Mortar and Adhesive Mixing and Application
Questions test the mix, the notch, the coverage and the timing - the four things that decide whether a tile stays bonded.
Rules that matter
- Follow the bag: measure water with a jug, add powder to water, mix 2–3 minutes with a helical paddle at low speed, let it slake for the stated time, then remix briefly.
- Respect pot life and open time. Once adhesive skins over it will not bond; scrape it off and apply fresh.
- Never re-temper by adding water to a stiffened mix.
- Comb adhesive in one direction with the notched trowel, then back-butter large-format tiles to achieve full contact.
- Coverage: at least 90% for dry internal floors and 100% in wet areas, external work and heavy traffic. Dot-and-dab is not an accepted bedding method.
- Adhesive class matters: C1 is standard, C2 improved; S1 and S2 describe deformability for movement and large format.
Worked example
A 20 kg bag covers about 4 kg/m² at a 10 mm notch, so 5 m² per bag. A 12 m² floor plus 10% waste needs 13.2 m², which is 13.2 ÷ 5 = 2.64, so buy 3 bags. The bag states 5.5 litres of water per 20 kg - measure 5.5 L, not 'about half a bucket'. Reasoning: coverage figures come from the manufacturer and scale linearly with area, while the water ratio is fixed by chemistry and cannot be judged by eye.
Traps that cost marks
- A wet, sloppy mix that shrinks, cracks and bonds weakly.
- Mixing more than can be used within the pot life.
- Laying into adhesive that has already skinned.
- Using a small notch on a large tile and leaving voids.