How to Slope a Shower Floor With Mortar

Updated on February 17, 2019
Bert Holopaw profile image

Bert spent 25 years working as a home-improvement and residential construction contractor in central Florida.

Some shower remodel projects, such as turning a tub into a standup shower, turn into a nightmare when the tile installer failed to create a properly sloped shower floor. A shower floor without enough pitch usually puddles and holds water. Soap scum and water deposits form in these wet areas, which makes the tile and grout look dirty. On the other end of the spectrum, the tile on a floor with too much pitch looks and feels uneven.

A properly sloped shower floor never puddles or holds water.
A properly sloped shower floor never puddles or holds water. | Source

Inspect the Shower Floor Membrane

Always verify the shower floor membrane holds water prior to starting this project. A finished shower floor's tile, grout and caulking directs most of the water toward the drain opening. However, over time small cracks often form around the drain opening and shower walls. The shower floor membrane, sometimes called a pan, catches any water that slips past the tile before it leaks into an adjacent room. A simple water test checks the pan's integrity.

1) Unscrew and remove the top portion of the shower drain from its base. Set the top aside.

2) Insert an expandable plug into the drain opening. The plug must extend down below the base's threaded section.

3) Fill the shower pan with a couple inches of water. Wait several hours before proceeding. This allows enough time for small leaks to soak the subfloor under the pan.

4) Inspect the outside perimeter of the shower for seepage. If the leak test exposed any signs of leakage, replace or repair the membrane. Some local building codes require licensed plumbers for this.

5) Remove the plug and drain the membrane. The shower floor project should start only after the membrane passes a leak test.

Prepare the Shower Drain

1) Assemble the drain fitting. The shower drain cap screws into the base.

2) Adjust drain height until the bottom measures approximately 1.5 inches from the base. Turn the drain clockwise to lower and counterclockwise to raise. This normally leaves enough room for the mortar to form a solid subfloor around the drain opening without compromising the slope from the furthest wall.

3) Protect the surface of all drain grates with masking tape. The tape must not extend past the finished surface. The tape protects the finished surface from scratches and prevents mortar from falling into the drain opening.

4) Cover the base's weep holes with small stones or leftover debris from the original floor's removal; small pieces of broken tile works well for this. The weep holes let the accumulated moisture resting on the pan enter the drainage system.

Notice the wall boards do not extend down to the bottom of the shower membrane.
Notice the wall boards do not extend down to the bottom of the shower membrane. | Source

Prepare the Collar and Curb

Some tile installers like to run the wall boards down to the bottom of the shower membrane. Unfortunately the folds in the shower membrane material normally push against the bottom of the wall boards, creating out-of-plumb walls which leads to uneven tile cuts along the corners. To avoid this the tile installer should stop the wall boards approximately 4 to 6 inches above the bottom of the membrane and fill the resulting space with a mortar collar. The collar forms a solid plumb surface for the bottom row of wall tile to adhere against.

1) Measure the distance from the pan to the bottom of the wall boards. Add one inch to this measurement and transfer the total to a piece of wire lathe. Cut the lathe with tin snips at the appropriate location. Measure the length of a wall and cut a piece of lathe at this length. The lathe gives the mortar something to grip to. Without lathe the mortar tends to sag and fall away from the membrane.

2) Tuck the top one inch of a piece of lathe between the bottom of the wall board and the membrane. Repeat this process for each remaining wall.

3) Measure the vertical side of the curb. Add approximately two inches to this measurement. Transfer this total to the lathe and cut at the correct location.

4) Fold the lathe strip into a "7" shape, keeping the long side of the fold the same length as the curb's vertical side. The "7" shape serves two purposes: the long side keeps the pan material tight against the curb and the short side provides a leak-free mounting location.

5) Mount the folded lathe onto the curb. Push the corner of the bend tight against the curb and secure the lathe's horizontal side to the curb with roofing nails.

6) Measure the curb's length and transfer the measurement to a 1x4 board. Cut the board approximately 1/8-inch shorter than the measurement.

7) Position the board on top of the curb with its inside edge overlaping the lathe approximately 1/2 inch. Tack the board in place with a couple nails or screws.

Add just enough water to the mortar mix to activate the cement.
Add just enough water to the mortar mix to activate the cement. | Source

Make the Mortar

The mortar used to pack a shower floor consists of a mixture of Portland cement, sand and water. Some contractors working on large projects buy the dry ingredients separately and mix them on the job. It often makes sense to buy pre-mixed mortar for smaller jobs. I prefer to use Sakrete Sand Mix for this application. The type and percentage of sand in this product works especially well when packing the curb and collar. However, any type-S mortar will work.

