What is a hot mop shower pan?

The part of the shower nobody sees

Most people have never heard the phrase until a tile setter or a general contractor says it to them. Then it becomes urgent, because nothing else in the bathroom can happen until the pan is done.

The pan is not the tile. It is not the drain. It is the waterproof layer underneath everything, and it is the only thing standing between the water in your shower and the floor structure below it. When a shower leaks into the room below, this is almost always what failed.

How one is actually built

Every pan we build goes in the same order. It takes about two hours.

1. The pre-slope

   Cement is mixed with mortar mix and laid so that every part of the floor drains toward the drain. Water that cannot reach the drain sits still, and standing water is what finds a weakness.

2. The layers

   Hot liquid asphalt is mopped on with a fiberglass mop. Three layers of 15lb felt paper go in, with four layers of asphalt — one between each layer of felt, and one over the top.

3. Up the walls and over the curb

   The membrane does not stop at the floor. It turns up the wall behind the tile and over the curb, so water that runs down the wall has nowhere to get behind it.

4. The corners

   Yellow fiberglass mesh is worked into every corner and seam. Corners are the first place a pan fails, and they are the reason we do tight, straight corners rather than round ones.

5. The flood test

   The pan is filled with water. If it has not leaked in 24 hours, it was built right.

Tight corners, not round ones

It is one of the five questions we get asked most, and the answer matters. A rounded corner is faster to form and leaves the membrane stretched thin exactly where it has to work hardest. We do tight, straight corners and reinforce them with mesh.

Why we still do it this way

There are several ways to waterproof a shower. Sheet membranes, liquid-applied coatings, plastic liners — they all have a place, and some of them are quicker.

In ten years of building pans and being called out to fix other people’s, hot mop is the one we have seen last the longest. That is not a laboratory claim; it is what we see when we open up failed showers. It is what we build, and it is the only thing we build.

What has to be ready before anyone can mop

A pan cannot go in over a shower that is not prepped. This is the list we give a contractor.

• The drain set and flushed, and the correct shower drain for the job.
• Blocking installed around the walls, typically 2x10 or 2x12.
• The curb — also called the dam or threshold — in place. Minimum two 2x4s; three is better.
• A solid floor. If it moves underfoot, it cannot be mopped. The material is heavy.
• Demolition finished. We do not demo, and we do not tile.

Does it smell?

Yes. Hot asphalt has a smell, and we tell people before we start rather than after. It clears, but if there is someone in the house who needs to be somewhere else for a couple of hours, plan for that.