How to Install Mortar Shower Base

What is a Mortar Shower Pan Base?

A mortar shower pan base consists of a thick yet flexible PVC membrane liner that is sandwiched between two layers of mortar installed in the base of the shower stall.

The first layer of mortar, known as the pre-slope layer, consists of a sloping layer of concrete mortar that varies from about 1/4 inch thick at the adjustable height shower drain to about 1 to 1.25 inches at the shower walls, depending upon the distance from the drain to the shower walls. It gets thicker toward the shower walls as the shower size gets larger.

Embedded in the mortar shower pan base is steel mesh to add reinforcement to the concrete mortar. The purpose of the pre-slope is to enable water to flow towards the drain on the finished tiled shower floor. It also enables water to flow to the drain if the membrane liner was ever to spring a leak, which is nearly impossible to happen if it was installed correctly and with care.

The thick PVC shower pan membrane liner is then installed over the pre-slope mortar layer. A hole is cut in it at the shower drain and the sides of the liner are folded into the shower corners.

The liner is held in place with roofing nails near the top of the liner, which should sit approximately 6 inches above the shower floor. The roofing nails are nailed into the framing lumber that represents the base of the shower area. A final layer of concrete mortar is then poured over the shower pan membrane liner to complete the mortar shower pan base installation. Note that pea size stones are placed right around the perimeter of the adjustable height drain prior to pouring the final layer of mortar.

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This prevents the weep holes associated with the adjustable drain assembly from getting clogged by the mortar. 

The weep holes enable water that may get through the tile and mortar base and down to the membrane liner to drain out the adjustable shower drain assembly. Note that when pouring the final layer of mortar it is important to main the pre-slope pitch towards the drain.

Once the final mortar layer has been poured it is important to adjust the height of the adjustable drain assembly so that the top of it will be flushed with the tiled shower floor surface. You can adjust the drain height by simply turning the top portion of the adjustable drain assembly with your hand.

This needs to be done while the concrete is still uncured. Once the mortar shower base has had a couple of days to fully cure the shower can then be tiled.

One final note. It is important during a mortar shower pan base installation that the subfloor be extremely solid with little to no deflection. When installing a mortar pan shower base on a wood subfloor, there should be a layered approach with the subfloor plywood. For example, exterior grade ½ inch plywood should be installed over the base ¾ inch plywood subfloor. Make sure that both layers of plywood are screwed tightly to each other and the floor joists.

This will help to minimize any deflection so that there is no risk of the ceramic floor tiles becoming loose and the mortar shower base failing.