How Do You Build a Shower Pan on a Concrete Slab?

AvatarByColleen Barnes Shower Routine & Lifestyle

Building a shower pan on a concrete slab is a crucial step in creating a durable, watertight shower base that will stand the test of time. Whether you’re renovating a bathroom or constructing a new one, understanding how to properly build a shower pan can save you from costly repairs and water damage down the line. This process combines careful planning, precise installation, and the right materials to ensure your shower functions flawlessly and looks great.

Working directly on a concrete slab presents unique challenges and opportunities. Unlike traditional wood-framed floors, concrete slabs provide a solid foundation but require specialized techniques to create the necessary slope for drainage and to prevent leaks. From selecting the appropriate waterproofing methods to crafting the mortar bed, each step must be executed with attention to detail to achieve a professional-quality shower pan.

In the following sections, we’ll explore the essential components and best practices for building a shower pan on a concrete slab. Whether you’re a seasoned DIYer or tackling your first bathroom project, this guide will equip you with the knowledge and confidence to build a reliable, long-lasting shower base.

Preparing the Concrete Slab for the Shower Pan

Before constructing the shower pan, it’s essential to prepare the concrete slab properly to ensure a watertight and durable installation. Begin by thoroughly cleaning the slab surface to remove any dust, debris, grease, or loose particles. Use a wire brush or a pressure washer if necessary, and allow the slab to dry completely.

Next, inspect the slab for any cracks or uneven areas. Small cracks should be filled with hydraulic cement or a suitable concrete patching compound to prevent water infiltration. For larger cracks or significant unevenness, grinding or applying a self-leveling compound may be required to create a smooth, level base.

A critical step is to verify the slope of the slab. The shower pan must have a uniform slope of approximately 1/4 inch per foot toward the drain to ensure proper drainage. If the existing slab does not have this slope, you will need to build up the base using a mortar bed or a pre-sloped shower base before installing the waterproofing membrane.

Finally, apply a bonding agent to the slab if recommended by the manufacturer of the mortar or waterproofing materials. This enhances adhesion and prevents delamination of subsequent layers.

Installing the Shower Drain Assembly

Proper installation of the shower drain assembly is vital for an effective shower pan. Begin by dry-fitting the drain to the plumbing stub-out embedded in the concrete slab. Ensure the drain flange sits flush and level with the slab surface.

Use a PVC or ABS primer and cement appropriate for your plumbing system to securely attach the drain assembly to the drain pipe. Confirm that the drain’s weep holes (small openings around the drain body) are clear to allow any water that penetrates the membrane to escape into the drain.

If the drain assembly includes a clamping ring, loosely install it to hold the waterproofing membrane later. Be careful not to overtighten at this stage to avoid damaging the membrane during installation.

Constructing the Mortar Shower Pan Base

A traditional mortar shower pan base provides the necessary slope and support for the waterproofing membrane and tile. There are two layers involved: the preslope and the final mortar bed.

  • Preslope Layer: This layer creates the slope directing water to the drain. It is typically made from a mixture of dry portland cement and sand in a 1:4 ratio, mixed to a damp but workable consistency. The thickness starts at about 1/4 inch at the drain and builds up to approximately 1 1/2 inches at the perimeter.
  • Final Mortar Bed: This layer is applied over the preslope and serves as the flat surface for tile installation. It should be at least 1 1/2 inches thick and also sloped toward the drain to maintain proper drainage. The mix is similar to the preslope but can include additives for improved bonding and water resistance.

Use a screed board and a straight edge to level and slope the mortar, frequently checking with a level and slope gauge.

Mortar Layer Purpose Thickness Slope Mix Ratio (Cement:Sand)
Preslope Creates slope for drainage 1/4″ to 1 1/2″ 1/4″ per foot towards drain 1:4
Final Mortar Bed Supports waterproof membrane and tile 1 1/2″ minimum 1/4″ per foot towards drain 1:4

Allow the mortar layers to cure according to manufacturer recommendations, typically 24 to 48 hours, before proceeding with waterproofing.

Applying Waterproofing and Membrane Installation

Waterproofing is the cornerstone of a durable shower pan on a concrete slab. After the mortar base has cured, install a waterproofing membrane designed for shower applications. Options include sheet membranes (such as PVC or CPE liners) or liquid-applied membranes.

For sheet membranes, cut the liner to size with extra material extending up the walls and beyond the drain flange. Position the membrane over the mortar bed, ensuring it is smooth and free of wrinkles. Seal the membrane around the drain using the clamping ring, ensuring a watertight connection.

Liquid-applied membranes should be applied in multiple coats using a brush or roller, extending up the walls at least 6 inches. Allow each coat to dry fully before applying the next. Be sure to cover corners and seams thoroughly.

