How much grout for a subway tile shower?

One shower, two joint widths — and the manufacturer says so

Daltile's spec sheet for Rittenhouse Square, the archetypal 3 × 6 subway, prints its installation recommendation as two lines: "Wall: 1/16″" and "Mosaic: 1/8″". One collection, one shower, two different joints, because the wall tile and the dot-mounted mosaic that usually goes on the floor are different products — the wall tile is 5/16 in thick, the mosaic 1/4 in.

Run the two separately and the floor is the surprise: at Daltile's own widths the mosaic takes 17.7 lb per 100 sq ft against 7.8 for the walls, 2.3 times the rate per square foot. A shower floor is small, so it does not overturn the total — on a 15 sq ft pan it adds roughly a quarter again on top of the walls, more if the chip is 1 in rather than 2 × 4. But you cannot take the wall figure down to the floor, and the calculator above only holds one tile at a time. Run it twice and add the results.

The same tile, beveled, is 20% more grout

The Rittenhouse sheet lists the flat 3 × 6 at 5/16 in thick, 100 pieces and 12.50 sq ft per carton, and the beveled 3 × 6 at 3/8 in thick, 80 pieces and 10.40 sq ft. Same name, same nominal size, different depth of joint: 7.8 lb per 100 sq ft becomes 9.3, exactly 20% more. Over this whole shower that is about 2 lb — not another box, but it is why two people tiling the same shower get different numbers from the same tile name. The thickness is on the carton; the tile name will not tell you.

What actually moves the number, in order

Joint width first, by a distance: taking the walls from 1/16 to 1/8 in takes 7.8 lb per 100 sq ft to 15.1, near enough double. Depth second, at 20% for the bevel. The long shape of a subway comes last and is smaller than its reputation — a 3 × 6 against a square tile of the identical face area, everything else held, is only about 6% more grout. And running bond adds nothing at all: offsetting the courses relocates the joints, it does not create any. Pick the joint width carefully and stop worrying about the pattern.

Grout is not the waterproofing, and the corners are not grout

Two things from LATICRETE's own data sheet for PERMACOLOR that decide whether the shower survives. Under Limitations: grouts "are not replacements for waterproofing membranes" — the membrane behind the tile does that job, and no grout choice substitutes for it. And under submerged and steam use: allow the grout 14 days at 70°F before filling with water or exposing to steam, which is the real answer to when the shower can be used.

Then subtract the corners from your order. TCNA's guidance is that "anywhere there is a change in substrate or backing surface such as the joint between walks and floor and wall joint, caulk should be used in place of grout since these surfaces move independently of each other"; grout there "will crack allowing moisture to penetrate". In a standard shower that is both vertical inside corners, the whole wall-to-pan junction and the curb edges — silicone, not grout.

Sources: Daltile Rittenhouse Square Glazed Ceramic sales sheet (sizes, thickness, sq ft per carton, grout joint recommendation), LATICRETE PERMACOLOR Grout, DS-250 (limitations, submerged and steam cure) and TCNA, Grout FAQ (caulk at changes of plane).