What is a hot mop shower pan?

A hot mop shower pan is a waterproof membrane built in place under a tiled shower floor. Layers of felt paper are bonded together with hot asphalt and mopped over the shower floor, up the walls and over the curb, forming one seamless liner. The liner is clamped into the drain, flood tested, and then covered by a mortar bed and tile.

If you're building or remodeling a shower in Southern California, there's a good chance your contractor or tile setter has mentioned "hot mop." Here's what it actually is, and why it matters even though you'll never see it once the shower is done.

Why a shower needs a pan at all

Tile and grout look solid, but they aren't waterproof. Grout and the mortar bed under the tile absorb water every time the shower runs. Without a waterproof layer underneath, that water would keep reaching the wood framing and subfloor, and eventually the ceiling below. The shower pan is that waterproof layer: a tray that catches any water that gets through and sends it to the drain.

What's inside a hot mop pan, layer by layer

Cross-section of a hot mop shower pan (simplified).
  1. Framing and subfloor. The shower floor and curb are framed solid and the drain body is set by the plumber.
  2. Sloped base. Where the job calls for it, a mortar base sloped toward the drain goes in first, so the membrane itself falls to the drain.
  3. Hot mop membrane. Layers of felt paper, each bonded and sealed with hot-applied asphalt, mopped into one seamless pan.
  4. Up the walls. The membrane runs up the walls well above the height of the curb, so water can't get behind it.
  5. Over the curb. It wraps up, over and down the outside of the curb in one continuous piece.
  6. Clamped at the drain. The drain's clamping ring locks the membrane into the drain body, so water that reaches the membrane drains away.
  7. Mortar bed and tile. After the flood test, the tile setter lays the mortar bed and tile over the membrane, which protects it.

How a hot mop pan is made

Once the framing is done and the plumber has set the drain body, the hot mop crew heats asphalt until it's liquid. A layer of felt paper is laid into the pan and sealed with hot asphalt, then the next layer goes on top, and so on. Because each layer is mopped in place, the membrane follows the exact shape of the shower: inside corners, the curb, a bench, a niche. There are no folded corners and no glued seams.

The membrane is set into the drain body, and the drain's clamping ring is tightened down over it. Then the drain is plugged and the pan is filled with water for a flood test. Only after it passes (and after the city inspection, if your permit requires one) does the tile setter lay the mortar bed and tile.

Hot mop vs. other shower pan methods

Hot mop isn't the only way to waterproof a shower. PVC and CPE sheet liners also sit under the mortar bed, while bonded sheet membranes (such as Schluter-Kerdi) and liquid-applied membranes go directly under the tile. All of them can work well when they're installed correctly and tested. Hot mop's strengths are its seamless, custom fit and its long track record in Southern California. We cover the differences in detail in hot mop vs. other waterproofing methods.

What homeowners should ask

Frequently asked questions

Why is it called hot mop?

Because the asphalt is heated until it's liquid and applied with a mop, sealing each layer of felt to the next.

Does a hot mop shower pan smell?

Hot asphalt has a noticeable smell while it's being applied. The work area is ventilated, and the smell fades after the asphalt cools.

Is hot mop still used in new homes?

Yes. Hot mop remains a common way to waterproof shower pans in Southern California, in new homes and remodels alike.

Planning a shower in Orange County? Get a free estimate or call (949) 446-4413. Learn more about our hot mop shower pans.

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