Lippage Under 0.5mm Corrected with a Raimondi RLS Levelling System on 120cm Slabs

Deviation between two slab edges becomes visible in grazing light once it passes roughly 0.5mm. On a 120cm x 60cm porcelain slab weighing close to 18kg, hand pressure cannot reliably maintain that line while the bed cures. The Raimondi RLS clip-and-cap system carries the load across the joint, pulling neighbouring surfaces into one plane during the set window. British Standard BS 5385-3 relates acceptable lippage to joint width, and for joints of 6mm or more the permitted deviation sits around 1mm plus a proportion of tile thickness. A floor kept under 0.5mm is tighter than that requirement.

The RLS clip slides under the tile edge. The reusable cap then threads down over the exposed stem and clamps two adjacent tiles to the same reference plane. Raimondi supplies clips for finished bed thicknesses up to 12mm and joint widths from 1mm to 3mm, while on slabs of this format the usual failure point sits in the adhesive bed and substrate below the cap.

Where the Screed Decides the Tolerance

A levelling system can pull edge deviation into line only when the substrate below is already flat enough for the slab format. Across a 120cm span, a screed running 3mm out of plane under a two-metre straightedge asks the adhesive to absorb the whole error. That amount of stored stress can lift a corner or hold an edge high even with caps tightened correctly.

BS 8204-1 sets surface regularity classes for cementitious screeds. SR1 permits a maximum deviation of 3mm under a 2m straightedge, SR2 permits 5mm, and SR3 permits 10mm. Large format porcelain over 60cm on any edge uses SR1 as the working minimum.

Many slab installers check the whole floor with a 2m aluminium straightedge before any adhesive is mixed. The straightedge is laid across the room in a grid, then turned through different directions. Where it rocks, the high point is found; where daylight shows below it, the low area is marked in pencil.

Low spots are filled with a levelling compound such as Mapei Ultraplan or Ardex K 15. Both can be brought to a feather edge and can cure to receive tile within hours, depending on ambient temperature. The aim is to remove the hidden load that would otherwise be transferred into the clip stems as the slab settles.

Dryness is checked with the same seriousness as flatness. A cementitious screed loses moisture at roughly one millimetre of depth per day for the first 50mm. A 75mm sand-and-cement screed therefore sits at around 90 days before it reaches the 75 percent relative humidity threshold measured with a hygrometer under BS 8203.

Bonding porcelain slabs above that reading traps moisture under an impervious surface. The adhesive then fails to gain full strength. Flatness and dryness are settled before the trowel is lifted, because both are hidden once the first slab is down.

Adhesive Coverage Carries the Load

A C2 classified cement adhesive under EN 12004 is the baseline for porcelain of this weight. C2TE S1 adds the deformability class S1, which means the cured bed can flex enough to absorb differential movement as temperature shifts. Over underfloor heating, that deformability is required.

Coverage under a 120cm slab needs to be effectively complete and void-free. Empty pockets under a slab edge are common starting points for impact cracks. They also leave the RLS cap with no firm bedding beneath the edge it is trying to pull flat.

The method is back-buttering. A thin scraped coat is pressed into the back of the slab. The substrate then receives a floated bed combed with a 10mm or 12mm half-moon notched trowel.

All combing ridges run in one direction. As the slab is beaten down, air is pushed out along the channels instead of being trapped below the porcelain. The movement of the air matters because large slabs give it a long route to escape.

Beating is done with a rubber-faced beating block and a mallet. The installer works from the centre outward, following the direction of the notch ridges. A test slab is lifted after beating so the transfer can be inspected across the back.

If the transfer is incomplete on a slab this size, the setting changes before the floor continues. The answer may be a larger notch or a wetter mix, depending on what the lifted slab shows. RLS clips are inserted only after full coverage has been proven.

On a 120cm side, clip spacing is typically one clip every 30cm to 40cm. The cap is tightened until the wedge or threaded head seats firmly. Over-tightening risks crushing the tile glaze, so the aim is firm clamping without damage.

Uncoupling Over Underfloor Heating

Underfloor heating makes the substrate and porcelain move at different rates. A warm-water system cycles the screed through temperature swings, and the screed expands and contracts on a schedule the porcelain does not follow. Where no separation layer is present, differential shear passes straight into the adhesive bond and then into the grout joints.

Schluter Ditra is a polyethylene matrix with square cavities and an anchoring fleece on the underside. Bonded to the screed with a suitable thin-set, it decouples the tile layer from substrate movement. The cavities also carry vapour laterally to the perimeter.

The 8mm Ditra-Heat variant integrates heating cable channels directly. Over an existing wet system, standard Ditra at around 3mm handles the uncoupling function. The tile is then bonded into the matrix cavities with a second coat of C2 adhesive, filling the squares so the slab sits on a mechanically keyed surface.

Movement joints still form part of the layout over this assembly. BS 5385 calls for movement joints at bay perimeters, at changes of substrate, and generally in field areas exceeding around 40 square metres or at intervals of 8m to 10m in continuous runs. The uncoupling mat reduces stress reaching those joints while the joints remain part of the floor design.

Commissioning the heating affects the sequence. The system stays off during tiling and throughout the full adhesive cure. After that, the temperature rises in stages of a few degrees per day, since taking a cold slab floor straight to operating temperature loads the fresh bond with thermal shock before it has reached strength.

Epoxy Grout in a Kitchen

Epoxy grout under EN 13888 as class RG resists chemical attack and absorbs no liquid in the way a cementitious CG2 grout does. In a kitchen where turmeric, red wine, and coffee reach the joints daily, that low absorbency is why installers use products such as Mapei Kerapoxy or Litokol Starlike.

When epoxy grout stains, residue from installation is usually the cause. Epoxy left on the tile face or in the joint texture dries as a haze, and that haze holds dirt. The wash-off window is short, often under 30 minutes at room temperature.

Cleaning uses an acidic epoxy-grout residue remover, with plain water unsuitable for the cured film. The remover is worked with a hard sponge in a circular motion before a clean pull-off.

Short Tolerance Note

A 0.5mm result starts with the substrate and adhesive bed. The RLS can hold only the plane that those layers already allow.

A Worked Sequence on 12 Square Metres

Take a kitchen floor of 12 square metres, laid in 120cm x 60cm slabs. That comes to 16 to 17 slabs plus cuts. The screed is checked with a 2m straightedge on a grid, and two low spots are filled with Ardex K 15 to bring the whole floor inside SR1.

A hygrometer reads 72 percent relative humidity. That is under the 75 percent threshold, so bonding proceeds. Ditra is bonded down first over the underfloor zone with a C2 thin-set combed at 4mm, rolled flat, and left to grab.

The slab adhesive is a C2TE S1, mixed to a slump that holds a comb ridge without slumping. Each slab is back-buttered and set into a 12mm notched bed. The slab is then beaten with a rubber block along the ridge direction.

A lifted check slab shows full transfer. RLS clips go in at roughly 35cm centres along each 120cm edge, four clips per long side, and the caps are tightened until the edges pull level. A straightedge laid across three slabs confirms deviation under 0.5mm at each joint.

The caps come off after 24 hours once the adhesive has set. They are snapped sideways with a rubber mallet along the joint line so the clip stem breaks below the surface. The joints are then grouted with Kerapoxy, washed within the window, and cleared of residue haze with an acidic sponge pass.

The heating stays off through the cure and rises a few degrees daily afterwards. The measured lippage remains unchanged after cap removal because the bed set flat while clamped.

Remaining Question

This sequence records the installation result, while the unresolved point is how a borderline-dry screed behaves after repeated heating seasons beneath an impervious porcelain surface.