Tile adhesive is sold by weight and used by area, and the rate is set almost entirely by the trowel: the notched edge leaves ridges of a certain height, and the height of those ridges is how much adhesive stays on the wall when the tile is pressed in. A 6 mm notch puts down about two kilos a square metre; a 12 mm notch puts down about four. The tile size decides which notch you need, because coverage has to be near complete across the back of the tile — and large tiles also need adhesive buttered on the back as well as the substrate, which is a second quantity on top of the first.
Area, trowel and bag
The trowel notch sets the quantity. The tile size sets the trowel.
Adhesive applied to the back of the tile as well. Twenty percent for 600 mm, forty for large format.
Left in the bucket, on the trowel, and skinning over before use.
Result
The trowel is the quantity
A notched trowel does not spread adhesive, it combs it. The notches leave parallel ridges of a controlled height, and when the tile is pressed and twisted into place the ridges collapse to fill the gap. The height of the ridge is therefore the thickness of the adhesive bed, and the bed thickness times the area is the volume — which is why the notch size on the trowel sets the quantity more firmly than anything else does.
The consequence is that using a bigger trowel than the tile needs is not a shortcut, it is waste; and using a smaller one is not thrift, it is a hollow bed. The rule is coverage: after pressing a tile down and lifting it off again, ninety-five percent of the back of an internal tile should be covered, and that figure rises to full coverage in a wet area or outside. Test it once at the start of every job.
Why large tiles need buttering the back
Combing the substrate gives ridges that collapse under pressure. On a small tile the pressure is even and the ridges collapse fully. On a 900 mm or 1200 mm tile, the tile itself is stiff enough to bridge, the pressure from a hand is spread over a large area, and the ridges in the middle stay as ridges — leaving channels of air under the centre of the tile.
The fix is to spread a thin, flat coat on the back of the tile as well, which is back-buttering. It adds material, typically twenty percent at 600 mm and forty percent above 900 mm, and it is not optional on large format: a tile with a hollow under it is a tile that cracks the first time something heavy is dropped on it.
Mixed adhesive skins over in about twenty minutes in a warm room, and once it has skinned it will not bond. Adhesive thrown away is more expensive than adhesive that cost more per bag.
Mix what you can use in twenty minutes, and never re-wet a mix that has started to set. Adding water to a skinning mix is the usual cause of tiles that come loose.
Choosing the adhesive
Standard cementitious adhesive covers most internal work on a sound, rigid substrate. Flexible adhesive — polymer-modified — is for anywhere with movement or anywhere the tile is large: timber floors, underfloor heating, and anything over 900 mm. Rapid-set is for when the job has to be grouted the same day, and it costs more for the privilege.
Ready-mixed tubs are for small wall jobs and for the occasional repair. They are several times the price per kilo of powder, they are only suitable for small tiles in dry areas, and they must not be used on floors or in wet rooms. Powder is the default for anything bigger than a splashback.
| Situation | Adhesive | Notch | Rate |
|---|---|---|---|
| Wall, ceramic to 300 mm | Standard | 6 mm | 2.0 kg/m² |
| Floor, ceramic to 600 mm | Standard | 8 mm | 2.6 kg/m² |
| Large format, 900 mm plus | Flexible | 10 to 12 mm | 3.3 to 4.1 kg/m² |
| Timber floor or underfloor heating | Flexible, two-part | 8 mm | 2.6 kg/m² |
| Wet room | Flexible, tanked | 8 mm | 2.6 kg/m² |
| Exterior, porcelain | Flexible, frost-proof | 12 mm | 4.1 kg/m² |
A worked example
- Comb12 m² at 2.6 kg/m² is 31.2 kg.
- Back-butteringTwenty percent adds 6.2 kg, so 37.4 kg goes on.
- AllowanceTen percent for the bucket takes it to 41.2 kg.
- BagsIn 20 kg bags that is 3 bags, or $54 at $18.
- The real rate41.2 kg over 12 m² is 3.43 kg/m², against the 2.6 the trowel suggests.
That gap between 2.6 and 3.43 is the whole point of doing this properly. The trowel rate is what goes on the substrate; the real rate is what you buy, and it is about a third higher once back-buttering and the bucket are counted.
Mixing and spreading
Add powder to water and mix slowly — a fast paddle whips air in and weakens the bed. Aim for a stiff, creamy consistency that holds a ridge when the trowel is drawn through it. Let it slake, then remix briefly, and do not add more water after that.
Hold the trowel at about sixty degrees and comb in one direction only, in straight passes. Combing in both directions folds the ridges over each other and traps air. Spread only as much as you can tile within the open time — usually about twenty minutes — and if the ridges have skinned, scrape it off and start again rather than bedding on top.
Press and twist rather than press and slide: a short twisting motion collapses the ridges and closes the channels between them. Then the joint width, which is set by the spacers, becomes the next quantity — the grout figure works from it, and it is a much smaller number than the adhesive. The tiles themselves are counted by the tile figure, and the same job in brick or block is sized by the mortar figure instead.
Do
- Choose the notch from a coverage test, not from habit.
- Back-butter large tiles and anything going outside.
- Mix slowly, let it slake, and never re-wet a skinning mix.
- Comb in one direction only, and only as far as you can tile in twenty minutes.
- Press and twist each tile to collapse the ridges.
Avoid
- Using a small notch on a big tile and leaving air channels underneath.
- Adding water to a mix that has started to set.
- Combing in both directions, which folds the ridges over and traps air.
- Spreading further than can be tiled within the open time.
- Using ready-mixed tubs on a floor or in a wet room.
Where the numbers stop being true
The rate per square metre assumes a flat substrate. Every hollow in the wall has to be filled with adhesive before the comb can do its job, and on a wall that has not been levelled the real rate can be double. If the straightedge rocks, fix the wall first — levelling compound is cheaper per square metre than adhesive used as filler.
Nothing here accounts for the substrate's absorbency. A very porous background sucks water out of the bed and shortens the open time dramatically, which shows up as waste rather than as a higher rate. Prime it, and the rate holds.
And the back-buttering percentage is an allowance, not a measurement. On a tile with a deeply profiled back, or on a mosaic sheet, the amount that goes on the back is different again. If you are working with anything unusual, weigh one tile's worth of adhesive and scale from that rather than from a percentage.
How the figures are produced
The comb rate is the area times the rate for the chosen notch. Back-buttering adds a percentage of that, and the waste percentage is applied to the total going on. Bags are the resulting weight divided by the bag size, rounded up, so the final bag is rarely full. The fill shows the share of what is bought that the job actually uses, and the rate output is the total bought divided by the area — the figure worth remembering for the next job.
Questions people ask
How much adhesive for 12 square metres?
About 41 kg with an 8 mm notch, back-buttering and a ten percent allowance — three 20 kg bags.
What notch should I use?
Whichever gives at least ninety-five percent coverage. Bed a tile, lift it, look at the back. Six millimetres for small walls, eight for floors, ten to twelve for large format.
Do I have to back-butter?
Above about 600 mm, yes. Below that it is optional indoors and worthwhile in a wet area.
Can I use ready-mixed adhesive on a floor?
No. Ready-mixed is for small wall tiles in dry areas only. Floors and wet rooms want powder.
Why is my rate higher than the bag says?
Because the bag's figure is the comb rate on a flat wall. Back-buttering, waste and an uneven substrate all sit on top of it.
How long does mixed adhesive last?
About twenty minutes of open time in a warm room. Mix less, more often.