Starting below the floor
Going curbless means dropping the shower floor's structure below the level of the rest of the bathroom, opening up room underneath the tile for the slope and drain body that a raised pan would otherwise hide inside its own walls.[1]
One manufacturer's bracket kit, built for a center point drain, drops the subfloor by 11/16 inch, works on joists up to 19.2 inches on center, and needs no cutting or notching of the joists at all.[1]
Slope, the part you can't skip
The standard slope for a shower floor, curbless or not, is 1/4 inch of fall for every foot of run toward the drain, which keeps water moving without creating a surface so steep it feels unstable to stand on.[2]
On a curbless shower, that slope has to be built into the same recessed area that handles the floor transition, which is more demanding than sloping a self-contained prefab pan, since the mortar bed is doing both jobs, drainage and a level-feeling transition, at once.
Point drain vs. linear drain
A point drain, centered in the shower, slopes the floor from every direction toward one spot, which the recessed-bracket system above is built specifically around.[1]
A linear drain, typically run along one edge or the entry itself, slopes the floor in fewer directions, often just front-to-back, which can simplify the tile layout since fewer cut angles are needed to meet the drain line.
Waterproofing has to extend past the shower
Because a curbless shower has no raised lip to physically stop water at the edge, the waterproofing membrane has to extend beyond the shower's footprint onto the surrounding bathroom floor, not stop at the boundary the way it might with a curbed shower.
This is one reason a curbless conversion of an existing bathroom often means pulling up flooring well beyond the shower itself, not just inside the new footprint.
What this adds to a timeline and budget
The framing and recessing step happens before any waterproofing or tile, which is time a standard prefab pan installation skips entirely, since a prefab pan simply sits on top of the existing subfloor.
National data puts a curbless build at $8,000 to $15,000, a noticeably higher band than a standard prefab conversion, largely because of this structural work and the additional waterproofing it requires before finish materials go in.[3]
Questions worth asking your contractor
Ask directly whether they've built a curbless shower before, and ask to see one, since the framing and slope work is easy to get subtly wrong in ways that don't show up until months later as a slow leak or standing water at the entry.
Ask what's below the bathroom: a wood-framed floor over a basement or crawlspace is the easiest scenario, while a concrete slab means cutting into the slab itself rather than working between joists, a bigger and dustier job.