How to Choose a Floor Gully Drain
Choose a floor gully by matching its rated flow, outlet and trap arrangement, grate, and waterproofing connection to your fixtures, plumbing code, floor structure, and finish height. Then build a stable, continuously sloped floor around it. A suitable drain is only one part of a tested assembly; no grate or drain alone can prevent hidden leaks.
A drain that looks right can still be wrong for the job. If it cannot handle the fixture’s discharge, connect to your chosen waterproofing system, or sit securely at the finished floor height, water may pool or escape below the tile. I treat drain selection as a coordination task: plumbing, structure, slope, membrane, tile, and grate must work together.
The details matter in a shower, laundry room, or other wet area. Before buying, check the drain maker’s installation instructions and confirm local plumbing and accessibility rules. When a project changes the structure or plumbing, involve the appropriate qualified trades.
Inspect the substrate, framing, and existing plumbing
Start by identifying what supports the floor and what is already below it. The slab or framed floor must be sound, the drain must connect to suitable plumbing, and the finished assembly needs enough height for its slope and waterproofing. Resolve movement, moisture damage, and conflicts before setting the drain.
I begin by recording the proposed drain location, outlet route, fixture discharge, and finished floor height. Locate utilities before cutting. On concrete, check for cracks, damage, moisture concerns, and whether the drain can be set at the required elevation. On a framed floor, check the joists, blocking, and sheathing for damage and stiffness.
Tile needs a stable base. For ceramic or porcelain, the design deflection is generally at least L/360; natural stone generally requires L/720. These ratings describe how much a floor can bend under load. If joists or the subfloor do not meet the needs of the finish, correct the structure first. An uncoupling membrane cannot make a weak floor structurally sound, and movement at the drain can break tile or the waterproof connection.
Check the existing pipe size and route, trap location, venting, and access for service. A drain outlet’s diameter does not prove that it can handle the fixture flow. If the drain location or pipe route must change, have the plumbing plan checked against local code before work begins.
Select the drain and execute the sloped, waterproofed installation
Choose a drain whose documented flow rating, plumbing connection, and waterproofing detail suit the project. Then set it securely at the right height and form a continuous slope into it. The membrane-to-drain connection must follow the drain maker’s approved detail; similar-looking parts are not automatically compatible.
Match flow, outlet, and trap
The rated flow is the amount of water a drain can accept under the maker’s stated test conditions. Compare that rating, including the specified water head and grate setup, with the expected fixture discharge and applicable plumbing code. Do not use outlet diameter alone as a capacity rating.
Provide a trapped and vented connection as required by local code. In places that have adopted the 2021 International Plumbing Code, the general trap-seal depth is 2–4 inches (51–102 mm). Local rules and approved trap-seal protection may differ, so confirm what applies to your site.
| Check | What to confirm | Why it matters |
|---|---|---|
| Flow | Rated capacity at the stated head and grate configuration | The drain must suit the fixture discharge |
| Plumbing | Outlet, trap, vent, and service access | The assembly must meet local code and be serviceable |
| Membrane | Approved bonding flange, clamp, or adapter | The waterproofing must connect as designed |
| Grate | Shape, opening, and load rating for the use | The grate must fit the finish and intended traffic |
Coordinate drain and membrane
Select the drain and waterproofing system as a pair. A bonded membrane needs a compatible bonding flange or approved adapter. A clamping-style system uses its specified connection. These approaches are not interchangeable based on appearance. Where applicable, use a membrane meeting ANSI A118.10 and follow the system’s instructions for corners and pipe penetrations.
Set the drain body so it cannot move and its top aligns with the planned finished floor. Build a continuous fall to the drain without low spots or “birdbaths.” A common shower-floor design range is 1:80 to 1:50, or about 1.25% to 2%, subject to local code, accessibility needs, and manufacturer instructions. Preserve that designed slope; do not flatten a wet floor to meet a level-floor tolerance.
For tile outside the sloped wet area, ANSI A108.02 flatness limits are typically 1/4 inch in 10 feet when all tile edges are under 15 inches, and 1/8 inch in 10 feet when any edge is 15 inches or longer. These are flatness limits, not permission to remove the drainage slope.
Install and test in sequence
- Plan: Confirm fixture flow, drain location, outlet, trap and vent, grate, finished height, structure, and local rules.
- Prepare: Repair the substrate and secure the drain. Make the required slope in the substrate or approved floor-forming layer, not by asking tile mortar to fix major errors.
- Waterproof: Connect the membrane using the drain system’s specified detail. Treat corners and penetrations as directed, then complete the required flood test before covering the work.
- Finish: Set tile with mortar suited to both the tile and membrane. Wet areas need at least 95% mortar coverage, with edges and corners fully supported. Keep the grate removable and clear of mortar.
- Commission: Grout and seal only as required by the selected materials. Provide movement joints at changes of plane and as directed by TCNA EJ171. Run water to check drainage, flow, trap function, and for leaks.
