How to Choose a Floor Gully for a Wet Room
Choose a floor gully only after you know the floor build-up, waterproofing system, waste-pipe route and shower’s peak flow. Match the gully’s outlet, trap, tested capacity and grate to those conditions, then confirm local code approval. A compatible membrane connection, sound substrate and continuous fall matter as much as the drain itself.
A wet-room drain is a small part of the budget, but a poor choice can put the whole investment at risk. Water can pass through a weak connection and stay hidden below tile, where it may damage framing or ceilings before you see a stain. I check the complete floor assembly before I recommend a gully, not just its shape or price.
“Floor gully” may mean a point drain with a grate, a channel drain, or a trapped floor outlet, depending on local usage. This guide focuses on selecting and fitting a floor gully for a tiled wet room. Local plumbing rules and the drain and membrane makers’ instructions take priority where they differ from general guidance.
Start with the floor and waterproofing system
The floor build-up is every layer between the structural floor and finished tile. Identify those layers first: substrate, slope-forming material, waterproofing, mortar, tile and drain. The gully must suit the whole assembly, including the waterproofing connection and available installation depth, or a seemingly suitable drain may not be safe to use.
Before ordering, establish whether the base is concrete or framed, and whether waterproofing will sit on top of the slope or below a mortar bed. Record tile thickness, drain location, outlet direction and pipe depth. Also check how the trap and hair catcher will be reached for cleaning.
For a framed floor, ask a qualified designer or installer to confirm stiffness under the expected loads. As a common tile-industry benchmark, floors should meet at least L/360 deflection for ceramic or porcelain tile and L/720 for natural stone, where applicable. Deflection describes how much the floor bends under load. The floor also needs room for the drain and sloped assembly without cutting or weakening framing improperly.
On concrete, inspect for cracks, loose areas and moisture concerns. A drain does not solve moisture moving through a slab. Check that the membrane and flooring products suit the slab’s condition, and follow their instructions for moisture testing or mitigation.
Match the gully to the membrane and drainage
A compatible gully is one designed or approved to connect to the selected waterproofing system. The membrane-to-drain joint is a critical part of the water barrier. A correctly sloped tile surface cannot make up for a connection that leaks, so confirm the exact connection detail before buying or installing the drain.
Choose the right drain connection
A surface-applied membrane needs an approved connection, often a bonded flange, collar or another system-specific detail. A traditional clamping drain is generally used with a different waterproofing arrangement. Do not pair a generic clamping drain with a surface membrane unless the drain and membrane manufacturers approve that detail.
Ask for written product information showing the compatible drain model and membrane method. Confirm how the membrane turns into or onto the drain, how corners and seams are treated, and whether the system needs a special primer, sealant or accessory. Keep the relevant instructions on site.
Check the trap, outlet and capacity
The trap blocks sewer gases with a water seal. A conventional trapped floor drain commonly uses a 50 mm water seal under EN 1253, but the applicable local standard and the particular gully’s certified configuration must be checked. Some low-profile or special traps differ and may need specific care or approval.
Use the gully’s published flow capacity at the test head relevant to your installation. Compare it with the maximum combined flow from the shower outlets, then confirm that the trap and waste pipe can handle that flow in the actual layout. Do not compare capacity figures measured under different conditions as if they were equivalent.
Check outlet diameter and direction against the pipe route, as well as the trap’s clean-out access. The grate or tile-insert top must suit your tile thickness and allow the finished height to be adjusted as the manufacturer requires.
| Feature to verify | What to check | Why it matters |
|---|---|---|
| Membrane connection | Approved detail for your exact membrane and gully | Reduces risk of leaks at the drain |
| Flow capacity | Published rate at a relevant test head | Must suit combined shower flow and pipework |
| Trap and outlet | Seal type, outlet size, route and cleaning access | Affects drainage, odor control and maintenance |
| Grate or insert | Tile thickness, adjustment and removal | Supports a neat finish and future cleaning |
| Load class | Rating suited to the room and use | Helps ensure the grate is appropriate for expected loads |
A flow-rate number alone does not establish suitability. Ask the supplier or installer to confirm the gully, trap, pipework and shower flow as a system. Check that the product is accepted under local plumbing rules.
Plan the slope, grate and safe floor finish
Floor fall is the slope that directs water toward the gully. It must be formed in the substrate beneath the waterproofing, not improvised by tilting individual tiles. A common planning range is 1:80 to 1:100, equal to 12.5 to 10 mm of fall per metre, unless local code or the system instructions require another slope.
Work out the required fall from the furthest point to the drain. For example, a point 1.2 metres from the gully needs about 12 to 15 mm of fall at that range. The finished surface must not have reverse falls or low spots that hold water. A flush test after installation helps reveal drainage issues.
Choose the grate style around the floor layout. A point gully often needs tile cuts shaped to meet the drain, while a linear gully can suit a single-plane slope when the room design allows it. Neither format is automatically better; location, pipe route, slope geometry and approved waterproofing details decide what works.
For slip resistance, use tile rated for wet barefoot use under the testing method and rules applicable in your region. Ask for the product’s test classification, and confirm it is suitable for the intended wet area. Small mosaics can offer more grout joints, but that does not by itself prove a floor is slip-resistant. Keep the grate flush with the tile and free of sharp edges or raised lips.
