Large format tiling
Specifying liquid screed as a base for large format tiling
Large format tiles are unforgiving of an uneven or insufficiently dry substrate, so the screed specification needs a tighter surface regularity class and a clear moisture and primer sequence.
Specifying a large format tile floor over screed?
Send us the tile size and adhesive system and we can confirm surface regularity and moisture requirements.
Or call our landline on 01392 237700
Large format tiles, generally taken to mean pieces significantly bigger than a traditional 300 millimetre tile, place greater demands on the substrate beneath them because there is less flexibility in the tile to accommodate minor lippage or an uneven surface across its span.
A liquid screed's inherent self-levelling properties are an advantage here, but the specification still needs to define the surface regularity class required, the moisture condition the screed must reach before tiling, and the primer or preparation sequence the tiling contractor will follow.
Coordination between the screed specification and the tiling specification matters more with large format products than with smaller tiles, since a mismatch discovered only once tiling begins is expensive and disruptive to correct at that stage.
Screed for large format tiling — the points that decide it
- A tighter surface regularity (SR) class than might be used for smaller tiles or resilient flooring is generally appropriate for large format tiling.
- The screed must reach the moisture level specified by the tile adhesive manufacturer, not just a generic figure, before tiling proceeds.
- Priming the screed surface according to the adhesive manufacturer's system is usually required to control absorption and improve bond.
- Movement joints in the screed should be carried through, or otherwise addressed, in the tiling layout rather than tiled straight over.
- Large format tiles are less tolerant of local dips or high spots, so localised surface checks may be more informative than a single average reading.
- The adhesive bed and fixing method should be confirmed as compatible with a liquid screed substrate, since not all adhesives are suited to every screed type.
- Any deflection or movement limits for the substrate relevant to rigid large format tiles should be discussed with the tile manufacturer where the floor is subject to vibration or significant live loading.
Why surface regularity matters more with large tiles
A small tile can accommodate a degree of undulation in the substrate without the joint lines becoming visibly uneven, but a large format tile spans a greater distance and shows up any dip or high spot as visible lippage between adjacent pieces, which is both a cosmetic issue and, in some cases, a trip hazard.
Specifying an appropriate surface regularity class for the screed, referenced against BS 8204, and confirming this with the tile installer's requirements before the pour, reduces the risk of a costly remedial levelling stage being needed once the screed has cured.
Moisture and priming before tiling
Tile adhesive manufacturers publish moisture criteria for the substrate before their products can be applied, and these should be checked specifically rather than assuming a generic screed drying rule applies. Calcium sulfate screeds in particular usually require a specific primer before a cementitious tile adhesive is applied, to manage the interaction between the two materials.
Skipping or substituting the specified primer is a common cause of adhesive failure under tiles, so the primer system should be specified explicitly and sourced from the same manufacturer range as the adhesive wherever possible.
Coordinating joints and layout
Movement joints in the screed do not disappear once tiling begins; they need to be carried through the tile layout, either as a matching movement joint in the tiling or otherwise accommodated, since tiling directly and rigidly across a screed movement joint risks cracking the tiles at that line.
Agreeing the tile layout and joint positions alongside the screed movement joint plan, rather than after the screed has already been poured, avoids a layout clash that is difficult to resolve once both trades have finished their work.
Specifying a large format tile floor over screed?
Send us the tile size and adhesive system and we can confirm surface regularity and moisture requirements.
Or call our landline on 01392 237700
Screed for large format tiling — your questions
In short
Large format tiles are unforgiving of an uneven or insufficiently dry substrate, so the screed specification needs a tighter surface regularity class and a clear moisture and primer sequence.
Specifying a large format tile floor over screed?
Send us the tile size and adhesive system and we can confirm surface regularity and moisture requirements.