Why 24x48 Porcelain Tile Requires Better Wall Preparation

For years the trade called any tile with an edge of 15 inches or more "large-format." That number was never an official definition. When the ANSI A108 and A118 tile standards were reissued in 2024, a new terminology section (A108.T) defined large-format tile as a tile "having a facial dimension greater than 23 in. (584 mm)". The 15-inch figure still matters, because it is the break point where the substrate flatness tolerance tightens, as Scott Carothers of the Ceramic Tile Education Foundation explains. A 24x48 clears both thresholds comfortably: it is large-format by the new definition and it triggers the tighter flatness rule by the old one.

What makes it different from a 12x24 is not just area. A 48-inch tile on a wall framed at 16 inches on center spans three stud bays. It cannot follow the wall; as Joshua Oduin put it in Fine Homebuilding, "large-format tile doesn't follow undulations in the substrate". It bridges them, and every bridge has two ends that stand proud of the tile next to it. Smaller tile forgives a wavy wall by bending the tile plane in tiny steps. A 24x48 does not bend, so the wall has to be right.

This article covers standard-thickness porcelain. Thin gauged panels are a different system; see our gauged porcelain installation guide. For the material choice itself, see Porcelain vs. Ceramic vs. Natural Stone for Shower Walls.

ANSI A108.02 is the industry workmanship standard that tile contractors and most manufacturers point to (it is not building code). Its substrate flatness rule has two bands, quoted here from Carothers' summary:

An eighth of an inch over ten feet is far flatter than the framing in a typical Denver-metro tract home. Studs crown, plates sit on uneven subfloors, and a wall that was fine for painted drywall can easily be out more than the tolerance between one stud and the next. Board follows the studs; it does not fix them.

So the flattening has to happen before the board goes on. Our sequence on every large-format shower:

1. Strip to the studs, then check each wall with a long straightedge and string lines, both across the studs and up each stud, and mark every high and low.
2. Correct the framing: plane or shim individual studs, sister badly crowned ones, and add blocking where the niche, bench and any grab bars will land. This is the cheapest point in the job to gain flatness.
3. Install the board so edges land on framing, with fasteners set flush. A proud screw head under a 24x48 is a guaranteed rocker.
4. Re-check the finished board surface against the 1/8-in.-in-10-ft and 1/16-in.-in-2-ft rule before any membrane goes on, and correct remaining lows with a skim of the setting mortar, within the mortar's rated thickness.
5. Waterproof, flood test the base, snap the layout, and only then open the first box of tile.

Three ways to get a large-format-ready wall, each with a different cost and a different failure mode. Product requirements below are the manufacturers' published instructions; follow the sheet for the product actually on your job.

We use more than one board type. Our Denver steam shower was waterproofed with a LATICRETE HYDRO BAN liquid membrane over every surface, walls, sloped ceiling, bench and niche; the Highlands Ranch shower rebuild, which replaced a failed pan from another contractor, used DensShield glass-mat gypsum backer with reinforced seams and a LATICRETE waterproofing membrane system over the wet-area walls. Which board we choose depends on the framing we find, the fixtures, and whether the homeowner wants a bench and niches built from the same panel system. The mud pan versus foam tray question is a similar trade one level down.

A standard thinset is formulated for a thin bed under small and medium tile. A 24x48 needs more mortar under it to absorb the tile's own warpage and the wall's remaining variation, and it needs that mortar to hold roughly 33 pounds of porcelain (one tile at 9.5 mm, by a typical retail spec of 66.92 lb per two-tile box) on a vertical surface without sagging while it cures. That is what the large-and-heavy-tile category exists for. On the bag, look for ANSI A118.15 or A118.4 followed by a string of letters that includes an H. Custom Building Products lists ProLite as exceeding ANSI A118.15HET; LATICRETE lists MULTIMAX Lite as exceeding A118.15 HET and 4-XLT as exceeding A118.15 with non-sag wall performance and a bed from 3/32 in. to 3/4 in. Our Premium shower tier is built with LATICRETE products; the Essential tier uses standard-format porcelain or ceramic where a conventional thinset is appropriate.

