Specifying a quadrant shower tray in a 900×900 Domlur guest bath: radius tolerance and the tile-alignment architects check on site

In a three-bedroom Domlur apartment with 8'6" floor-to-floor heights, the guest bathroom often measures 5'×7' or smaller—just enough for a water-closet, a vanity, and a corner shower. The quadrant enclosure, with its curved glass arc and matching acrylic or stone-resin tray, fits the geometry. But the detail that telegraphs on handover is the joint line where the tray radius meets the glass: a 2 mm mismatch reads as a silicone seam that widens and narrows, and the tile border—if you've run a 12"×24" rectified porcelain up to the tray edge—will show the error. This note walks through tray-first versus glass-first sequencing, how to write radius tolerance into the shop drawing, and why the 800 mm versus 900 mm footprint decision usually comes down to the tile layout, not the plumbing rough-in.

Tray-first sequencing and why the radius dimension governs the glass order

Most Bangalore contractors install the shower tray after the floor tile is laid but before the wall tile is grouted to full height. The tray sits on a mortar bed or adjustable feet, the waste trap drops through a 90 mm core-hole in the structural slab, and the tray flange tucks under the first course of wall tile by 10–15 mm. Once the tray is levelled and the silicone bead cures, you measure the installed radius at three points—front left, apex, front right—and average them. That averaged radius, not the nominal dimension on the manufacturer's data sheet, is what you send to the glass atelier for the curved panel.

A typical 900×900 mm quadrant tray from European or Turkish tooling will specify an arc radius of 550 mm, but the as-moulded part can run 548–552 mm depending on the acrylic sheet thickness (typically 4–5 mm for a reinforced base) and the vacuum-forming shrinkage. If you order the glass to a nominal 550 mm radius without verifying the installed tray, you risk a 2–4 mm gap at one quadrant and a near-zero gap at another. The silicone bead width then varies, the joint line looks uneven under a ceiling downlight, and the client notices. Tray-first sequencing—measure, then cut glass—avoids that.

When to spec glass-first: the pre-tiled niche detail

In a handful of Sadashivanagar and Indiranagar projects where the architect wants a flush threshold—no tray lip, just a linear drain set into a tiled wet-room floor—the curved glass panel is sometimes anchored to a half-height pony wall and the floor tile is sloped 1:80 toward the drain. Here you reverse the sequence: template the glass radius from the RCP, install our low-iron clear curved panel with black spigots, then tile up to the glass base with a 3 mm joint. The drain sits 150–200 mm back from the arc, and the floor screed is built up in two pours to hit the fall. This detail adds three days to the wet-area schedule and requires a waterproofing membrane that laps 300 mm up the wall, but it reads as a single sculptural gesture rather than a tray-and-enclosure assembly. We've fitted eight this year; all were 1000 mm radius or larger because the tile fall becomes difficult to achieve in an 800 mm footprint.

Radius tolerance: the 2 mm rule and how to write it into the shop drawing

The shop drawing for a quadrant enclosure should call out the radius dimension with a bilateral tolerance: R550 ±2 mm, measured at installed tray centreline, verified on site before glass fabrication. That ±2 mm window is tight enough to keep the silicone joint visually uniform—typically 4–5 mm wide along the full arc—but loose enough to accommodate normal acrylic moulding variation and the minor out-of-plumb that occurs when a tray is bedded onto a tiled floor with 2–3 mm lippage.

We template every quadrant with a flexible aluminium rule or a digital radius gauge; the measurement is taken at the tray's top surface, not the flange, because that's where the glass will bear. If the as-built radius falls outside the ±2 mm band, we note it on the site dimension sheet and either adjust the glass curve—our CNC waterjet can interpolate a three-point spline—or, if the error exceeds 5 mm, recommend replacing the tray. The latter has happened twice in four years, both times because the tray was specified at 800 mm but an 850 mm unit was delivered and installed without checking the carton label.