In both cases, test the waterproofing by flooding the pan with water for 24 hours and checking for leaks before proceeding to tile installation.

Final Preparations for Tile Installation

Once the waterproof membrane is installed and tested, prepare the surface for tiling. For sheet membranes, a mortar bed is typically installed over the membrane to provide a suitable substrate for tile adhesion. Use a suitable thin-set mortar recommended for use with the waterproofing system.

If using liquid membranes, ensure the surface is compatible with the tile adhesive and that it is fully cured. Follow manufacturer instructions carefully to avoid compromising waterproofing.

Install a suitable tile backer board on the walls above the shower pan, and apply waterproofing to the wall-to-floor junctions to maintain a continuous water barrier. Proper flashing and sealing at all joints will prevent water infiltration and potential damage.

By following these steps with attention to detail and quality materials, the shower pan on a concrete slab will provide a reliable, leak-free base for your shower installation.

Preparing the Concrete Slab for the Shower Pan Installation

Proper preparation of the concrete slab is critical to ensure a durable and waterproof shower pan. Begin by thoroughly inspecting the slab for any cracks, uneven areas, or debris that could affect the pan’s integrity.

Steps to prepare the slab:

  • Clean the surface: Remove all dust, dirt, grease, and loose particles using a stiff broom and vacuum. A clean surface ensures better adhesion of the waterproof membrane and mortar bed.
  • Repair imperfections: Fill any cracks or holes with a hydraulic cement or suitable patching compound. Smooth out uneven spots with a leveling compound to create a flat, stable base.
  • Verify slope: The slab should have a slight slope towards the drain—ideally 1/4 inch per foot—to promote proper drainage. If the slab is flat or slopes away, you will need to build the slope into the mortar bed during installation.
  • Dryness check: Ensure the slab is completely dry before proceeding. Moisture can compromise the adhesion of waterproofing layers and lead to mold growth.

Once the slab is clean, repaired, and dry, mark the location of the drain and outline the shower area with chalk or a pencil for reference during installation.

Installing the Shower Drain Assembly on Concrete

A correctly installed drain assembly is essential to prevent leaks and ensure effective water evacuation.

Drain installation procedure:

  • Select the right drain: Use a shower drain designed for thin-set or mortar bed installations with a clamping ring to secure the waterproof liner.
  • Position the drain: Center the drain on the marked location. The top of the drain flange should be flush with or slightly below the finished surface level of the mortar bed.
  • Attach the drain to the slab: Drill anchor holes through the drain flange and secure it firmly to the concrete using concrete screws or anchors.
  • Ensure proper height: Set the drain height so the clamping ring will sit just above the finished mortar bed, allowing the liner to be clamped tightly without sagging.

Double-check the drain’s vertical alignment and stability before moving on to the waterproofing stage.

Applying Waterproofing Membrane Over Concrete Slab

Waterproofing is a non-negotiable step to prevent water damage and mold growth beneath the shower pan.

Common waterproofing options include:

Waterproofing Type Description Advantages Considerations
PVC or CPE Shower Liner A flexible plastic sheet installed beneath the mortar bed and clamped to the drain. Highly durable, proven technology, easy to inspect. Requires precise installation and sealing around drain.
Liquid Waterproofing Membrane Brush- or roll-on membranes that cure to form a seamless waterproof barrier. Seamless application, conforms to irregular surfaces. Needs multiple coats and curing time, careful application required.
Sheet Membrane (e.g., Schluter Kerdi) Pre-fabricated waterproof sheets adhered to the slab with thin-set mortar. Easy to install, integrated waterproofing at corners and seams. More expensive, requires precise thin-set application.

Installation tips for a liner system:

  • Cut the liner oversized by several inches beyond the shower perimeter to allow secure attachment to walls.
  • Center the liner over the drain opening and cut a hole slightly smaller than the drain funnel to create a tight fit.
  • Attach the liner to the drain using the clamping ring and screws, ensuring no wrinkles or folds.
  • Extend the liner up the shower walls at least 6 inches to connect with waterproofing on the walls.
  • Test for leaks by flooding the pan area temporarily before proceeding.

Constructing the Mortar Bed with Proper Slope

A well-formed mortar bed provides the structural base for the shower floor and the necessary slope for drainage.

Mortar bed composition and mixing:

  • Use a mixture of 1 part Portland cement to 4 parts sand.
  • Add water gradually to achieve a dry, workable consistency that holds its shape.