Coverage

Manufacturers' sheets, following ANSI A108.5, require mortar contact of at least 80 percent in dry areas and 95 percent in wet areas and exteriors, with all tile edges supported. A shower wall is a wet area, so the target is 95 percent with no hollow corners. Voids are where water that gets through the grout collects, and an unsupported corner is where a 24x48 cracks if anything leans on it. The only honest way to know you hit 95 percent is to pull a tile now and then and look; MAPEI's data sheet says exactly that.

Trowel and back-buttering

Custom recommends against the common 1/2-inch square-notch trowel for this work and instead a 3/4 x 9/16 x 3/8 in. U-notch held at about 30 degrees, because rounded ridges collapse into a continuous bed when the tile is pushed perpendicular to the ridges. Then the back of the tile gets its own flat coat. Fine Homebuilding's rule is blunt: "With larger tiles, you must always back-butter the underside of each tile". The back coat fills the texture of the tile, wets the surface so the bond coat grabs, and lets the installer make up the small thickness differences that would otherwise become lippage.

Lippage is the height difference between the edges of adjacent tiles. ANSI A108.02 ties the allowance to grout joint width, because a narrow joint makes any step visible. The 2023 edition, as summarized by Donato Pompo in The Construction Specifier:

Three consequences follow for a 24x48 wall. First, the joint cannot be as tight as the showroom sample. The standard's running-bond rule, quoted in LATICRETE's TDS 164, requires an average joint of at least 1/8 in. for rectified tile with a long side, plus the measured edge warpage of the actual tiles. We hold 1/8 in. as the floor on these walls; narrower joints, in LATICRETE's words, "greatly exaggerate" lippage and are hard to fill with grout.

Second, the pattern is limited. Fired porcelain is never perfectly flat; the high point is usually at the center or the corners. In a 50 percent brick offset, the center (highest point) of one tile sits against the end (lowest point) of the next, which is the worst possible pairing. ANSI A108.02 therefore caps the offset at 33 percent when the side being offset is longer than 15 in. (the earlier edition said 18 in.), unless the manufacturer allows more. Stacked (grid) layouts and one-third offsets are both fine with a 24x48; a true half-offset is a request we talk a homeowner out of, or build a mock-up for first.

Third, leveling clips help, within limits. Clips and wedges pull two adjacent edges flush while the mortar sets and act as spacers, which is why they are in nearly every photo of our large-format work. But they require about 1/16 to 1/8 in. more mortar to work in, they "are not designed to take the bow out of tiles", and, as TileLetter notes, they are "often misused" and "not intended to level" a substrate. On a wavy wall a clip can make two tiles flush at the joint while their faces tilt in opposite directions, which looks fine at noon and terrible under a vanity light. Flat wall first, clips second.

A 24x48 at 9.5 mm weighs about 4.3 pounds per square foot by the retail listing above, so a 5-by-8-foot shower wall of it carries roughly 170 pounds of tile before mortar. That is not a structural problem for a properly framed wall, but it does raise the bar on three details. The framing must be stiff: USG's cement-board guide requires studs at 16 in. on center and deflection no greater than L/360 for tile, and notes that large format tile may need a more rigid assembly than the minimum. The board must be thick enough for the stud spacing: Schluter specifies 1/2 in. KERDI-BOARD at 16 in. o.c. and 3/4 in. at 24 in. o.c.. And the mortar must hold the tile in place while it cures; that is the "non-sag" property every large-and-heavy-tile mortar advertises, and it is why we do not substitute a floor mortar on a wall.

Weight also changes handling: two people set a 24x48 on a wall, and a tile dropped into place rather than beaten into the bed leaves voids. The framing around an older tub surround was never checked for any of this, which is why a shower remodel that moves to large-format tile should include a framing inspection in the scope, not just a tile swap.

Every shower has outside corners: the edge where the wall tile wraps a knee wall, the face of a niche, the front of a bench, the curb. With a 24x48, those edges are long, straight and very visible. There are two honest ways to finish them.

A metal profile such as Schluter-JOLLY is a 90-degree trim whose perforated anchoring leg sets in the mortar under the tile, leaving a slim metal edge that, in the manufacturer's words, "prevents tile edges from chipping." It installs fast, forgives imperfect cuts, and comes in brass, aluminum and stainless finishes. Its downside is visual: a metal line at every corner.

