Tile a Shower Niche With Schluter KERDI Board in 7 Steps for a Watertight Recess

Why rear corners crack before the tile face

A tiled niche set into a stud wall usually leaks where the horizontal shelf meets the vertical back. Water runs along grout joints, reaches the bond line, and follows any break in the waterproofing at that inside angle. KERDI Board handles the surface portion of that problem because the board itself is the waterproofing layer: an extruded polystyrene core, reinforcing fleece on both faces, and a cementitious skin that accepts thinset directly while resisting water absorption.

The prefabricated option is the Schluter KERDI-BOARD-SN, a ready-made niche sold in sizes such as 305 by 508 mm and 305 by 711 mm. Its bottom shelf is moulded with a slope of roughly 2 to 3 degrees toward the shower. When a niche is built from board offcuts, that pitch has to be cut into the assembly by hand. A flat shelf holds standing water, and that water can wick into the grout over time.

The recess is also in a harsh part of the enclosure. It sits in the spray pattern and holds bottles that drip after the shower is off.

Step 1 and 2: frame the opening and dry-fit the panel

Locate the recess between two studs at 406 mm centres so the opening stays within one stud bay. Cutting structural members creates a different framing problem, so a standard 356 mm niche width is a practical fit between 406 mm studs once the framing allowance is included.

Install horizontal blocking at the top and bottom of the planned opening. A 38 by 89 mm timber screwed to the studs with structural screws gives the box solid support on all four edges. The blocking also gives the flange a surface to pull against when the unit is fastened.

Set the KERDI-BOARD-SN into the rough opening before mortar is mixed. The prefabricated flange is designed to sit proud of the surrounding wall board by the thickness of the substrate, leaving the niche face and the wall panel flush when everything is seated.

Lay a straightedge across the opening. If the niche sits too deep, pack the blocking forward. If the niche stands proud, the tile will step at the transition, and grout will leave the defect visible.

Mark the four flange positions on the surrounding KERDI Board wall with pencil. Those marks show exactly where bonding mortar needs to be placed. During this dry fit, also confirm that the moulded sill drains toward the room. With the box upside down on a bench, it is easy to reverse the orientation and aim the slope into the wall.

Step 3: bond the niche with unmodified thinset

Schluter specifies unmodified thinset mortar, an ANSI A118.1 dry-set, for bonding KERDI products. Polymer-modified mortar cures partly through water evaporation. Between two non-porous fleece surfaces, that moisture has little path out, so the mortar can stay soft for weeks. Unmodified mortar cures by hydration and sets reliably inside the sealed assembly.

Mix the thinset to a consistency that holds a notch. Schluter ALL-SET is the modified alternative the company permits as an exception. Comb the wall opening and the rear flange of the niche with a 6 mm notched trowel.

Press the KERDI-BOARD-SN into place and embed the flange fully. Drive corrosion-resistant screws through the flange into the blocking at roughly 200 mm spacing. Clean squeeze-out from the inside corners while the mortar is wet, since cured ridges in those corners make the next waterproofing step harder.

Step 4: band every seam with KERDI-BAND

The board surface is waterproof, while the joints need fleece banding to receive the same protection. Cut KERDI-BAND, the 125 mm fleece strip, for each of the eight seams where the niche flange meets the wall board, plus the four inside corners of the recess itself.

Spread a thin coat of unmodified thinset over each seam. Embed the band with the flat side of the trowel, then squeeze out excess mortar until the strip lies flat and shows no air pockets. At the inside corners, use KERDI-KERECK preformed corners. These moulded fleece pieces are shaped to a 90 degree angle and seal the three-plane junction at the back of the shelf, where flat band can bridge and lift. Any band joints need at least 50 mm of overlap.

A void behind the band gives water a route under the fleece and along the seam. Starved thinset can make a strip look sealed on the surface while leaving a hidden channel underneath. Run your fingers across every banded line and feel for the slight ridge of embedded fleece. A patch that feels like bare board lacks mortar below the fleece.

A note on timing

Let the bonded and banded assembly cure for 24 hours before flood-testing or tiling. The unmodified mortar in the sealed seams needs that curing window.

Step 5 and 6: flood-test, then tile the recess

Before tile covers the work, confirm that the waterproofing holds. Plug the niche and surrounding floor section, or run the shower against the wall for 15 minutes. If the back of the wall cavity is accessible, inspect it after the spray. Dry framing after sustained water exposure confirms that the bond line and bands are continuous.

Set the niche tile so cut edges land at the back inside corners and factory edges remain at the front opening, where they are seen. Many installers tile the back wall of the recess first, then the sill, then the two side panels. That order lets the sill tile lap over the side tiles and shed water forward off the moulded slope.

Use the same thinset across the field. In the niche, a slightly stiffer mix helps hold the sill tile on its pitch without slumping. The sloped shelf deserves careful coverage, because a tile set on a pitch wants to slide before the mortar firms up.

Where the layout allows, keep the grout joints in the recess aligned with the surrounding wall field. A grid that drifts away from the wall pattern makes the niche read as an afterthought. Schluter RONDEC or QUADEC can finish the front lip of the opening, covering the exposed tile edge with metal or PVC trim and avoiding a vulnerable mitred corner that can chip when struck by a shampoo bottle.

Watch the front edge of the sill as the mortar cures. If the tile slumps overnight, it can leave a lip that traps water at the opening, defeating the purpose of the moulded slope.

Step 7: grout and seal the joints, not the field

Grout the niche with the same product used on the surrounding wall. Treat the perimeter joints differently. The four corners where the niche meets the wall plane, along with the inside corners of the recess, are movement joints. Use colour-matched silicone in those locations. Cement grout in a change-of-plane corner cracks as the assembly moves with temperature and load, and Schluter and most tile bodies call for flexible sealant at every change of plane.

Natural stone changes the sealing routine. Marble, travertine, and limestone are porous and can absorb both water and pigment from coloured grout. An impregnating sealer such as Lithofin MN Stain-Stop goes on the stone before grouting to help keep the grout from staining the tile face.

Lithofin stone sealer penetrates the stone and leaves no surface film, so it does not alter the finish. In a niche, reapplication is part of maintenance because the recess receives more direct water contact than the open wall.

Porcelain and glazed ceramic need no stone sealer. Their vitrified, non-absorbent surfaces leave a sealer sitting on top with no useful role. On a porcelain niche, the silicone at the corners is the joint that later needs attention, since daily spray causes it to degrade and discolour over a few years.

The waterproofing is complete before any tile sealer is applied. Stone sealing is a stain-control and maintenance step for the visible surface. A porcelain niche built on KERDI Board remains watertight without sealer on the tile face.

What the prefabricated unit decides for you

The KERDI-BOARD-SN fixes the niche dimensions, sill slope, and flange depth. Those built-in details remove three common sources of installation error, while the tile layout must adapt to a fixed opening.

A niche cut from board stock can be sized to a whole-tile module so cut tile does not land inside the recess. With that choice, the installer has to create the slope, corner geometry, and flush flange, then band every joint correctly.

Both routes still rely on full mortar coverage behind the preformed KERECK corners. After trim and grout cover the work, one point remains hidden inside the rear shelf corner: did the fleece bond tight all the way through the bend?