Why the tile border often dictates 800 mm versus 900 mm

In a 5'×7' (1525×2135 mm) guest bathroom, an 800 mm quadrant leaves a 725 mm wide aisle in front of the vanity; a 900 mm quadrant narrows that to 625 mm. The Building Code minimum is 600 mm, so both work. But if you're running a 600×1200 mm rectified porcelain on the floor in a brick-bond pattern, the 900 mm tray will land mid-tile on one axis, forcing a 150 mm cut piece along the curved edge. An 800 mm tray, by contrast, can align to a grout joint if you shift the layout origin by 100 mm. The tile alignment—not the shower's internal dimensions—often settles the question. We ask for the floor-tile shop drawing before quoting the glass, because a misaligned tray edge reads as a layout error even if the radius tolerance is perfect.

Drain offset and the reflected-ceiling-plan check for the overhead rain head

The waste outlet on most quadrant trays sits 120–150 mm from the back corner, offset along the diagonal. That position keeps the trap accessible through a removable grate but pushes the drain centreline 200–250 mm away from the geometric centre of the arc. If you're specifying a ceiling-mounted rain head—common in Koramangala and Whitefield apartments with 9' slab-to-slab heights—the shower valve and head should be centred on the user's standing position, not the tray's geometric centre. The RCP needs to show the rain-head rough-in at approximately 400 mm from the back corner (assuming a 900 mm tray), which places it forward of the drain by 150–200 mm.

That offset has two consequences: first, the ceiling chase for the shower arm must dodge any recessed downlights you've planned for general bathroom illumination, so coordinate the RCP with the electrical rough-in early. Second, if the rain head diameter is 250 mm or larger, part of the spray will land outside the tray footprint unless you angle the arm slightly backward. We've seen three Domlur projects where a 300 mm head was centred on the tray's geometric middle; the forward spray hit the glass door and ran down onto the threshold, creating a puddle. The fix was to rotate the arm 5° toward the back wall—a site adjustment that should have been caught in the shop drawing.

Electrical rough-in: the GFCI box and the LED strip detail

If the rain head includes an integrated LED ring or colour-therapy module, the transformer and GFCI breaker must sit outside the wet zone—typically in the vanity cabinet or in a false ceiling above the door. The low-voltage cable (12 V or 24 V DC) runs through the ceiling void and drops down the shower-arm chase. The RCP should call out a 4"×4" junction box at the chase entry point, with a service loop of 300 mm so the electrician can pull the cable for termination. We've coordinated this detail on our bronze-tint quadrant enclosures with brass hardware, where the warm glass tone pairs with a 3000 K LED strip along the ceiling cove; the transformer sits in the vanity plinth, and the cable is sleeved in black PVC conduit that reads as a shadow line against the bronze panel.

Silicone joint width and the hard-water maintenance note for Cauvery supply

A 4–5 mm silicone joint between the tray edge and the glass base is wide enough to absorb the differential movement between acrylic (coefficient of thermal expansion ~70 µm/m·K) and toughened glass (~9 µm/m·K) but narrow enough that the bead doesn't sag or collect grime. We use a neutral-cure silicone with a Shore A hardness of 20–25, which stays pliable through Bangalore's June–September monsoon humidity swings. The joint is tooled with a 6 mm radius former to leave a shallow concave profile that sheds water toward the tray, not into the seam.

Cauvery water in Domlur and Indiranagar typically runs 200–250 ppm TDS, with calcium and magnesium as the dominant dissolved solids. Over six to eight months, lime scale will coat the silicone bead if the joint isn't squeegeed after each shower. The white crust is more visible on a dark tray—charcoal or graphite acrylic—than on white, and it's nearly impossible to remove without a dilute citric-acid solution that can degrade the silicone if left in contact for more than two minutes. The handover document should include a one-page maintenance note: squeegee the glass and tray after use, wipe the silicone joint weekly with a damp microfibre cloth, and avoid abrasive creams or bleach-based cleaners. That note isn't decorative; it's the difference between a joint that lasts eight years and one that needs re-sealing in three.

As-built documentation: the joint-line photograph and the tolerance sign-off

Before the final silicone bead is applied, we photograph the dry-fit joint line at four points around the arc, with a steel rule in frame to show the gap width. Those images go into the as-built folder alongside the site dimension sheet and the shop drawing. If the client or the architect queries the joint appearance six months later—usually because lime scale has made a 4 mm joint look like 6 mm—the photographs confirm that the original tolerance was met. We also ask the site architect to sign off on the dry-fit before sealing; that signature doesn't transfer liability, but it does close the loop on any radius discrepancy that might have crept in during tray installation.