Install and test the assembly in order
The installation sequence protects the membrane and helps catch defects before they disappear under tile. Follow the gully and waterproofing manufacturers’ instructions at each stage. Local requirements may also set inspection, testing and threshold rules, so confirm them before work begins.
Prepare, connect and waterproof
- Inspect the base. Confirm substrate condition, framing stiffness where relevant, drain position, pipe route and available depth. Repair defects and make sure the drain body can be fixed firmly.
- Form the fall. Shape the substrate toward the gully, check it with a level or suitable slope gauge, and correct reverse falls before waterproofing.
- Connect the gully. Install the specified trap and connect the outlet to the waste line. Keep the drain stable and follow the approved connection detail.
- Apply the waterproofing. Waterproof the full wet-area floor and required upstands. Join the membrane to the gully with its approved flange, collar or clamping detail. Do not cover the work before any required inspection or flood test is complete.
A flood test must follow the system instructions and local rules, including the required test time and water level. Protect the test area and check below or around it for signs of leakage. Do not assume a visual check of the tile surface proves the membrane is watertight.
Set tile, finish and verify drainage
Use compatible mortar and give the tile full, even support. Keep the planned fall running to the grate, match grate height to the finished tile, and keep mortar and grout out of the drain opening. Make sure the grate and trap remain removable for service.
Grout tile as specified. Use flexible sealant at movement joints where the design calls for it rather than filling those joints with rigid grout. Detail the doorway or threshold to contain water while avoiding a trip hazard and following local accessibility and building requirements.
After curing as directed, run water across the floor and check that it reaches the gully without persistent ponding. Confirm the drain clears freely, the grate can be lifted, and the trap can be cleaned. A tiled floor with good fall is not proof that the membrane connection is sound; that is why the waterproofing must be tested before it is concealed.
Field examples and common selection mistakes
These examples reflect practical decisions I encounter during remodels, rather than a promise that one drain type suits every room. They show how the floor structure, waterproofing detail and outlet route can change the right choice. Use them as a checklist, then confirm the details for your own room.
In one framed-floor remodel, a low-profile gully looked attractive because it reduced the height build-up. But the limited depth also affected the trap and outlet route. We checked the approved membrane connection and trap access before selecting it; choosing by overall height alone would have missed two important constraints.
In another shower, the proposed drain had a published capacity, but the quoted figure was measured under a different test head from the comparison product. Treating the figures as directly comparable could have led to the wrong choice. We confirmed the relevant flow data, shower outlet rates and pipe layout before proceeding.
Common warning signs include:
- The drain is ordered before the membrane method and tile thickness are known.
- The installer cannot show an approved drain-to-membrane detail.
- The grate or hair trap cannot be removed for cleaning.
- The slope is planned only in tile mortar, with no designed fall beneath the waterproofing.
- The gully will be concealed before a required inspection or test.
Conclusion: choose the whole system, not just the grate
A reliable wet-room floor starts with a gully that matches the substrate, waterproofing, flow demand, outlet route and finished tile. Confirm certified details and local approval, build the fall below the membrane, and test the waterproofing before covering it. Then verify drainage, grate removal and cleaning access before handing over the room.
Frequently asked questions
Is a point gully or a linear drain better for a wet room?
Neither is always better. A point gully may require several floor planes and angled tile cuts, while a linear drain can suit a single-plane slope. Choose based on drain position, pipe route, floor layout, available depth and the waterproofing system’s approved details.
What slope should a wet-room floor have toward the gully?
A common planning range is 1:80 to 1:100, or 12.5 to 10 mm of fall per metre. Local codes or product instructions may set another requirement. Check the fall before waterproofing and confirm the finished floor drains without reverse slopes or persistent ponding.
How do I know whether a gully can handle my shower?
Compare its published capacity at the relevant test head with the shower’s maximum combined flow. Then confirm that the trap, outlet and waste-pipe layout can carry that flow. A capacity figure on its own is not enough if the test conditions or installation differ.
Does every wet-room gully need a 50 mm trap seal?
No. A conventional trapped floor drain commonly has a 50 mm water seal under EN 1253, but local rules and the exact gully certification govern. Special or low-profile traps can differ, so check their approval, installation requirements and maintenance instructions.
Can I use any floor gully with a surface waterproofing membrane?
No. Use a gully with a documented, approved connection detail for the selected membrane. A generic clamping drain and surface membrane should not be combined unless the manufacturers approve that exact assembly. The drain-to-membrane joint is a key leak-risk area.
Should the floor slope be made with tile adhesive?
The designed fall should be formed in the substrate below the waterproofing, unless the approved system specifies another method. Do not rely on tilting tiles to correct a flat or wrongly sloped base. Check the fall before applying the membrane.
How can I avoid water pooling after tiling?
Check the substrate fall in several directions before waterproofing, then verify tile and grate levels as the finish is set. Keep the outlet clear of mortar and grout. After installation and curing, run water over the floor and correct drainage problems before use.
How often should I clean the gully?
Follow the gully maker’s care instructions and clean the grate, hair catcher and trap when debris slows drainage. Keep the water seal functioning, and avoid cleaners that can damage the grate, seals, membrane or grout. Ensure the parts remain accessible after tiling.
(This article was written by one of our staff writers, Carlos Rivera. Visit our Meet the Team page.)
