Shower pans built to drain

Why the floor of a shower has to slope

A shower pan is the floor you stand on while you wash. Water hits that floor and has to travel to the drain. If the floor is flat, water sits there. It does not evaporate fast enough. Soap scum and body oils stick to the tile. Mold starts in the grout lines. Over time the grout crumbles and water gets under the tile. That is when you get a soft spot in the floor and a leak into the room below or the crawl space. A flat pan is a callback waiting to happen. It is the one thing that will bring a homeowner back to you unhappy every time.

The fix is simple. The floor has to slope. The standard is a quarter inch of drop for every foot of distance from the farthest wall to the drain. On a typical three foot by three foot shower with the drain in the center, the floor at the walls sits about three eighths of an inch higher than the drain. That does not sound like much, but it is enough to send the water where it needs to go. We measure that slope with a level and a straightedge before any tile goes down. If the slope is not right, we fix the mud bed before we set a single piece of tile.

Two ways to build the slope: a traditional mortar bed or a foam pan

The old way is a mortar bed. We call it a dry pack. We mix sand and cement dry enough that you can ball it up in your hand and it holds its shape. That mix gets spread over the shower floor and shaped by hand to the right slope. Under that mud bed we put a pre slope, which is a thin layer of mortar that slopes the same way. The pre slope sits on top of the waterproof membrane. The membrane goes over a second layer of mortar that also slopes. So you get three layers, each one pitched toward the drain. It is a lot of work, but it has been done that way for decades and it works if it is done right.

The newer way is a foam pan. That is a factory made piece of rigid foam that is already cut to the right slope. It comes in standard sizes. You set it on the subfloor, glue it down, and put a bonded waterproof membrane over the top. The membrane bonds directly to the foam and to the drain flange. That system is faster than a mortar bed because you do not have to mix and shape mud. It is also lighter and thinner. But it only works if your shower is a standard size and your drain is in the right spot. If the shower is an odd shape or the drain is off center, we have to cut and shim the foam or go back to a mortar bed.

• A mortar bed takes a full day to set up and another day to cure before you can waterproof over it.
• A foam pan goes in in a couple of hours, but the bonded membrane has to be rolled with a heavy roller to get full contact.
• Both systems get a final slope check with a level and a straightedge before tile starts.
• The drain type matters. A clamping drain works with a traditional mortar bed. A bonding flange drain works with a bonded membrane system. They are not interchangeable.

The curb and what happens at the edge of the pan

The curb is the step you step over to get into the shower. It has to be waterproof too. On a mortar bed shower we build the curb out of stacked mortar or concrete block and cover it with the same waterproof membrane that goes under the pan. The membrane runs up the inside of the curb, over the top, and down the outside face. That way any water that splashes onto the curb runs back into the shower. It does not soak through the curb and rot the framing on the outside.

On a foam pan shower the curb is usually a prefabricated foam piece that gets the same bonded membrane over it. The membrane wraps the curb the same way. You do not want a gap between the pan membrane and the curb membrane. That gap is a leak path. We overlap the membrane at the corner and seal it with the manufacturer's recommended seam tape. Then we tile over it. The tile on the curb is usually a small format tile, like a mosaic or a two inch by two inch, because the curb is narrow and has corners that need to be wrapped. Large format tile on a curb tends to crack at the corners because the curb is a weak point in the structure.

Mosaic versus large format tile on a sloped floor

You can put any tile on a sloped floor as long as the tile is small enough to follow the slope. Mosaic tile, the kind that comes on a mesh sheet with one inch by one inch pieces, conforms to the shape of the mud bed or foam pan. The grout lines between each tiny piece act like little drains. Water runs through the joints and down to the membrane. Large format tile, say twelve inches by twenty four inches, does not bend. If you set a big tile on a sloped floor, the tile has to sit flat. The slope has to be built into the mud bed under the tile. That is harder to do because the mud bed has to be perfectly flat in one direction and sloped in another. It is possible but it takes more skill and more time.

If you want large format tile on the shower floor, we have to talk about it before we start. We might need to use a different drain or a different thickness of mud bed. We also have to think about the pattern. A running bond pattern on a sloped floor can look bad because the long edge of the tile does not follow the slope. A stacked pattern or a diagonal pattern works better. We have done it both ways. It depends on the tile size, the drain location, and the shape of the shower.

What happens if the pan is not done right

A pan that does not slope will hold water. That water will find the lowest spot in the grout and start to wick into the mortar bed. Over time the mortar bed stays wet. The waterproof membrane underneath is there to catch that water and send it to the drain, but if the membrane was not installed right, the water goes through the floor. You get a wet spot on the ceiling below or a musty smell in the bathroom. You might not see it for a year. By then the grout is loose and the tile is starting to pop off.

The only fix is to tear out the floor and start over. There is no way to patch a bad pan. You cannot inject grout or caulk to fix a slope that is not there. That is why we check the slope three times: once when the mud bed is still wet, once after it cures, and once after the waterproof membrane is down. We do not tile until we are sure the water will run to the drain. If you call us to look at a shower that is holding water, we will tell you the truth. It needs to come out. We can quote you a straight price after we look at the room and measure the actual space.