Frameless shower with a curbless drain in a Hennur master: waterproofing overlap, slope and the false-ceiling coordination architects miss

The Hennur master we fitted last month had a 1220×1520mm wet zone, a linear drain set 80mm from the threshold line, and a frameless 10mm panel mounted 6mm proud of the finished floor. The architect had specified a curbless entry—zero threshold, continuous Kota stone across the wet and dry zones—but the shop drawing revealed a coordination gap: the RCP showed a recessed linear drain, the structural drawing showed a 150mm slab, and the waterproofing detail showed no upturn behind the glass U-channel. We caught it at site-dimension stage, ten days before the screed pour.

The structural question: can you slope a 150mm slab without a step-down?

A curbless shower needs a 1:60 fall from the threshold line to the drain. Over a 1220mm depth, that's 20mm of drop. If your structural slab is 150mm uniform and you're laying 40mm screed, you have 40mm to play with—enough to build the slope into the screed without touching the slab. But if the architect has already detailed a 150mm slab and specified 20mm porcelain on 15mm bed, you're left with 15mm of screed—not enough to accommodate a 20mm fall plus the drain recess.

The fix is a step-down in the slab itself: drop the wet-zone slab by 50mm at the structural stage, pour it as part of the main slab, and waterproof the entire sunken zone as a single tray. That gives you 90mm of build-up to work with: 40mm screed with slope, 15mm adhesive bed, 20mm stone, and 15mm tolerance for the linear-drain body. The step-down happens under the finished floor, so the threshold reads as continuous stone. We see this missed in roughly half the RCPs that come through for curbless showers in Whitefield and Sarjapur Road projects—engineer draws a flat slab, architect assumes the tiler will "figure it out," and the tiler calls us three days before handover asking whether the glass can sit on a 12mm curb.

Coordination at tender stage, not at tile stage

If you're specifying a curbless threshold, call the structural coordination at tender stage. Mark the step-down on the structural drawing, dimension it on the RCP section, and note it in the plumbing spec so the drain supplier knows the recess depth. The linear drain needs a 60mm recess from finished-floor level; if you're building slope into screed, that recess eats into your slab unless you've planned for it. We've seen Hennur row-house projects where the structural slab was poured uniform, the architect discovered the issue at first-fix plumbing, and the only solution was a 15mm raised threshold in brushed steel—functional, but not the detail that was sold.

Waterproofing upturn behind the U-channel: the 150mm overlap architects forget

A frameless shower panel sits in a floor-mounted U-channel, typically 12mm wide × 25mm deep, anchored with chemical anchors every 300mm. The glass slots into the channel with a 2mm side tolerance and a 6mm bottom gap—enough to let water pass under the panel if the waterproofing doesn't extend behind the channel. The correct detail: waterproofing membrane (two-coat cementitious or single-ply sheet) turned up the wall behind the U-channel by 150mm minimum, lapped under the channel base, and continued across the wet-zone floor to the drain. The U-channel is then anchored through the membrane into the structural slab, and the anchor holes are sealed with the same membrane compound.

What we see missed: the waterproofing stops at the threshold line, the U-channel is anchored to bare concrete, and the glass is assumed to be the water barrier. It isn't. A 6mm floor gap and a 2mm side tolerance mean water will track under the panel and behind the channel if there's no membrane upturn. In a Cauvery-supply bathroom—TDS around 250ppm, high calcium—you'll see efflorescence along the threshold joint line within six months, and delamination of the screed within eighteen. The membrane upturn also gives you a secondary line of defence if the silicone bead at the glass-to-floor joint fails; water hits membrane, not substrate.

Tiling sequence and the membrane lap

Tile the wet zone first, then anchor the U-channel. That way the membrane is continuous under the tile, the tile is cut to the U-channel line, and the channel sits on top of the finished floor with the membrane lapped underneath. If you anchor the channel first and tile around it, you're cutting tile to a 12mm tolerance and trying to seal a three-way joint—tile, channel, membrane—with silicone. It will fail. The correct sequence: waterproof, tile, anchor U-channel through tile into slab, seal anchor points, set glass. We specify this in every shop drawing, but site supervisors trained on curbed showers often reverse it.

The RCP coordination: linear drain, false ceiling and the trap-access question

A linear drain is typically 600–1200mm long, 80–100mm wide at the grate, and requires a 75mm P-trap immediately below the drain body. That trap needs to vent, and it needs access for snaking. In a ground-floor bathroom with a crawl space, access is straightforward. In a first-floor bathroom above a false ceiling—common in Indiranagar and Koramangala stacked floors—the trap sits in the false-ceiling void, and the RCP needs to show: trap location, vent route, and access-panel position in the ceiling below. We see this missed when the architect details the shower in plan and section but doesn't coordinate with the ground-floor RCP.

The vent is the critical piece. A 75mm P-trap on a 1200mm linear drain will siphon if not vented, especially if the drain is running a 1:60 slope over 1220mm and discharging into a 110mm soil stack. The vent should be a 40mm AAV (air-admittance valve) or a 40mm vent pipe taken up through the wall cavity to the roof. If you're working in a row house with a common soil stack, coordinate the vent route with the plumbing consultant before you finalise the glass line—we've had to shift a frameless panel 150mm to the left because the vent pipe was boxed into the wall and the glass would have landed on the boxing.