The as-built folder also includes the tray manufacturer's care-and-warranty card, the glass toughening certificate (BS 6206 Class A or equivalent), and a one-line note of the silicone batch number. If a joint fails in year two or three—rare, but it happens if the substrate wasn't degreased properly—that batch number lets us cross-reference the cure profile and tensile strength, which helps determine whether the failure was material, application, or substrate-related. The folder lives with the project O&M manual; we've handed over forty-three this year, and not one has come back with a tolerance dispute.

Questions we get asked

Can you fit a quadrant enclosure if the corner walls are out of square by more than 3 mm?

Yes, but the glass panel will need to be scribed to match the wall angle rather than cut to a true 90°. We measure the installed tray's back-corner angle with a digital protractor; if it reads 89° or 91°, we adjust the panel's side edges by up to 5 mm over the 1950 mm height so the top rail sits parallel to the ceiling. Beyond 5 mm of out-of-square, the curved panel starts to look visibly trapezoidal, and we recommend shimming the tray or re-setting one wall. The tolerance note on the shop drawing should read: Walls to be plumb within 3 mm over 2000 mm height; corner angle 90° ±1°.

What's the lead time for a curved panel once you have the as-built radius?

Fourteen working days from template approval to site delivery, assuming the glass specification is our fluted panel with brass hardware or another standard atelier finish. If you're commissioning a custom low-iron tint or a screen-printed pattern, add another seven days for the coating run. The curved panel is cut on a five-axis waterjet, the edges are seamed by hand to 1 mm radius, and the panel is toughened in a horizontal furnace with a custom jig to prevent optical distortion along the curve. We don't hold curved stock because every radius is site-specific.

How do you handle the threshold detail if the bathroom floor tile is 10 mm thicker than the tray flange?

The tray flange is typically 30–40 mm wide and sits 3–5 mm proud of the floor screed. If your floor tile plus adhesive bed totals 22 mm (12 mm porcelain + 10 mm adhesive), the tile will finish 12–14 mm higher than the tray flange. We run the tile up to the flange edge, leaving a 3 mm expansion joint, then fill the joint with a flexible epoxy grout or silicone in a colour-matched shade. The result is a small step-down into the tray—acceptable under most residential codes—but if you want a flush threshold you'll need to build up the tray on a plinth or recess the floor screed by 15 mm within the shower footprint, which requires a second waterproofing pass.

What happens if the tray cracks during installation—does the glass dimension change?

If the tray cracks before we template the radius, you replace the tray and we re-measure. If it cracks after the glass is cut but before installation, we can usually salvage the panel by scribing it to the replacement tray's radius, provided the new tray is within ±5 mm of the original. If the radius difference exceeds 5 mm, the panel must be re-cut. We've had one tray crack in four years—a stone-resin unit that was dropped during delivery—and the replacement tray measured 551 mm radius versus the original 549 mm. The glass panel was scribed at the base by 2 mm along a 30° arc, and the joint closed to 4 mm uniform width. No re-cut was needed.

Do you install the tray, or does the civil contractor handle it?

The civil contractor or the tiling subcontractor installs the tray, sets the waste trap, and applies the first waterproofing layer around the flange. We template the radius once the tray is fixed and the wall tile is grouted to at least 300 mm above the tray edge—that gives us a stable reference plane. We return to install the glass panel, the hinges or spigots, and the door, then apply the final silicone bead along the tray-to-glass joint. The handover happens after the silicone cures for 24 hours and we've run a water test: five minutes of overhead spray with the door closed, checking for any seepage at the threshold or the back-corner seam.

Commissioning a quadrant enclosure for your next Bangalore project

If you're specifying a corner shower in a compact guest bath—Domlur, Jayanagar, Koramangala, or anywhere the floor plan demands a tight radius—send us the RCP, the floor-tile layout, and the tray data sheet once the civil drawings are frozen. We'll mark up the shop drawing with radius tolerance, drain offset, and the threshold detail, then schedule the site template two days after the tray is installed. Visit the atelier at HSR Layout to see the curved-panel jig and the five-axis waterjet, or reach us to discuss your next commission.