A 45-degree miter cuts the back of each meeting tile at 45 degrees so the two faces wrap the corner with only a hairline joint. On a marble-look porcelain, the result reads as a solid slab. This is our standard on Premium-tier showers, and the photos from our Denver steam shower show the process: the niche and outside-corner edges were cut with a diamond blade, then wet-ground and polished by hand. The trade is divided on it, and you should hear both sides. In TileLetter's August 2025 pros-and-cons piece, one contractor argues mitered edges bring "increased vulnerability to chipping" and that movement can crack the tile just beyond the miter point; another says they give a continuous look that "especially look[s] great with large-format tile and panels" and that a correctly built assembly handles the movement.

Both are right. A mitered porcelain edge is thinner than a full tile edge and has no metal protecting it. A dropped shampoo bottle that would bounce off a profile can chip a miter, and the chip shows the tile's body color under the printed face. We reduce the risk by leaving a hairline of the glazed face rather than grinding to a knife edge, by fully supporting the corner with mortar behind both tiles, and by treating the corner joint as a movement joint rather than packing it with cementitious grout. We do not promise a miter can never chip. If that worry outweighs the look for your household, a profile is the better choice, and we say so. The grout question (our Premium tier uses epoxy) is covered in Epoxy vs. Cementitious vs. Premixed Grout.

Large tile punishes afterthoughts. Three places where it matters most:

• Niches. A niche cut into the middle of a 24x48 field leaves narrow slivers around it unless the opening is sized and positioned to the tile module before framing. We set the niche so its edges land on or near grout lines, frame and block it at the stud stage, waterproof it as part of the wall, and finish its face edges either mitered or with a profile to match the rest of the shower. In our Highlands Ranch master bath the niche interior was lined in penny mosaic, set into the large-format field, so the small tile did the work of turning the corner.
• Benches. A floating or built-in bench top in large-format tile is usually one or two pieces, which looks right but means the top must be cut precisely and must slope slightly toward the shower so water does not sit on it. The bench front is an outside corner; see the miter discussion above. Blocking for a floating bench goes in before the board.
• The curb. The curb top slopes into the shower; that is our practice on every shower so water on the curb drains back to the pan. A 24x48 cut to a 4- to 6-inch curb cap is a long, thin piece of porcelain; it needs full mortar support and is often better cut from a tile with the same face but a slightly thicker body, or from a solid-surface or stone cap the homeowner chooses to match. Curbs are also where the pan slope and liner height meet the wall tile, so the bottom course of 24x48 is cut to that geometry, not the other way around.

Everything above sits on the parts that keep the shower dry. Whatever the tile, every shower we build gets a code-listed membrane, a sloped base and a flood test before the first tile goes on. Flatness makes the 24x48 look right; the membrane makes it last. Site conditions, product compatibility and your local building division can all change the right method, so treat this as the standard we build to, not a universal spec.

Can 24x48 tile go over my existing cement board or drywall?

Not over drywall in a shower, for waterproofing reasons unrelated to tile size. Over existing cement board, only if it meets the 1/8-in.-in-10-ft flatness rule and the wall has a working membrane. An existing shower wall almost never meets both, so we open it up.

Is a 50 percent brick-pattern offset ever acceptable with 24x48?

Only if the tile manufacturer explicitly allows it for that product, and even then we recommend a mock-up under the actual bathroom lighting first. The industry standard caps the offset at 33 percent for tiles with a long side over 15 inches because the warped center of one tile lands against the flat end of the next. Stacked and one-third offsets look modern and avoid the problem.

Why does your quote for large-format tile include framing work?

Because the wall is where flatness comes from. Shimming or sistering a few studs costs hours; a wavy large-format wall costs a redo. We price the framing corrections as their own line so you can see them.

Will the mitered corners chip?

They can. A mitered porcelain edge has no metal protecting it, and a hard knock can take a flake off that shows the tile body. We build them to minimize that risk, but if the possibility bothers you, a Schluter-style profile is the right answer and we will install it without argument.