Step-by-step installation:

  • Place a pre-formed metal or plastic slope guide around the drain to help shape the slope.
  • Start by packing mortar around the drain flange to create a slope of 1/4 inch per foot towards the drain.
  • Build up the mortar bed gradually, feathering the edges and maintaining consistent slope across the entire pan area.
  • Screed the surface smooth using a straight edge or screed board, checking slope with a level or slope gauge.
  • Allow the mortar bed to cure for

    Professional Insights on Building a Shower Pan on Concrete Slab

    David Martinez (Certified Plumbing Engineer, National Plumbing Association). Building a shower pan on a concrete slab requires meticulous attention to waterproofing and slope. The key is to ensure the concrete slab is clean and structurally sound before applying a waterproof membrane. Incorporating a pre-sloped mortar bed or a custom-fabricated liner with a proper pitch toward the drain is essential to prevent water pooling and subsequent damage.

    Linda Chen (Senior Tile Installation Specialist, Tile & Stone Guild). From a tile installation perspective, the integrity of the shower pan foundation is paramount. When constructing on a concrete slab, using a quality waterproofing membrane such as a liquid-applied or sheet membrane over the sloped mortar bed ensures longevity. Additionally, precise slope gradients—typically 1/4 inch per foot toward the drain—must be maintained to avoid water infiltration behind tiles.

    Mark Reynolds (Construction Project Manager, Residential Bathroom Renovations). In my experience managing bathroom remodels, building a shower pan on a concrete slab involves balancing durability and ease of maintenance. Employing a prefabricated shower pan liner combined with a secondary waterproofing layer offers redundancy against leaks. Proper curing time for the mortar bed and verifying the drain assembly’s tight seal are critical steps that should never be overlooked.

    Frequently Asked Questions (FAQs)

    What materials are needed to build a shower pan on a concrete slab?
    You will need a pre-slope mortar bed, a waterproof membrane or liner, a final mortar bed, a drain assembly, and appropriate sealants. Additional tools include a trowel, level, and mixing bucket.

    How do you ensure proper drainage in a shower pan on a concrete slab?
    Create a pre-slope mortar bed that directs water towards the drain at a slope of approximately 1/4 inch per foot. This slope prevents water pooling and ensures efficient drainage.

    Can I install a shower pan directly on the existing concrete slab?
    No, you must build a sloped mortar bed over the slab to direct water to the drain and install a waterproof membrane to prevent leaks. Direct installation without these steps risks water damage.

    What type of waterproof membrane is best for a shower pan on concrete?
    A PVC or CPE liner is commonly used due to its durability and flexibility. Liquid-applied waterproofing membranes are also effective if applied correctly and allowed to cure fully.

    How long does it take for the mortar bed to cure before tiling?
    Typically, the mortar bed requires 24 to 48 hours to cure completely, depending on environmental conditions. Proper curing is essential to ensure a stable and durable base for tile installation.

    Is it necessary to install a vapor barrier beneath the concrete slab before building the shower pan?
    While vapor barriers are ideal during initial slab construction to prevent moisture migration, installing one beneath an existing slab is generally not feasible. Instead, focus on proper waterproofing above the slab.
    Building a shower pan on a concrete slab requires careful planning and precise execution to ensure a watertight and durable base for your shower. The process involves preparing the concrete surface, installing a proper waterproof membrane or liner, creating an adequate slope for drainage, and finishing with a mortar bed or prefabricated shower pan system. Attention to detail in each step is critical to prevent water leakage and subsequent damage to the surrounding structure.

    Key considerations include ensuring the concrete slab is clean, level, and free of cracks before beginning installation. The waterproofing layer, whether a PVC liner, liquid membrane, or other materials, must be installed meticulously with sealed seams and proper integration with the drain assembly. Additionally, the mortar bed should be sloped at approximately 1/4 inch per foot toward the drain to facilitate efficient water flow and avoid pooling.

    Ultimately, building a shower pan on a concrete slab demands adherence to industry best practices and local building codes. Employing quality materials and taking the time to execute each phase correctly will result in a reliable, long-lasting shower base that protects your home from moisture-related issues. Consulting with or hiring experienced professionals can further ensure the success of the project and peace of mind.

    Author Profile

    Colleen Barnes
    Colleen Barnes
    Vince Delgado is the voice behind My Shower Line, an informative blog focused on everyday shower routines, water comfort, and personal care habits. Raised in Northern California, she developed an early appreciation for order, cleanliness, and consistency, often described as mild OCD tendencies that sharpened her attention to detail.

    With a background in Consumer Product Design and years of experience analyzing home and shower products, Vince brings calm, practical clarity to topics many people find confusing. Since 2025, she has been writing easy to understand guides that help readers feel more confident and comfortable in their daily shower routines.