False-ceiling access panel and the handover detail

Specify a 300×300mm access panel in the false ceiling directly below the linear-drain trap, and mark it on the ground-floor RCP. Frame it in the ceiling grid, hinge it if the ceiling is gypsum, and make it removable if the ceiling is grid-and-tile. The plumber will need to access that trap at least once in the first year for a cleanout, and the homeowner will need it again when they call a plumber for a slow drain. If there's no access panel, the plumber will cut the ceiling, and the ceiling contractor will bill a reinstatement—usually around ₹8,000 for a 600×600mm patch in a coffered ceiling. We note this in the handover document for every curbless shower we fit, but it's the architect's detail to call.

Glass specification: thickness, tint and hardware finish for a wet zone

We fit curbless showers with 10mm toughened glass as standard—12mm if the panel is over 2100mm tall or if the client has specified a floor-to-ceiling panel in a double-height bathroom. The additional 2mm gives you stiffness against deflection when the panel is cantilevered from a single U-channel with no top rail. For a 1830mm tall panel, 10mm is sufficient; deflection under hand pressure is under 3mm at mid-height, well within tolerance for the silicone joint at the glass-to-wall return.

Tint and texture are taste, but they affect maintenance. A low-iron clear panel with brushed-brass hardware will show water spots more readily than a bronze-tint panel with black hardware, but it also reads as lighter in a north-facing bathroom. Fluted glass hides spots but traps soap in the grooves; we recommend it for powder-room enclosures, not for daily-use master showers in hard-water zones. The hardware finish—brushed brass, matte black, polished chrome—should match the basin taps and the linear-drain grate. We stock all three, but brushed brass and matte black are the two we fit most often in Hennur and Yelahanka projects.

The silicone joint and the bead profile

A frameless shower has three silicone joints: glass to wall (vertical), glass to floor (horizontal), and glass to U-channel (if the channel is surface-mounted). All three should be tooled with a 6mm concave bead, using a neutral-cure silicone rated for wet-area use. Acetic-cure silicone will outgas and stain natural stone; we've seen it yellow Jaisalmer limestone in a Sadashivanagar master within eight weeks. The bead should be continuous, with no gaps at the corners, and it should be applied after the tile grout has cured—wait 48 hours after grouting before you silicone, or the silicone will bridge over uncured grout and fail at the bond line.

Slope verification at screed stage: the 1:60 fall and the straight-edge test

Once the screed is laid and the waterproofing is cured, verify the slope before you tile. Lay a 1500mm straight-edge from the threshold line to the drain, and measure the gap at the drain end—it should be 25mm for a 1500mm run (1:60 fall). If it's less than 20mm, the slope is insufficient and water will pool at the threshold. If it's more than 30mm, the slope is too steep and the drain grate will sit proud of the finished floor unless you recess the drain body deeper into the slab. We've seen both errors in Sarjapur Road projects where the tiler was working to a verbal brief, not a dimensioned section.

The straight-edge test also catches high spots. A single high spot 300mm from the drain will create a dam, and water will pool behind it even if the overall slope is correct. Grind down any high spots with a rubbing stone before you tile—it's a ten-minute fix at screed stage, and a two-day re-tile if you catch it after the stone is laid.

Questions we get asked

Can you fit a curbless shower in a 150mm slab without a structural step-down?

Only if you have at least 40mm of screed to build the slope, and only if the linear drain can recess into the slab by 60mm without cutting rebar. In most Bangalore residential slabs—150mm thick, M25 grade, 12mm rebar at 150mm centres—you can recess a drain by 50mm if you coordinate with the structural engineer and avoid the rebar grid. Beyond 50mm, you're cutting steel, and you need a structural sign-off. The safer detail is a 50mm step-down in the slab at the pour stage.

Does the waterproofing membrane need to lap under the U-channel, or can it stop at the threshold line?

It must lap under the U-channel and turn up the wall behind the channel by 150mm minimum. The glass is not a water barrier—the 6mm floor gap and the 2mm side tolerance allow water to pass under and behind the panel. The membrane is your primary barrier. If the membrane stops at the threshold, you'll see efflorescence and screed delamination within a year in a hard-water zone.

How do you coordinate the linear-drain trap with the false ceiling below?

Mark the trap location on both the bathroom RCP and the ground-floor RCP, dimension the vent route, and specify a 300×300mm access panel in the false ceiling directly below the trap. If the ceiling is already built, locate the trap from above, measure down to the ceiling plane, and cut the access panel before you close the ceiling grid. The plumber will need that access within the first year.

What's the correct glass thickness for a 1830mm tall curbless panel?

10mm toughened is standard for panels up to 2100mm tall. If the panel is floor-to-ceiling in a double-height space, or if it's cantilevered with no top rail, specify 12mm. The additional 2mm reduces deflection at mid-height and gives you a stiffer joint at the glass-to-wall return. We stock both thicknesses and can fit either within the same lead time.

Can you retrofit a curbless threshold into an existing bathroom with a tiled curb?

Only if you're willing to re-tile the wet zone and re-waterproof. You'll need to remove the curb, verify the slab level, build a slope into new screed, re-waterproof the floor and the wall upturn, re-tile, and then anchor the U-channel. It's functionally a full wet-area renovation. If the slab is already level with the dry zone and you don't have 40mm of build-up to create slope, you'll also need to cut a step-down into the slab—possible, but it requires structural approval and dustless cutting to avoid disturbing adjacent rooms. Most clients in that position choose to keep the curb and replace the glass only.

If you're specifying a curbless shower for a Bangalore residential project and need shop drawings, slope verification or a site dimension survey before the screed pour, talk to the atelier. We work with architects and interior designers across Hennur, Whitefield, Indiranagar and Yelahanka on frameless enclosures fitted to the millimetre.