The most important part of the entire shower project to get right is absolutely the floor pan!
And, not surprisingly, this is where most do-it-yourselfers, make a mistake, try to make a shortcut, or just flat out get it wrong. Building the floor pan incorrectly can lead to leaking, of course, but also mold growth underneath the tile and mortar bed if a proper “pre-slope” isn’t installed. But, I don’t want to get ahead of myself, I’ll cover all the details how to do this later in the article. Let’s start here: What is a shower pan?
The term “pan” originates from when contractors used to install a copper pan in the base of where the shower was being built.
The use of the copper pan transitioned into using 4 lb sheet lead (weighs 4lbs per sq ft, giving it that name), because it’s very malleable and could be shaped easily on site. A contractor could measure up the size needed, go out to the garage or driveway, bend up the corners as needed with a 2×6 and rubber mallet, folding corners over each other and overlapping so all the edges were at the top. Weighing 4lbs per square foot, it was quite a chunk to lug through the house, but could be done with two guys, and it could be folded in on itself, since it’s so malleable, in order to get through tight hallways and around corners.
Using the rubber mallet again, they would hammer an impression of the drain into the lead, giving a mark to cut out the drain hole. Once the hole was cut out, the drain flange could be attached, making a watertight seal. An adjustable shower drain was then threaded into the flange, and pea gravel placed around the weep holes to protect them from the deck mud that was installed next. Deck mud is a dry cement, wetted just enough to let the cement hold shape, allowing it to be packed in creating the slope needed for water to flow toward the drain.
Here is where many who take on the task of building their own shower, without any experience doing so, go wrong. The pre-slope is a slight slope of the floor draining toward the shower drain, created with dry-pack cement before the shower pan is installed. Despite what you may think, water can and will penetrate all the way through the tile, mortar and concrete above the shower pan, making its way down to the shower pan. In the absence of a pre-slope below the shower pan, the pan will be flat on the floor surface, keeping any of that water in the concrete from percolating down and into the weep holes of the shower drain. When the concrete remains moist, mold growth will occur over time, eventually causing considerable damage.
To create the pre-slope on a plywood surface you must first lay down a layer of felt paper (isolates concrete from floor movement), then staple down a layer of Metal Lath. Mix cement with enough water to get it to hold shape, and pack it down creating a slope from 1/8″ thick at the drain, up toward the shower edge at a slope of about 1/4″ per foot. On a concrete floor, the felt paper is not needed, concrete can be directly applied to floor.