How to Make a Shower Pre-Slope

Construct the frame for the shower pan. Remove any materials necessary to expose the subfloor in the area. Measure the opening at the shower threshold where the curb will be placed, transfer this measurement to 2-by-4s and cut three lengths of 2-by-4 to this size. Stack the 2-by-4s at the threshold and secure them to floor joists and wall studs using 16d galvanized nails. Measure the stud bays around the shower perimeter, then transfer this measurement to 2-by-10s. Cut the 2-by-10s to this size and install a section of 2-by-10 in each stud bay to form a low wall around the shower.

Staple a section of building paper to the subfloor. It should be cut to fit the floor, with a small hole to expose the drain opening.

Glue the bottom piece of the shower drain into the drain opening using PVC cement. Unlike standard drains, shower drains consist of three pieces. The bottom two are held together by drain bolts. Remove the top two sections before installing the lowest portion. Stuff a rag into the drain opening. This will keep loose mortar from falling into the drain opening when the mortar layer that establishes the slope is placed.

Mark the height of the drain flange on the wall using the level and carpenter's pencil.

Determine the total rise that will be required from the floor. Measure the distance, in feet, between the center of the drain opening and the wall farthest from the drain. The floor should rise at least 1/4 inch per foot. So, for example, if the distance between the drain and the farthest wall is 4 feet, the total rise required is 1 inch.

Draw a line around the shower perimeter that reflects the total required rise. On the wall, measure the height of the rise up from the marked height of the shower drain and mark this point. Use a level to guide the creation of an even line on the wall around the entire shower.

Cut a section of metal lath to fit on the floor. Cut a hole for the shower drain, leaving a 1/2-inch gap around the drain piece.

Staple the metal lath to the floor.

Prepare the mortar for the pre-pan. Mix mortar so it has a fairly dry consistency. If desired, prepare the mortar with a polymer additive to improve strength or other characteristics.

Pack the mortar into the metal lath using a trowel. Build it up between the drain and the line on the wall so it roughly reflects the planned slope.

Run a straightedge or level over the mortar. Keep one end on the shower drain and extend the other end to the line drawn on the wall. Identify any high or low spots and remove excess mortar, or fill in low spaces as needed. Move the end of the straightedge around the shower's perimeter to identify and correct high and low spots throughout the entire shower.

Finish the surface of the mortar pre-pan by running a float over the surface to make it smooth. Let the mortar cure for an adequate amount of time, usually about a day, before continuing shower construction.