Technical guide
Large-format tiles and porcelain slabs: a specification guide
A plain-language guide to porcelain beyond one metre: what the thicknesses mean, why the substrate decides everything, how the adhesive and joints work, where the format fails, and what to check before it reaches a drawing.
By Mihail Cheptene · 25 July 2026
What counts as a tile, and what counts as a slab
UK trade practice calls anything with a side over 600 mm large format. BS 5385-1:2018 is stricter: a ceramic panel has a surface area over 1 m² and an edge over 1200 mm, and a thin tile is 5.5 mm thick or less. This guide covers that material, gauged porcelain, sold as slabs or panels in formats from 1000 x 3000 mm to 1620 x 3240 mm. It is also marketed as sintered stone, a wider label that includes non-porcelain products.
The brands are a short list. Laminam and Kerlite by Cotto d'Este opened the category; Neolith, Florim, Fiandre, Inalco and SapienStone followed. There are fewer factories than names: Fiandre and SapienStone are both Iris Ceramica Group. The technology is shared, ceramic powder compacted and sintered into very large sheets. The products are not. Mesh-backed and unbacked versions cut, handle and bond differently, and flexural strength varies by brand and thickness.
Figures here follow Laminam's technical guides as representative of the category, cross-checked against BS 5385: Part 1 for internal walls, Part 3 for internal floors, Part 4 for wet areas including showers. Principles transfer between brands; certificates and tested adhesive combinations do not. The data sheet of the specified product has the last word.
Thickness, and why it varies
The range runs 3.5, 5.6, 6, 12.5 and 20.5 mm; each thickness is a use, not a preference. 3.5 mm is wall-only, mesh-backed. 5.6 mm is the floor minimum: it tolerates impact and substrate imperfections that 3.5 mm will not. 6 mm mesh-backed covers walls and floors in the biggest sizes. 12.5 mm takes heavy traffic. 20.5 mm goes outdoors, pedestal paving included.
The thin end exists for weight, not economy. At 12.5 mm porcelain weighs about 30 kg per square metre, so a full 1620 x 3240 mm slab is roughly 160 kg, beyond safe handling on most sites. At 5.6 to 6 mm the same slab is about 75 kg, movable by two people on a vacuum frame. The factory-bonded fibreglass mesh is what makes that thinness workable: on thin slabs it is the structure, not an extra.
Not a bigger tile. A different object.
A 300 x 600 tile is rigid relative to its bed: it bridges small hollows, and every joint interrupts what the substrate is doing. A three-metre slab at 6 mm is closer to a membrane: it follows the surface exactly, transmits every void and every movement, and has almost no joints to absorb them.
The precision is real, rectified edges hold ±0.5 mm and flatness sits within 2 mm on the biggest slabs, and it cuts both ways: a slab this flat and thin hides nothing underneath. Every rule in this guide follows from that. It stops being a covering that helps and becomes a surface that reports.
Substrate and waterproofing
The governing number: 3 mm maximum deviation under a 2 m straightedge, checked in all directions. Corrected before installation with levelling compound, never by varying adhesive thickness on the day. On a slab that follows the substrate exactly, a ridge becomes lippage and a hollow becomes a void.
Moisture. Cement screeds dry at roughly 7 to 10 days per centimetre for the first 40 mm, slower beyond, and need to be under 3% residual moisture; anhydrite under 0.5%, sanded and primed. On a programme this is sequencing: a screed poured late cannot take a slab on time.
Walls. Gypsum plaster carries 20 kg of tiling per square metre under BS 5385-1, and a 6 mm slab plus adhesive approaches that: plaster must be sound, primed and solidly bedded, no dabs. Plasterboard takes a primer first. Direct tiling to plywood is no longer recommended; rigid backer boards replaced it.
