, see do I need a permit to remodel a bathroom in Phoenix.) The pan also has to sit inside a shower built to the right footprint, so it is worth checking the minimum shower size code before you frame anything.
Get the pan right and the rest of the shower is just finish work. Get it wrong and you pay for it twice: once to build it, and again to rip it out and dry the house. The code rules are the cheap version of that lesson.