1) Pour the dry ingredients into wheelbarrow or other flat surface. The amount of ingredients depends on the shower's size and the depth of the mortar bed. A 3-foot by 4-foot shower uses approximately one 80-pound bag of Portland cement and 10-gallons of coarse sand, or two 80-pound bags of pre-mixed mortar. Expect to discard a small amount of leftover material. Thoroughly mix the dry ingredients with a shovel.

2) Push the mixture into a pile and create a bowl in the top, creating a volcano shape.

3) Slowly pour approximately 1/2-gallon of water into the crater.

4) Work the water into the mortar mix with a shovel. Start with stabbing motions in the center of the pile. The stabbing motions create a pathway for the moisture to reach the bottom of the pile. Once the moisture activates the center, scoop some dry mortar mix from the outside edge of the pile and place it in the crater. Work the dry mix into the crater. If the mixture needs additional water, add 1 cup to the crater. Continue to stir the dry mix into the crater until the entire pile changes color from a light grey to a dark grey. When complete the mortar mix should hold its shape when squeezed.

Mud the Collar and Curb

1) Distribute about 10 shovel fulls of mortar mix around the perimeter of the shower pan.

2) Force the mortar mix into the wire lathe with a grout float, or other similar tool. Continue to fill the void below the wall board until the mortar starts to bulge out into the shower area. Start at one corner and work around the shower.

3) Shave the excess mortar from the collar and curb with a straight edge, using the wall boards as a guide to keep the collar plumb. Cover the stones protecting the drain fitting's weep holes with the excess mortar.

4) Clean any leftover mortar from the wall boards with a damp sponge. Failure to clean the wall boards often leads to raised out-of-plumb wall tile.

Prepare the Shower Pan for Mortar

1) Measure from the drain opening to the furthest wall, then round up to the nearest foot. Calculate the slope by multiplying the measurement by 1/4 inch. For instance, if the drain opening measures 2-foot 8-inches from the furthest wall, then the shower floor should slope 3/4 inch from that wall to the drain.

2) Place a bubble level on the drain opening and extend it to the furthest wall. Adjust the wall's side of the bubble level to match the calculated slope. Place a mark on the collar at the appropriate height. Visually verify the slope does not rise above the curb. If so, lower the drain opening the appropriate amount.

3) Score a line around the shower floor's perimeter at the mark representing the floor's slope, using a bubble level as a guide.

Dry pack the shower floor with either a grout float or a flat trowel.
Dry pack the shower floor with either a grout float or a flat trowel. | Source

Dry Pack the Shower Floor Mortar

1) Distribute about a gallon of mortar mix next to each corner of the shower and in the center of each wall.

2) Form a 2-inch wide mortar shelf around the shower's perimeter, using the scored line in the collar and curb that represents the slope as a height guide. This should form a perfectly level shelf around the perimeter of the shower.

3) Spread one half of the remaining mortar mix across the back half of the shower pan.

4) Compress the mortar with a grout float or flat trowel, working from the shelf toward the drain opening. Tamping the mortar removes air pockets and stiffens the mortar bed. Add extra mortar if dips appear in the bed.

5) Add the remaining mortar to the near side of the pan and compress it until it matches the far side. When complete, the shower floor should pitch roughly from the shelf to the top of the shower drain and contain high spots. Do not expect a perfect finish at the end of this step.


Finish the Shower Floor Mortar

A true craftsman's artistry shows up in this step. A well formed mortar bed makes quality tile installation a breeze. However, a sloppy bed often leads to puddle-causing dips and valleys.

1) Position a piece of cardboard on the mortar bed to kneel on within reach of the furthest wall.

2) Hold one end of a straightedge on one corner of the bed's shelf and the other end next to the shower drain. Move the straightedge across the length of the wall, removing all high spots in the mortar bed. Clean your tools repeatedly. Mortar often sticks to dry or dirty tools.

3) Hold the straightedge on the shelf and shave the mortar bed to achieve the final slope. Avoid removing mortar from the shelf near the shower wall. Remove a small amount with each pass. Discard the debris as the project proceeds. The final slope of the mortar bed should finish approximately 3/16-inch below the top of the shower floor drain. The exact height depends on the tile's thickness.

4) Smooth any rough areas on the floor or walls with a flat trowel. Keep the trowel wet to avoid pulling mortar.

This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.

© 2018 Bert Holopaw


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