15 Litres of Adhesive Saved by Notch Trowel Selection on 300 Square Metres of Porcelain

Why the notch profile changes the volume

A 10 mm square-notch trowel leaves ridges that produce a theoretical wet-bed depth of about 5 mm after combing. A 6 mm half-round profile is nearer to 3 mm once the tile is placed and beaten in. Porcelain changes the calculation because the biscuit absorbs almost nothing, so extra depth under the tile does little for bond strength. It remains as wet material trapped below the slab.

A cementitious adhesive such as Mapei Keraflex Maxi S1 covers roughly 3 to 4 square metres per 25 kg bag at a 3 mm bed. At a 5 mm bed, the same bag is closer to 2 to 2.5 square metres. Across 300 square metres that spread is visible on site with a wet-film comb, especially when the substrate is already level and the tile is beaten in evenly.

The half-round notch collapses cleanly and pushes air sideways as the tile is bedded. On porcelain in a wet area, the target is still the 90 to 95 percent contact ratio, so the thinner bed only saves material when it reaches that contact without leaving voids. A floor that is out of flat gives the half-round profile no spare depth to absorb hollows, which brings the substrate into the adhesive calculation before any trowel choice is made.

Reading coverage from a lifted tile

Lift a freshly laid tile within the first ten minutes and check the back. Ridges that have transferred and smeared into a near-continuous film show that the notch, adhesive open time, and bedding pressure are working together. Sharp ridges with bare valleys between them usually mean the adhesive has skinned over before the tile went down, or the bed is too deep for the tile weight to collapse.

That back-check remains the most useful field measurement because it shows contact directly. A 600 by 600 mm porcelain tile has enough weight for a 3 mm half-round bed to collapse under hand pressure and a rubber mallet. A 1200 by 600 format often needs more help, which is why large formats frequently get back-buttered whatever notch is used. Back-buttering adds a thin skim to the tile face, filling ridge valleys from the tile side as well as from the floor.

Ridge collapse is sometimes mistaken for waste when the aim is to reduce consumption. In practice, standing ridges are the ones spending material badly: they use adhesive while leaving voids. Full contact at the shallowest bed that achieves it is where the saving appears.

The substrate sets the limit

A floor that deviates 6 mm under a 2 metre straightedge turns the adhesive bed into a levelling layer. The installer compensates with a deeper notch, extra top-ups, or both, and the saving from the 6 mm half-round profile disappears into the hollows.

Uzin NC 175 is a fibre-reinforced smoothing compound rated for pour depths from around 2 mm up to 20 mm in one application. That range covers much of the deviation found in an old screed. Poured over a primed substrate, it self-levels to a flatness that allows the half-round notch to work without local patching under individual tiles.

With a timber floor the priority shifts, because a plywood subfloor destined for LVT or tile needs mechanical stability before it can be levelled at all: 18 mm WBP plywood, screws at 150 mm centres, staggered joints, and a perimeter gap for movement. Levelling plywood before LVT is mainly about removing deflection. A smoothing compound over a board that flexes underfoot will crack.

Prime, level, cure, and then choose the notch for the flattest bed the floor can support. If the levelling stage is skipped, trowel selection loses most of its meaning because the adhesive is being used to fill the floor as well as bond the tile. Every millimetre of substrate deviation asks the adhesive to make up the difference, and adhesive is the expensive way to level a floor when a smoothing compound can bring the surface into tolerance before the first tile is set.

A flatter substrate also changes the pace of the installation. Lippage corrections take less time, levelling clips need fewer adjustments, and fewer tiles have to be lifted and re-bedded. Those labour savings rarely appear in the coverage calculation, yet they are part of the same flatness decision.

Ditra over a floor that will move

Schluter Ditra sits between the tile bed and a substrate expected to move, commonly a timber floor or a young screed still releasing moisture. The polyethylene membrane is roughly 3 mm thick. Its keyed underside bonds into the adhesive below, while the square-cavity upper face locks into the adhesive above.

When the substrate expands or contracts, the cavities absorb shear before it reaches the tile. The membrane changes adhesive use in two places: the bed underneath must suit the fleece and underside geometry, and the bed above must fill the top cavities before the tile is set.

That top fill is the mechanical key. Installers new to the system sometimes spread too little material and fail to bond properly into the fleece. Schluter’s own coverage guidance points to filling the cavities completely first, followed by combing the surface for the tile.

Grout volume starts at the joint

A cementitious grout in a shower floor absorbs water and holds it. Over time it develops the residue and growth that tilers often scrape out of wet rooms after a couple of years. An epoxy grout such as Mapei Kerapoxy CQ is non-absorbent, resists staining, and is installed without sealing. It costs several times more per kilogram, and its working time is counted in tens of minutes instead of hours.

Joint width drives the grout quantity more sharply than floor area. A 300 square metre floor with 2 mm joints between 600 mm tiles uses only a fraction of the grout needed for the same floor at 5 mm joints. In a wet room, the installer has to read the joint width and the product chemistry together before ordering volume.

Epoxy in a 2 mm joint is manageable to work. In a 10 mm joint, the same material can set before it is struck off cleanly. That difference affects labour as much as product cost.

Because epoxy does not absorb water, it also holds its shade, while cementitious grout in a wet area can lighten as it dries unevenly and darken again when wet. On a large floor read as one plane, that variation is visible from the doorway, so the material that reduces cleaning labour also reduces callbacks about patchy joints.

Undercutting the skirting

Cut the door casing and skirting so the tile can slide underneath; an oscillating multi-tool with a flush-cut blade, run flat on a tile offcut used as a height gauge, removes the bottom of the timber cleanly and leaves the porcelain disappearing under the joinery.

This also keeps the coverage discipline at the perimeter, since a tile scribed tight to skirting usually has a hand-cut edge with weaker adhesive contact, while a tile slid under an undercut casing keeps its factory edge and full bed to the wall line.

The worked number for 300 square metres

Take Keraflex Maxi S1 in a 25 kg bag, mixed to roughly 15 litres of usable adhesive. At a 5 mm bed from the 10 mm square-notch trowel, coverage near 2.2 square metres per bag puts a 300 square metre floor at about 136 bags. At a 3 mm bed from the 6 mm half-round notch on a levelled substrate, coverage near 3.5 square metres per bag puts the same floor at about 86 bags.

The difference is 50 bags. At roughly 15 litres of mixed adhesive per bag, the material saved is about 750 litres across the job, or about 2.5 litres per square metre. The 15 litre figure belongs to the mixed yield of one bag; used as the total saving, it understates the result by a full pallet-scale amount.

That number depends on the Uzin pour producing a substrate flat enough for the shallow bed to hit more than 90 percent contact without local thickening. If the floor needs the adhesive to correct dips, the calculated saving moves out of the coverage sheet and into the bed.

A large-format tile then turns the saving into a labour question. When a 1200 by 600 tile will not collapse a 3 mm bed, back-buttering is the right way to keep contact, and the litres saved by the shallower notch have to be weighed against the extra handling. Whether the 750 litre figure survives contact with a floor full of 1200 mm tiles is the part no coverage sheet settles in advance.