Installing a shower waste in a concrete floor is a critical step in any wet room or bathroom renovation project. Done correctly, it ensures a waterproof, stable surface that prevents water damage and maintains the integrity of the structure for years. Done incorrectly, it can lead to persistent leaks, mold growth, and costly repairs down the line.
The process requires precision and planning, particularly because concrete is an unforgiving material. Unlike timber joists, you cannot simply cut into a solid slab without a strategy. This guide walks through the essential steps, from initial planning to the final waterproofing seal, to ensure a successful installation.
Planning the Drain Location
Before any cutting begins, the position of the waste outlet must be determined with absolute accuracy. Unlike standard bathtubs which have a designated waste location, a shower on a concrete slab often allows for more flexibility, but this freedom requires strict adherence to plumbing codes.
You must consider the final floor finish. The waste body will sit slightly below the finished surface, so you need to calculate the total height occupied by the shower tray or the slope and tile layer. The center of the waste body should align with the planned position of the shower drain. Mark this spot clearly on the concrete using a marker and a straightedge to ensure the flange sits perpendicular to the flow of water.
Creating the Structural Opening
With the location marked, the next phase involves cutting through the concrete to create the opening for the waste body. For a standard residential slab, a core drill is the most efficient and cleanest method. Using a diamond-tipped core bit, drill a hole large enough to accommodate the outer diameter of the pipe.
If the installation is in an upper floor with a suspended slab, you must proceed with extreme caution. The goal is to remove the minimum amount of concrete necessary to fit the waste body without compromising the structural integrity of the slab. Drill a pilot hole for your main cut and use a demolition hammer for the initial breakthrough, followed by a rotary hammer for cleaner edges.
| Tool | Best For |
|---|---|
| Core Drill | Clean, precise holes for main pipe entry. |
| Rotary Hammer | Breaking through concrete to create the main cavity. |
| Angle Grinder with Diamond Blade | Trimming and smoothing the edges of the cut. |
Preparing the Internal Framework
Once the hole is cut, you will see a void where the concrete used to be. To support the weight of the future shower and prevent cracks in the waterproofing membrane, you need to build a supportive framework. This is often referred to as creating a "mud sill" or support box.
Cut timber boards (pressure-treated timber is ideal for damp environments) to line the interior walls of the cut concrete. These boards provide a solid nailing surface for the waterproofing membrane and create a contained space for the slope mortar that will direct water toward the waste body. Secure these boards deeply into the surrounding concrete structure using appropriate masonry fasteners.
Installing the Waste Body and Slope
With the framework in place, you can install the main waste body into the concrete opening. Ensure the flange is level and the outlet pipe is positioned correctly to connect with the main drainage stack. Use a non-shrink cementitious grout or heavy-duty sealant to secure the body tightly to the concrete, preventing any movement that could stress the waterproofing later.
After the body is set, the floor must be sloped. A minimum slope of 1% (10mm per meter) is generally required to ensure water flows efficiently toward the waste. Mix a slope mortar specifically designed for waterproofing and build up the floor from the edges, directing the flow perfectly toward the cut-out waste body. Allow this mortar to cure completely before proceeding to the next step.
Waterproofing and Finishing
The most crucial step in protecting the concrete is applying a seamless waterproofing membrane. Roll out a waterproofing sheet (like TPO or PVC) or apply a liquid rubber membrane that bonds directly to the slope mortar. Pay special attention to the edges, heating the material to weld it tightly to the waste body and the surrounding walls.
Create a "curtain wall" by applying a continuous bead of waterproof sealant up the sides of the waste body where it meets the membrane. This prevents water from migrating upward behind the fixture. Finally, reinstall the cover plate and test the system with a water leak test before laying the final floor tiles.