Weight and logistics decide the format
The numbers are modest, about 8 kg per square metre at 3.5 mm and 15 at 6 mm; the sheet sizes turn them into a handling problem. A 1000 x 3000 mm slab at 3.5 mm is a 25 kg sail; a full 1620 x 3240 mm slab at 6 mm is 75 kg of glass-stiff ceramic: it stays vertical, never travels flat, and once cut it moves on a suction frame.
This is a design input. The slab must travel from van to room: stair turns, lift dimensions, door heights. On-site cutting needs a surface that carries the whole sheet. In an existing upper-floor bathroom the real question at design stage is whether a three-metre slab enters the building at all; if not, the format changes on the drawing, not on delivery day.
A vacuum suction frame spreads the grip across the sheet — a slab is carried by its face, never by its edge.
Adhesive: class, coverage, direction
The adhesive class is C2 with a deformability rating, S1 or S2: an improved cementitious adhesive that flexes with the differential movement between slab and substrate. Deformability is the point, not a premium option. Standard or quick-setting depends on substrate and size; the manufacturer tables name specific products per combination.
Coverage is non-negotiable: full-bed, double-coated. A 6 to 8 mm notched trowel on the substrate, 3 mm on the back of the slab, both combed the same direction, parallel to the short side, so trapped air has a straight path out when the slab is beaten in with a rubber float from the far edge. Spot fixing does not exist in this format: every unfilled pocket is a future fracture point, and no joint nearby stops the crack.
Thermal movement: small numbers, long lengths
Porcelain's coefficient of linear thermal expansion is about 6.6 x 10⁻⁶ per °C (EN ISO 10545-8). Over a 300 mm tile that is nothing. Over a 3 m slab with a 30 °C swing it is roughly 0.6 mm, and it never acts alone: the substrate expands at a different rate, and the difference between the two loads the adhesive line.
Where this stops being academic: heated screeds cycling daily, dark slabs in sun, external walls, any junction fixed hard to a dissimilar material. This is the physics behind every joint rule that follows. Movement joints exist because 0.6 mm with nowhere to go becomes stress with somewhere to go: through the slab.
How it behaves while the adhesive cures — and after
Porcelain absorbs around 0.1% water, so the adhesive under it loses moisture in one direction only: into the substrate. Cure runs slower than under a standard tile, and slower again over non-absorbent substrates such as existing tiles or a membrane. Traffic or grouting inside that window puts micro-movement into a bed that has not gripped yet: the classic origin of the edge that lets go a year later. Quick-setting adhesives close exactly this window, which is why they are the default for the largest sizes.
Once cured, the material is inert. Colour is UV-stable, geometry does not drift, the surface does not craze or fade. Everything that moves after month one is the building: seasonal moisture, thermal cycles, settlement. The system either absorbs that movement at the joints or delivers it to the slab.
Where the format is vulnerable: cuts, niches, corners
Every cut concentrates stress, and an internal corner concentrates it most. L-shaped cuts around doors, windows and niches are the most common origin of cracks, enough that Laminam treats a crack starting at an L-cut as within tolerance, not a material defect. The manufacturer's advice is to avoid the single-piece L and build those areas from butt-jointed pieces. Where an L is unavoidable, a hole of at least 5 mm radius is drilled at the internal corner and the cuts run into it: a rounded corner spreads the stress a sharp one collects.
Cut-outs for sockets and valves follow the same logic: four drilled corner holes joined by a grinder, never a punched square. Drilling is diamond, no percussion. Cut edges are eased with a diamond pad, and a slab weakened by an opening moves on a suction frame only. Niches and internal corners are butt-jointed and sealed flexible, not grouted rigid: an internal corner is a junction of two structures, not a decorative line.
An unavoidable L: the internal corners are drilled first and the cuts run into the holes, leaving a radius instead of a stress point.
