How to Build a Shower Mudpan

Remove any materials necessary to expose the subfloor.

Build the curb at the shower threshold and frame the perimeter. Measure the threshold opening between the wall studs and cut three lengths of 2x4 to this size. Stack the 2x4s in the threshold and attach them to the floor joist and studs at the threshold using 16d galvanized nails. Measure the spaces between the studs, cut 2x10s to this size and in stall them at the bottom of the stud bays to create a low wall around the perimeter.

Staple a section of building paper to the subfloor in the shower.

Glue the bottom piece of a three-piece shower drain into the drain opening using PVC cement. Stuff a rag into the drain opening to prevent any loose mortar from accidentally falling into the drain.

Plan for the establishment of the slope. The pre-pan layer should create a rise of at least 1/4 inch per foot between the drain and wall. Mark the height of the bottom drain piece on the wall. Measure the distance between the center of the drain and the farthest wall and calculate the total required rise. Measure the distance of the total rise up the wall from the height of the drain and mark this point. Use a level to draw a line around the shower perimeter at this height.

Staple metal lath to the subfloor over the building paper. Cut it to fit the floor space and leave a 1/2-inch gap around the drain.

Prepare the mortar, or mud, for the pre-pan layer. Mix thinset mortar so that it has a fairly dry consistency using a polymer additive to increase strength, if desired.

Pack in the mud for the pre-pan using a trowel. Build the mud up between the drain and the line drawn around the wall. Hold a level or straightedge between the drain and the line on the wall around the perimeter to locate any low spots. Correct any inconsistencies and smooth the surface by running a float over the mud. Let the mud cure adequately before continuing pan construction.

Measure and cut waterproofing membrane to cover the shower floor and curb. The membrane should extend 8 inches up each wall and up and over the curb. Lay the membrane where it will be placed and cut slits for the drain bolts and an opening for the drain. Cut the material for the curb at the wall studs so it will fit over the curb easily.

Attach the membrane to the underlying material. Use an adhesive suitable for use with the specific membrane material and glue it down, working from the drain outward. For added security, staple the membrane to the underlying wood but only place staples above the top of the curb and on the top and outside of the curb.

Glue dam corners into the inside corners of the curb to provide reinforcement.

Attach the middle and top drain pieces. Install the middle drain piece using the drain bolts and tighten them to create a watertight seal. Screw in or otherwise insert the top drain piece, the strainer, and adjust its height to reflect the planned depth of the mortar bed plus the thickness of the tiling or other surfacing that will be placed.

Measure the planned height of the mortar bed up the wall, mark this point and draw a line around the perimeter at this level.

Place enough mud in the shower pan to build up the bed to about half of its thickness.

Cut a section of metal lath to fit in the pan, leaving a gap around the drain, and press it into the mud.

Cover the metal lath and build up the mud bed to the line drawn on the wall. Use a level or straight edge to check for unevenness and move the mortar around as needed. Smooth the mud surface with a float. Let the mud cure and finish the pan with the desired surfacing.