How to lay tiles properly: the NF DTU 52.2 rules explained

NF DTU 52.2 and NF DTU 52.1: the reference texts

NF DTU 52.2, a French unified technical document published as a standard, sets the rules for bonding ceramic tiles and natural stone to floors and walls, indoors and outdoors: accepted substrates, preparation, adhesives, coverage, joints and tolerances. It covers the vast majority of jobs. NF DTU 52.1 covers floors laid on a thick mortar bed, still used for natural stone, terracotta or large differences in level.

Their scope is limited, in particular by tile size depending on location and substrate. Beyond it, for very large slabs, thin porcelain or an old timber floor, installation follows a technical assessment (Avis Technique) or the manufacturers’ instructions.

Checking the substrate first

Concrete slab, screed, existing tiles, plasterboard or render: the substrate carries the tiles for decades. Before laying, we check:

• flatness, with a 2 m straightedge and a short rule: the professional rules set a tolerance, stricter for large tiles;
• cohesion: a dusty substrate or a render that scratches easily will not hold the adhesive;
• cleanliness: dust, grease, old adhesive or paint prevent bonding;
• the residual moisture of a recent screed, measured before laying, especially for an anhydrite screed;
• stability: active cracks, a floor that deflects, expansion joints to be carried through.

Depending on the findings, we apply a primer, then a levelling compound if flatness requires it. Loading on extra adhesive to fill a dip is not accepted practice: tile adhesive is a thin bed. This preparation is part of our screed and levelling service; for a substrate that is already tiled, see our article on tiling over existing tiles.

Choosing the adhesive: what C1, C2, S1, E, T and F mean

Tile adhesives are classified under the European standard EN 12004. The first letter gives the type of product: C for a cement-based adhesive, D for a ready-mixed paste, R for a resin-based adhesive. Then come the performance codes, explained in our glossary entry on adhesive classes:

A C2 S1 adhesive covers many porcelain situations, but the final choice cross-checks three data sheets: the tile’s, the adhesive’s and that of the system underneath, such as a membrane or waterproofing. Grouts have their own classification too, under EN 13888: CG1 or CG2 for cement-based grouts, RG for resin grouts.

Bonding with no voids: notched trowel, open time and back-buttering

What holds a tile is the contact area between its back and the adhesive: the coverage. The notched trowel is chosen according to the tile size and the flatness of the substrate; the ridges are combed in one direction, then the tile is slid across them and beaten in. At regular intervals, we lift a freshly laid tile to check that its back is fully covered.

Adhesive also has an open time, during which it can still wet the tile. After that, a skin forms and the tile no longer really bonds, even if it seems to hold. Sun, draughts and absorbent substrates shorten that time: you only spread what you can cover in time.

Back-buttering, adhesive on both the substrate and the back of the tile, is required above a certain tile size and outdoors. We also use it whenever a tile is long, heavy or exposed to water, for void-free contact where water or frost would find the weak spot.

Perimeter joints, movement joints and grout joints

Tiling moves little, but it does move: with temperature, underfloor heating and shrinkage of the substrate. The laying rules therefore provide for three types of joint:

• the perimeter joint, against walls, columns and door frames, hidden by the skirting or filled with a flexible sealant;
• movement joints, which divide large areas and mark changes of substrate, on top of the building’s expansion joints, which are always carried through;
• the joints between tiles, whose minimum width depends on the tile: even rectified tiles are never laid without a joint.

In the internal corners of a shower or against a bath, cement grout gives way to a flexible sealant. The position of movement joints is decided at the layout planning stage, so that they fall on a grout line.

Respecting drying times

Each step has its own waiting time, given on the data sheet and lengthened by cold or damp. As orders of magnitude, depending on the products and conditions:

• before grouting: usually the next day with a standard adhesive, a few hours with a fast-setting one;
• before walking on it: light foot traffic first, then a longer wait before heavy furniture;
• before turning underfloor heating back on: a gradual rise in temperature, as set by the manufacturers;
• in a bathroom, waterproofing under the tiles dries between coats and before tiling, and the grout must be dry before the first shower.

Cutting these times short to save a day is a classic cause of cracked grout: we set them out in the schedule of the quote.

Flatness, lippage and handover: what you can check

At the end of the job, a few simple checks let you judge the work:

• in grazing light, in front of a window or with a torch laid on the floor, the edges stay flush, with no marked lippage;
• gentle tapping with a hard object reveals no hollow-sounding areas, especially at the corners of the tiles;
• grout lines are full, even and consistent in colour, with clean, well-distributed cuts;
• there is a flexible sealant wherever the tiles meet another material.

The professional rules set tolerances for flatness and lippage, which are the reference in case of disagreement. After handover, the French ten-year builder’s liability, the garantie décennale, covers damage that undermines the soundness of the work or makes it unfit for its purpose; our work is covered by it. In Évreux as in Île-de-France, our floor and wall tiling page describes how we work.

Frequently asked questions