Joints: the minimums, and why seamless is a photograph
BS 5385-1:2018 sets grout joints by size: at least 2 mm up to 600 mm, 3 mm up to 1200 mm, wider pro rata for panels; a three-metre slab wants 5 mm, the same minimum Laminam sets for its floors. Rectified edges make a 2 mm line look machine-made, but the joint does mechanical work, and it shortens the free length between interruptions.
Around the field: a 5 to 7 mm perimeter gap to every wall and fixed element, left free or sealed flexible, never grouted hard: a slab pinned at its edge has nowhere to move. In the field itself the manufacturer's grid is tight: expansion joints every 9 square metres, no side over 3 m, filled elastic, mirroring every screed joint and repeating at thresholds; external work runs at 3 to 5 m centres under BS 5385-3:2024. The jointless surface in the render is achievable, but by placing joints where the photograph does not look, not by deleting them.
What actually fails, and why
Cracks. Almost never the material's own doing. The usual chain: substrate movement crossing a joint that should have existed, or a point load over a void. The arithmetic of voids is unforgiving at this scale: 5% missed coverage under a full-size slab is a quarter of a square metre of hollow, a stool leg away from a star fracture.
Debonding. A deficit built in on day one: single-sided spreading, adhesive that skinned before the slab landed, dust on the back, an S1 where the movement needed S2. It holds through the first year, then releases along an edge after a full seasonal cycle. The failure date is misleading; the cause was laid at installation.
What does not happen: porcelain does not fatigue, craze, fade or change dimension. Practically every failure lives at the interface between slab and building, which means the material performs exactly as its data sheet promises, and everything else is execution.
A slab that released its internal tension during cutting — on the bench, not on the wall. Reserve slabs in the order exist for exactly this.
Common questions, straight answers
Can large-format slabs go over existing tiles?
Yes, with conditions. The existing layer must be fully bonded, checked rather than assumed, then abraded and degreased. If old joints cannot be mirrored or the floor shows cracks, an anti-fracture membrane goes between. Quick-setting adhesive is strongly advised, because nothing in that sandwich absorbs moisture.
Does underfloor heating change the installation?
The screed is commissioned through its full thermal cycle before installation, then returned to room temperature for laying. Shrinkage cracks are repaired first, an anti-fracture membrane is advisable for the largest slabs, and the 5 to 7 mm perimeter joint stays unfilled so the heated field can move.
If one slab cracks in two years, can it be replaced alone?
Mechanically yes: cut out, clean back, rebond. The real constraint is colour, shade varies by production batch, so a replacement bought later reads differently in raking light. The manufacturer's advice is to order reserve slabs from the original batch at purchase.
Does large format make a small bathroom feel bigger?
Visually, yes: fewer joints read as more surface. The limits are practical, whether a slab physically enters the room, and the waste percentage when most pieces are cuts. In many existing bathrooms a mid-size format delivers the visual effect a three-metre sheet cannot deliver through the door.
Why does installation cost more than standard tiles for the same area?
Because labour scales with risk, not square metres: two-person handling on vacuum frames, full double-spread coverage, levelling systems, slower cure management, and one mistake costing a whole slab rather than one tile.
Can it go on existing plaster without stripping?
Sound, keyed and primed plaster works within the weight limit: gypsum carries 20 kg of tiling per square metre under BS 5385-1, and a 6 mm slab with adhesive approaches it. Paint, wallpaper and anything friable comes off first, and the bedding is solid, never dabs.
Can a 6 mm slab hold a basin or wall-hung furniture?
The slab holds nothing: every fixing passes through it into structure behind, noggins, pattresses or a mounting frame positioned before cladding. Holes are drilled with diamond bits, never percussion, and the load path is designed at drawing stage.
Summarised from Laminam S.p.A.'s published technical guides and data sheets (laminam.com), representative of the gauged porcelain slab category, together with BS 5385-1:2018, BS 5385-3:2024 and BS 5385-4:2015. Principles transfer across brands; figures, certificates and tested adhesive combinations do not. Always verify against the technical data sheet of the specified product.