Tiling a Bathroom Floor on Concrete with DMX Membrane and Grout

Jeff of Home RenoVision DIY tiles a basement bathroom floor over concrete with large-format rectified porcelain tile, working from prep to grout. The system rests on DMX tile underlayment, an uncoupling membrane with a fleece back and a dimpled front, bonded with Schluter All-Set thinset. His claim for it: it "is an underlayment system that's guaranteed to work."

Layout: square to the door

The aim is to finish at the door with a full tile, so the first tiles are set square to the doorway. Jeff cuts a piece of lumber to the door's width and accounts for the drywall thickness. The door midpoint on a 1 3/8 jamb is about three-quarters of an inch, and he uses the quarter-inch side of a carpenter pencil to mark it. That lumber marks where the finished Schluter edge should sit. A laser line is aligned to a square held against it and slid until it hits the planned grout line. Every later measurement comes from that first square tile. Cuts go on edges that trim or other components will hide later.

Measuring for the toilet flange

For the flange, he uses a toilet flange extension kit purely as a measuring tool. The top of the flange should sit flush with the stone or up to a quarter inch above it. His first measurement was 13 3/8 to the lead edge of the flange, and 19 3/4 off the wall. He then caught a mistake: measurements must come from the tile line, the only square reference, not from the wall. He extends the line with a factory-cut scrap and remeasures, correcting by half an inch to three and a half. A temporary tile flange is screwed over the hole, onto the ABS and not just the cement. The membrane is rolled over it, the ridge is found with the back of a knife, and the opening is cut roughly, since it need not be perfect.

Thinset and membrane

The membrane is bonded with a 1/4 by 1/4 trowel, and the tile above with a 1/2 by 1/2 trowel. Jeff points out that this membrane takes the same thinset above and below, where other fleeces need a thin mix underneath. A finger drag test shows the right thickness: the ridges should collapse behind the finger. He keeps the trowel angle consistent around the room so everything ends up the same thickness. He does not wet the concrete, because All-Set suits wet or dry conditions. A bucket of water and a sponge stay at hand, since the thinset sets up thick and cleaning as he goes keeps tools workable. The waterproof tape at the joint is included in the package, and he applies it for a monolithic layer, because the membrane also isolates cracks in the concrete. He compares the job to taping drywall.

Setting the tile

For the large tile, he back-butters a thin layer over the whole tile to guarantee a bond to the thinset. He then slides the tile into position to collapse the ridges, pushing and pulling, and thinset squeezing out between tiles is expected. Grout joints are 1/16 inch, held with leveling clips, which he sets before collapsing the ridges. The clip system is semi-reusable: the base is reusable, but each clip leaves half its body behind.

His tip is a number one pencil, pre-sharpened, to clean thinset out of the 1/16 joints. Wood and graphite can't chip the glaze. Clean joints let the clips break off the next day. Jeff calls leaving thinset in the joint the number one homeowner mistake. Clips are squeezed together after twisting, because threads push things apart.

A miscut

Near the end of the day, around 3:30, one door-side tile was cut wrong. Jeff says his own measurement was the error: he wrote 22 1/2 when it should have been 24 1/2. He pulls the tile and washes it, hoping to salvage it for the shower. He says he doesn't do plumbing after lunch for the same reason. His advice is to cut tiles slightly short at the wall, since baseboard or casing covers the gap. The cut at the door is the one that matters most, because thin casing can expose the grout if the tile is cut too short.

Trim and grout

The Schluter edge at the door is cut with a QEP tile square, which has a groove that holds the trim for the miter saw. Jeff says this is safer than putting a hand there. He adds thinset beneath the trim for structure and keeps the trim out of contact with the tile. A pencil forms the grout line, and thinset on the trim face washes off for up to 48 hours.

For the grout, he uses a polymer grout in a 10 lb bag, mixed with a rubber float and a small mixer. Mixing more than 10 lb calls for a bigger paddle. He mixes the whole bag for consistent color, pouring it into about half an inch of water. It sits about 10 minutes before a second mix. "The more you touch it, the looser it gets." Grout goes in at 45 degrees in both directions, and is not applied between the tile and the shower. Two buckets, each with a clean sponge, prevent haze. The ideal joint dries with a slight indent, and too-wet grout shrinks. If so, it can be resurfaced four or five hours later. Wiping uses a squeezed-out sponge at 45 degrees. Knees and toes stay out of the grout lines. Air bubbles, which polymer grout is prone to, are patched with dry hands and given about 10 minutes before wiping.