A floor sink is a recessed drain receptor, typically set flush with the finished floor, that collects indirect waste from commercial kitchen equipment, ice machines, dishwashers, and similar fixtures. Installing one requires cutting into a concrete slab, connecting to the building’s drain system, and ensuring the finished assembly meets plumbing code requirements for indirect waste. The job involves concrete work, plumbing, and waterproofing, so it’s best suited for experienced DIYers or licensed professionals.
How a Floor Sink Differs From a Floor Drain
A floor drain sits flush with the floor and accepts water that flows across the surface. A floor sink is a deeper, open-top receptor designed to receive indirect waste. Equipment like a prep sink or walk-in cooler drains into the floor sink through a pipe that terminates above the rim, creating a visible air gap. That air gap prevents contaminated water from backing up into clean equipment. The most common size for commercial applications is 12 inches by 12 inches square with an 8-inch sump depth, though 6-inch deep models are also available.
Choosing the Right Size and Location
Before you buy anything, identify every fixture that will drain into the sink and check the total flow rate. Floor sinks come in various outlet sizes to handle different volumes, and undersizing will cause backups. A 12″ x 12″ receptor with an 8-inch sump depth handles most commercial kitchen applications.
Location matters just as much as size. The sink needs to sit close enough to the equipment it serves so discharge pipes can maintain a downhill slope, but far enough from foot traffic to avoid tripping hazards. You also need to verify there’s a viable path to connect the sink’s outlet to the building’s sanitary drain line below the slab. If you’re unsure about existing underground plumbing, have the slab scanned or x-rayed before cutting.
Cutting and Preparing the Concrete Slab
The most labor-intensive part of the job is opening the slab. You’ll need a concrete saw to cut a rectangular section large enough to fit the sink body plus room for the drain connection underneath. Mark the cut lines on the floor, allowing at least a few inches of clearance on each side of the sink. After cutting, break out the concrete with a demolition hammer and excavate the soil below to the depth needed for the sink body, its outlet pipe, and the connecting drain line.
If you’re pouring a new slab, the easier approach is to box out the opening before the pour. Set the floor sink and its PVC drain pipe in position first, brace them securely, then pour the concrete around the assembly. This avoids the mess and risk of cutting through an existing slab. Either way, the drain pipe below the sink must slope at a minimum of 1/4 inch per foot toward the building’s main drain.
Setting the Sink Body
With the opening prepared, dry-fit the sink to confirm depth and alignment. The top flange of the sink body should sit at the level of your finished floor surface so the rim ends up flush. If you’re working with an existing slab, the initial concrete backfill around the sink body should come up to the level of that top flange.
Connect the sink’s outlet to the drain line below using the appropriate PVC fittings. Make sure the connection is fully cemented and given time to cure before backfilling. Then pour concrete around the sink body, packing it firmly to eliminate voids. Vibrate or tamp the concrete to prevent settling later. The sink needs to be level in all directions so water flows evenly toward the outlet rather than pooling on one side.
Waterproofing the Flange
In installations where a waterproofing membrane is present (common in commercial kitchens and any space where moisture protection matters), the membrane must extend up to and over the sink’s top flange. This creates a continuous waterproof barrier that prevents water from seeping between the sink and the surrounding concrete.
Once the membrane is in place over the flange, a clamping collar or combination flashing clamp is set on top and secured with bolts. This sandwiches the membrane between the clamp and the flange, creating a watertight seal. Some sink models use a strainer frame that doubles as the membrane clamp at the finished floor surface. Skip this step and you’ll eventually get water migration under the slab, leading to erosion, odor, and structural problems.
Air Gap Requirements
The defining feature of a floor sink installation is the air gap. Every indirect waste pipe that terminates above the sink must maintain a minimum vertical distance of 1 inch between the lowest point of that pipe and the flood-level rim of the sink. This measurement is specified in the Uniform Plumbing Code and is non-negotiable for inspections.
In practice, most installers leave 1 to 2 inches of clearance. The pipe simply ends above the open sink, and water falls through the air into the receptor. No direct connection is made between the discharge pipe and the sink. That visible gap is the whole point: it’s a physical barrier that makes it impossible for sewage to back up into the equipment above.
Installing the Strainer or Sediment Basket
Every floor sink needs an internal strainer or sediment basket to keep solids out of the drain line. Which one you use depends on what’s flowing into the sink.
- Dome strainer: A rounded, perforated cover that sits over the outlet opening inside the sump. It catches large debris while allowing water to pass through freely. Suitable for clear-water waste from condensate lines, ice machines, or similar equipment.
- Sediment basket: A removable metal basket that sits inside the sump and collects food particles, grease, and other solids before they reach the drain. Required when the sink receives anything other than clear water, which includes most commercial kitchen applications.
The sediment basket needs regular cleaning, often daily in a busy kitchen. A clogged basket causes the sink to overflow, which defeats the entire purpose of the installation. Make sure the basket fits snugly inside the sump and is easy to lift out for emptying.
Finishing the Floor Around the Sink
The finished floor surface needs to slope gently toward the sink from all directions. A slope of about 1/4 inch per foot in the immediate area around the sink ensures spills and wash-down water flow into the receptor rather than pooling nearby. Use a level and screed to get this right when pouring the final concrete topping or setting tile.
If the floor finish is tile or epoxy, bring it right up to the edge of the sink flange for a clean, seamless transition. Any gap between the floor finish and the sink rim becomes a place for bacteria and moisture to collect. In food service environments, the health inspector will flag visible gaps or uneven transitions around floor sinks.
Once everything is set, run water into the sink from the discharge pipes to confirm proper drainage, check the air gap measurement, and verify the strainer or basket is in place. Fill the trap by running water for a minute to establish the water seal that blocks sewer gas from entering the space.