A pre-slope is the sloped bed of cement laid under the waterproofing so that every part of the shower floor drains toward the drain. Without it, water that gets through the tile has nowhere to go and sits against the membrane. It is the step most often skipped, and it is invisible once the tile is on.
What a pre-slope is
Before any waterproofing goes down, we mix cement with mortar mix and lay a bed that falls toward the drain across the whole floor. Then the hot mop goes on top of that shape, so the waterproof layer itself is sloped.
It is the first thing we do on every pan, and it is the reason the rest works.
What happens without one
Tile and grout are not waterproof. Water gets through them; that is expected, and it is exactly what the pan is there for. On a properly sloped pan that water runs to the drain and leaves.
On a flat pan it does not leave. It sits on the membrane, under the tile, indefinitely — working on every seam and every corner, all day, for years.
Second-floor showers and movement
On an upper floor there is a second problem to watch. If the shower floor moves — if it feels like it could give way when you step on it — a hot mop pan should not go on it at all. The material is heavy, and that is genuinely dangerous.
On a first-floor slab there is a different one: if the plumber has set the drain but not filled the p-trap with cement, the waterproofing will crack. We check for it, because it is not visible once the mop is down.
You cannot inspect it later
Once the tile is on, nobody can see whether there is a pre-slope under it. The only way to find out is the way nobody wants: the shower starts leaking and the tile comes off.
This is why it is worth asking the question before the work, not after.
Common questions
- What is a pre-slope in a shower pan?
- A bed of cement laid under the waterproofing that slopes toward the drain, so water reaching the membrane runs off instead of sitting on it.
- Does every shower pan need a pre-slope?
- Yes. Without one, water that passes through the tile sits on the waterproofing instead of draining away.
- Can you tell if a shower has a pre-slope after it is tiled?
- No. It is under the tile and the mortar bed. It only becomes visible if the shower is opened up.
About Figueroa Shower Pans
Figueroa Shower Pans waterproofs shower pans across San Diego County, based in Escondido. Francisco Figueroa has worked in the trade for 10 years, and shower pans are the only thing the business does — no demolition, no tile. CSLB License #1092967 · C-61/D-12, with general liability insurance and workers’ comp.