How to fix large-format tiles without hollows (S03 / S04)

Large tiles cannot bridge a bad background. Confirm the substrate is sound, cured and clean, and flat to within about 3 mm under a 2 m straightedge. Grind high spots and fill lows with a levelling screed.

SUMO S03 (C2TE) is formulated for large-size tiling. For commercial floors and heavy traffic, step up to SUMO S04 (C2TES1), which is deformable and absorbs shock. Both suit large-format porcelain, vitrified and stone.

Dry-lay and mark reference lines first. Mix the adhesive to a firm, lump-free paste — powder into water — and let it slake, then remix. Only mix what you can lay in the open time.

Key the substrate with the flat trowel edge, then comb straight ridges in one direction with a 10–12 mm notch. S03/S04 have extended open time, but do not let the bed skin over before you place the tile.

For 600×600 mm and larger tiles, also spread a thin even coat on the tile back. Combed bed + back-buttered tile collapses the ridges into full contact and pushes out air — aim for 90%+ coverage with no gaps at corners.

Press and slide the tile across the ridges, beat it in from the centre with a rubber mallet, then set spacers and levelling clips to keep faces flush. Check with a straightedge as you go.

Sponge faces and joints before the adhesive sets, cure per the datasheet, tap-test for solid contact, then grout — epoxy grout in wet zones.