Installation of Large Format Porcelain Tiles (over 60cm)

With small tiles, the grout joints can hide a slightly uneven surface. A large tile cannot adapt. It sits like a rigid board. If there is a bump or a dip underneath, you get a hollow under the tile or an edge that sticks up above its neighbour. That is why the substrate and the fixing technique matter more than anything else here.

The price list has three related items:

• Porcelain tiles over 60 cm is the general term for tiles with at least one side longer than 60 cm, such as 60 × 120, 80 × 80 or 120 × 120 cm
• Laying 60 × 120 cm tiles covers the most common large size in homes. Two people with suction lifters can handle it, and it fits well into a typical bathroom
• Oversized slabs (for example 120 × 260 cm) are a separate category. They need transport frames, a bigger team and an almost perfectly flat substrate

Thickness matters too. Standard porcelain is usually around 9 to 10 mm thick. Thin slabs of about 6 mm are lighter but break more easily when moved. Thick outdoor slabs of around 20 mm are often laid on pedestals or gravel, which is a different method from bonding them with adhesive indoors. The finish also plays a part. Polished porcelain shows every height difference at the edges. Matt and textured finishes are more forgiving and less slippery, so they are a safer choice for bathroom floors and terraces.

1. Measure the room and plan the layout: tile direction, joint lines and where the cut tiles go, so that you avoid thin slivers along the walls.
2. Check the substrate for flatness and strength with a long straightedge, and level any unevenness with a skim coat or self-levelling compound.
3. Clean off the dust and prime the surface. In bathrooms and showers, apply waterproofing, including sealing tapes in the corners and around drains.
4. Mix a flexible adhesive with a slow-speed paddle mixer and spread it with a notched trowel on both the substrate and the back of the tile (back-buttering).
5. Carry the tile with suction lifters, set it in place, press it down and align it using a levelling system with wedges or clips.
6. Lift a tile now and then to check that the adhesive covers the whole back.
7. Let the adhesive cure as the manufacturer specifies, remove the levelling clips and grout the joints.
8. Fill the movement joints around the edges and in the corners with flexible sealant, then clean the surface.

On walls, the first row often rests on a temporary batten so the heavy tiles cannot slide down before the adhesive sets.

The substrate must be firm, dry and flat. Manufacturers of large formats usually ask for a flatness tolerance of roughly 2 mm under a 2 m straightedge, and some are stricter. The screed must have dried and cured properly. With anhydrite (calcium sulphate) screeds, the residual moisture is also measured and the surface is sanded before tiling.

The adhesive should be a flexible cement-based type, usually class C2TE S1 or S2 under EN 12004. The "S" means the bond can cope with small movements in the substrate and with temperature changes. For large formats, a trowel with a notch of roughly 10 to 12 mm is common. The aim is almost full coverage of the tile back with no voids.

The joints should be at least as wide as the tile manufacturer recommends. For rectified tiles (with precisely ground edges), this is usually 2 mm or more. A joint of zero is a bad idea, because the surface needs a little room to move. As well as normal joints, you need movement joints around the edges of the room, at doorways between rooms and across large areas.

What about underfloor heating? Porcelain works well with it because it conducts heat well. Before tiling, the pipes should be pressure tested and the screed heated up gradually according to a heating schedule. After tiling, switch the heating on step by step.

The amount of work, and so the final cost, depends mainly on these things:

• Condition of the substrate: a new, flat screed saves a lot of time, while old uneven plaster and floors have to be levelled first
• Tile size and weight: two people can manage 60 × 120 cm, but oversized slabs need more hands and frames
• Shape of the room: niches, recesses, a shower with a fall to the drain, window reveals and lots of cuts around pipes all take longer
• Walls and floor together: if the joints on the wall should line up with those on the floor, the planning is more demanding
• Pattern: in an offset layout, the shift is usually no more than a third of the tile length, because large tiles are often slightly bowed. Where the pattern has to match, for example marble effect, the tiles need sorting
• Access: a narrow staircase or a small lift in a block of flats makes carrying in large boxes awkward

In a block of flats, it is wise to agree noisy work and the delivery of materials with the building management or owners' association in advance. Replacing tiles on its own does not usually need a permit. If the work is part of bigger changes, such as removing partition walls or altering shared risers, check with your local building authority what the local building regulations require.

• Tiling on an uneven substrate in the hope that the adhesive will make up the difference
• Applying adhesive only to the substrate or in dabs, which leaves voids that sound hollow when tapped and crack under load
• Using cheap, non-flexible adhesive that cannot cope with substrate movement or underfloor heating
• Working without a levelling system, so neighbouring edges end up at different heights
• Forgetting movement joints around the edges and across large areas
• Walking on the floor or grouting before the adhesive has cured
• Careless handling, so a corner chips off or a tile cracks before it is even laid

When the material arrives, store the boxes upright or on a flat base and check that all tiles come from the same production batch.

• Run a straightedge over the surface in several directions to check flatness
• Slide a finger or fingernail across the joints, neighbouring edges should not stick up noticeably
• Tap the tiles gently, especially at the corners and in the middle, and listen for a hollow sound
• Check that the joints are straight, even in width and completely filled
• Look at the cuts along the walls, door frames and around pipes
• Make sure there are flexible movement joints around the edges and in the corners
• In the shower, check the fall towards the drain and the connection to the waterproofing
• With underfloor heating, ask when and how to switch it on gradually
• Keep spare tiles from the same batch for any future repair