Coordinate grate, thresholds, accessibility, and movement joints
A grate must suit the drain, tile layout, and expected use, while remaining accessible for cleaning. Thresholds and slopes also affect safe entry and drainage. Plan these parts together before setting the drain; late changes can leave an awkward tile cut, a raised edge, or a floor that does not drain well.
A point drain collects water at one location, so the floor must fall toward it from all sides. A linear drain collects water along a line and may allow a single-plane slope, depending on the system and layout. Neither type is automatically better: room shape, tile size, plumbing location, and accessibility needs help guide the choice.
Check that the grate can be lifted for cleaning and is rated for the intended use. Make sure its frame suits the selected drain and finish. Choose a grate opening and cover style that will not create an avoidable trip or foot-catching concern, and follow accessibility requirements where they apply.
Set the threshold and finished floor elevations with the slope in mind. A shower should shed water toward its drain, not out toward the bathroom or into an adjoining room. Confirm movement joints at changes of plane and other required locations; grout is not a substitute for those joints.
Verify drainage and protect the assembly through maintenance
A completed floor needs a working drain and an intact waterproofing connection, not just neat tile. Test the assembly at the stage required by the system and local rules, then check drainage again after tiling. Routine cleaning helps keep the grate and drain open, while prompt attention to movement or leaks can limit damage.
A flood test checks the waterproofed area before tile hides it. Follow the membrane and drain manufacturer’s test method and the local requirements for duration and water level. Do not cover the assembly until the test passes and any leak or loss of water has been investigated.
After installation, run water across the finished floor. Look for persistent pooling, slow drainage, movement at the grate, or water escaping the wet area. A small amount of surface water may remain due to tile texture or layout, but a clear low spot that holds water needs attention.
Keep the grate removable and clean out hair, lint, and debris. In laundry areas, confirm that the drain and plumbing are suited to the equipment and expected discharge. If you find a loose drain, cracked tile, recurring dampness, or a ceiling stain below, investigate the cause rather than covering the symptom with fresh grout.
Floor gully drain examples: two planning scenarios
These examples show how the same selection checks lead to different decisions. They are planning scenarios, not reports of measured project results. Use the actual fixture data, site conditions, product instructions, and local rules for your installation.
Walk-in shower: A remodeler plans a point drain beneath small-format tile. Before ordering, they check the shower’s expected flow against the drain’s rated capacity, confirm the outlet and trapped, vented connection, and choose a membrane-compatible drain. They then set the drain height, form the continuous fall, and test the membrane before tile covers it.
Laundry floor: A remodeler plans a floor drain near a washer but finds that the proposed outlet and floor build-up have not been coordinated. The next step is to confirm the plumbing route, trap and vent requirements, rated drain capacity, and available finished height. If the floor framing or drain support is unstable, that must be addressed before waterproofing and tile.
Both cases point to the same decision rule: settle plumbing and floor geometry before choosing the final grate. A late change to drain height or location can disrupt the slope and membrane connection.
Frequently asked questions
Is a larger drain outlet always better?
No. Outlet diameter alone does not establish capacity. Compare the drain’s rated flow at the stated head and grate configuration with the fixture discharge, piping, and local plumbing requirements.
What slope should a shower floor have?
A common design range is 1:80 to 1:50, or about 1.25% to 2%. The required slope depends on local code, accessibility needs, and drain and membrane instructions. Keep the fall continuous and avoid low spots.
Does every floor gully need a trap?
A trapped connection is generally required, but confirm the applicable plumbing code and approved system. Venting and any permitted trap-seal protection also depend on local requirements.
Can I connect any waterproofing membrane to any drain?
No. Use a drain designed for the specific membrane connection, such as its approved bonding flange, clamping detail, or adapter. Follow the system instructions rather than matching parts by appearance.
What tile coverage is needed in a wet area?
Use mortar suited to the tile and membrane, and aim for at least 95% coverage in wet areas. Fully support tile edges and corners to reduce voids beneath the tile.
Can tile mortar create the floor slope?
It should not be used to correct major plane errors. Form the planned slope in the substrate or approved floor-forming layer, then set tile while preserving that drainage path.
What if the floor is framed with wood?
Check joists, blocking, and sheathing for condition and stiffness before building the sloped, waterproofed floor. Design deflection is generally at least L/360 for ceramic or porcelain tile and L/720 for natural stone.
Do I need to flood-test the membrane?
Follow the membrane system’s instructions and local requirements for testing before covering the assembly. Investigate any leak or unexplained water loss before installing tile.
How do I prevent the grate from clogging?
Keep it removable and clear of mortar during installation. Clean hair, lint, and debris from the grate and drain as needed, and check that water flows freely.
Is a point drain or linear drain better?
Neither is best for every floor. A point drain needs fall from multiple directions; a linear drain may suit a single-plane slope. Choose based on room layout, tile, plumbing, accessibility, and the compatible waterproofing system.
The reliable choice is the drain that fits the fixture flow, code, structure, slope, grate, and waterproofing detail as one assembly. Verify each part before tile goes down, and test the waterproofing while it remains visible. That is the practical way to reduce the risk of pooling and concealed leaks.
(This article was written by one of our staff writers, Carlos Rivera. Visit our Meet the Team page.)
