Most clients walk into the studio asking, in some form, about slip rating. It's a sensible question. It's also rarely the first thing we look at.
The order we evaluate porcelain for a wet room, in roughly fifteen years of doing this, runs body, finish, format, then slip rating — not the other way around. The reason is that the body of the tile, the part you can't see, decides almost everything about how the wet room will hold up. The face you can see is paint over a structure.
Body first: water absorption and rectified edges
Porcelain is defined, in most regions, by water absorption under 0.5 percent. Below that, the tile is effectively impervious. In a wet room you want comfortably below that — we look for ranges that publish absorption under 0.1 percent, ideally with a press density that produces a homogeneous body rather than a glaze applied to a softer biscuit.
You also want rectified edges. A rectified tile has been cut to a precise dimension after firing, allowing tight grout joints. In a wet room, a 2 mm rectified joint catches and holds far less water than a sloppier 5 mm one with chamfered edges.
Finish: how the surface will read when it's wet
Porcelain finishes range from full polish to lappato (semi-polish) to matte to structured. Each behaves differently when water hits it. A polished face is a mirror — pretty in dry rooms, treacherous in showers. A lappato will hide drift marks but is still slick. A good matte will keep its character wet or dry. A structured finish — the kind with a slight texture you can feel — is the most forgiving in a wet room.
The trade-off with structured finishes is cleaning. The microscopic texture that gives the tile its grip also catches soap. We'll talk you through realistic care before specifying one.
Format: the grout-joint argument
Counter-intuitively, larger tiles are usually better in small wet rooms. A 60×120 cm tile in a 5×8 ft shower means three or four grout joints, not thirty. Less grout means less maintenance and a calmer-reading room.
The constraint is substrate. Large-format porcelain demands a flat substrate — flatter than most builds offer out of the box. We measure with a long straight edge before committing to a format. If we're seeing more than 1.5 mm deviation over 2 metres, we'll specify floor levelling before tile day.
Slip rating: the floor of the conversation
For most residential wet rooms, the right slip rating is R10 to R11 on the floor and any value on the walls. Rising into R12 territory is usually overkill for residential and produces a texture that catches dirt aggressively.
Where we go higher: outdoor showers, pool surrounds, and wet rooms used by clients with mobility considerations. Where we don't: the interior of a primary bathroom shower used by two adults.
Things we get asked, that don't actually matter much
Colour stability under UV. Porcelain doesn't fade. The exception is a handful of red and orange pigments that aren't UV-stable, but no reputable mill has shipped one in years.
Frost rating. Relevant for outdoor showers in cold climates. Almost irrelevant in a Bay Area primary bath.
The brand. Most large-format porcelain is pressed in a small number of mills in Sassuolo and Castellón, then sold under multiple labels. The body is what matters; the label often doesn't.
What we'd ask you, in a studio session
How many people use the room. Whether anyone is inclined to slip. Whether the floor is at grade or above an occupied space. What the substrate is. How long you intend to live there. Whether you want a curbless transition. What the room next door is finished in.
Most of those answers narrow the choice from forty options to four. Then we lay the four against the floor plan and pick.
If you're starting a wet-room specification, book a studio session. We'll have the right ranges on the table.