The substrate rules
Before you open a bag, the substrate has to be **flat, sound, clean and dry**.
- Flat: with a two-metre straightedge, no more than three millimetres out. If there is more, correct it first with self-levelling compound or render; adhesive is not there to fill hollows.
- Sound: nothing that moves, crumbles or sounds hollow.
- Clean: no dust, no plaster residue, no flaking paint.
- Dry and stable: mortar that has just gone on is still shrinking and will drag the tiles with it.
On plaster, on paint or on very absorbent substrates, you need a primer.
Choosing the adhesive
The letters on the bag mean something specific:
- **C1** — standard adhesive. Fine for porous ceramic indoors and in small formats.
- **C2** — improved adhesion. This is what porcelain, exterior work and large formats require.
- **S1 / S2** — deformable and highly deformable. For underfloor heating, terraces, timber and any substrate that moves or expands.
- **F** — fast setting. When the floor has to be walked on soon.
- **E** — extended open time. Very welcome in summer.
Rule of thumb: **porcelain or outdoors, C2 as a minimum**. Saving money here is expensive, because porcelain barely absorbs water and a C1 will not hold it.
The right trowel
The notch size decides how much adhesive ends up under the tile, and that decides whether it stays there.
- Small tiles, up to 20×20: 6 mm notch.
- Medium format, up to 40×40: 8 mm.
- Large format and porcelain: 10 or 12 mm.
- Very large formats, external floors or pools: **back-butter as well**, that is, adhesive on the back of the tile too.
Contact coverage should be at least 80% indoors and **100% outdoors**. A void under the tile means trapped water and a cracked tile at the first frost.
Set out before you stick anything
Half an hour of setting out saves a day of regret.
- Find the centre and share the cuts out, so you do not end up with a two-centimetre strip in the most visible place.
- On a wall, start from a **marked level line**, not from the floor: floors are almost never level.
- Fix a batten to support the first course and take it off once the adhesive has set.
- Decide where the cuts will fall before you start.
Joints are not optional
Butt-jointing looks very smart in a photograph and is a mistake.
Ceramic expands. Without a joint the tiles push against each other and end up tenting or cracking. The joint also absorbs the size differences between tiles, which exist even when you cannot see them.
- Indoors, small format: 2 mm minimum.
- Large or rectified format: 2 or 3 mm.
- Outdoors: 5 mm.
- Perimeter, against every wall: 5 to 10 mm, covered by the skirting.
- Intermediate movement joints every 15 to 20 m² indoors, and every 9 to 12 m² outdoors or over underfloor heating.
Grouting
Wait until the adhesive has set, usually 24 hours. Clean the joints out properly, work the grout in diagonally with a rubber float and take off the excess before it hardens.
In wet areas and outdoors use an improved cementitious grout; in shower trays, worktops and pools, epoxy grout, which absorbs no water and does not stain.
The whitish haze left afterwards comes off with a damp, well-wrung sponge, changing the water often. If you let it dry completely you will need a dedicated cleaner.
Mistakes that keep happening
- Using C1 with porcelain.
- Putting dabs of adhesive on instead of combing it with the trowel.
- Spreading more adhesive than you can tile before it skins over. If you touch it and nothing comes off on your finger, it will not bond: scrape it off.
- Butt-jointing.
- Grouting too soon.
- Walking on the floor before the adhesive has set.
Come in and we will look at it
Give us the area, the tile format and where it is going (indoors, outdoors, shower, underfloor heating) and we will tell you which adhesive and which grout you need, and how many bags. We also stock trowels, spacers and levelling systems.