Granite tile adhesive thickness: separate the notch depth from the bed
Granite tile adhesive thickness is decided by the trowel, not by the adhesive. With a square-notched trowel whose teeth are as wide as the gaps between them — the pitch twice the tooth width — the ridges cover half the area and flatten to half the notch depth: 6 mm gives 3 mm, 10 mm gives 5 mm. What the site calls a 5 mm bed is the output of a 10 mm notch.
How thick should the adhesive bed be under granite? Separate notch depth from bed thickness
Granite tile adhesive thickness is two numbers, not one: how deep the trowel notch is, and how much is left once the ridges are flattened. With a square-notched trowel whose teeth are as wide as the gaps between them — that is, the pitch equals twice the tooth width — the ridges cover half the surface. Comb the bed, press a tile down, and the ridges spread to half the notch depth: a 6 mm notch leaves 3 mm, 8 mm leaves 4 mm, 10 mm leaves 5 mm, 12 mm leaves 6 mm. So when the site says "a 5 mm bed", that is what a 10 mm notch produces, not a layer measured with a rule on the back of a tile. The drawing asks how thick the finished bed is; the gang answers with the trowel in hand.
| Trowel notch depth | Bed after flattening | Paste per 1 m² | Typical use (examples, not a grading) |
|---|---|---|---|
| 6 mm | 3 mm | 3 litres | Thin-bed work, small floor tiles |
| 8 mm | 4 mm | 4 litres | General floors and low walls |
| 10 mm | 5 mm | 5 litres | Large floor tiles |
| 12 mm | 6 mm | 6 litres | Substrate 3 mm out of level; on small-format walls, comb only what one tile needs, and scrape off any paste that has skinned over |
Why does the bed end up half the notch depth? Work through the ridge section
A ridge is a prism of constant width, so its cross-section per unit length is notch depth × tooth width. Combed flat, that material spreads over one pitch, and the pitch is twice the tooth width; area divided by twice the width leaves half the depth. That half holds only for square teeth, a pitch of twice the tooth width, and a single pass in one direction; change the tooth profile, or comb a cross pass, and it has to be worked out again. Turn it into paste volume: 1 m² at 1 mm is exactly 1 litre, a 5 mm bed over 1 m² is 5 litres, and a 1.2 × 0.6 m tile takes 3.6 litres. Bags come from one formula only: litres per bag = the square metres printed on the bag × the bed thickness in millimetres, after the stated figure has been converted to a bed thickness; bags = total litres ÷ litres per bag, rounded up, with a little waste. First settle which of the two thicknesses the bag is stating. If it states the bed thickness, multiply by it directly. If it states the notch depth, halve it into a bed thickness first, then multiply. An example, not a code value: a bag marked 5 m² with 10 mm notch in brackets flattens to a 5 mm bed and gives 25 litres per bag; 20 m² at 5 mm needs 100 litres, 4 bags by arithmetic; add waste to the order.
If the substrate is uneven, can I simply comb the adhesive thicker?
Measure before you thicken. Hold a 2 m straightedge against the substrate, check it in two directions, and take the larger of the two readings. Unevenness is a local high point: measured in one direction only, the straightedge can ride between two high points and read small. Read 3 mm and do the subtraction: the bed must fill that 3 mm first, then leave 2~3 mm to bond on. That is a site starting point, not a code value; the lower limit is the usable bed range on that adhesive's data sheet. A 10 mm notch flattens to 5 mm, so filling 3 mm leaves only 2 mm, at that lower limit. Go to a 12 mm notch: it flattens to 6 mm, and filling 3 mm still leaves 3 mm. The action is a different trowel, not two more passes with the same one.
A 30 mm granite slab weighs 81 kg per square metre. What does the adhesive do on a wall and on a floor?
At 2.7 t/m³ a 30 mm slab is 0.03×2.7 = 0.081 t/m², or 81 kg/m²; a 20 mm slab is 54 kg/m². On one 0.72 m² tile that is about 58 kg and about 39 kg. Where that weight goes splits into two cases. On a wall, a wet-fixed tile is held by the adhesive and its own weight pulls it down; the slide is one-way, so the bed carries a steady one-way shear. On a floor the tile lies flat and its weight presses through the bed into the substrate; compression is not this layer's weak point, and what has to be managed is full paste, no voids, no movement. Neither case asks for thickness; both ask for fullness: on a wall the bonded area resists the slide, on a floor full paste seats the tile. One calculation (an example, not a code value): a 0.72 m² tile at 90% coverage bonds 0.648 m²; at 50% it bonds 0.36 m², and 0.648 is 1.8 times 0.36. That 1.8 is a ratio of bonded area. The line depends on three things — wall height, backing and panel size: facades, tall buildings, large panels and old-wall renovation go dry-hung; wet fixing only reaches low walls, a solid backing and small units. Go one notch size up and, once flattened, the bonded area does not change; what grows is shrinkage.
Lift a tile and tap it: what does the adhesive layer tell you?
Fix the two readings first. Tap and it feels solid, the sound is short and does not ring: the back is fully covered. The sound comes back hollow with a trailing tail: there is a void underneath. Lift while the paste is still soft; once it has set, lifting means breaking. Tap in more than one place as well, the four corners and the middle. Tap only the middle and the band of void along the edge stays hidden. Where two points on one tile sound different, the line between them is the edge of the void, and that is where the tile should be lifted.
| What you see on lifting | What it means | What to do on site |
|---|---|---|
| Only a few ribbons of paste, ridges still sharp | Ridges never flattened, bonded area small | Press until the ridges flatten; go one notch size up only where the substrate is too uneven for the bed to fill, then lift a tile and check the ridge marks |
| Hollow trailing sound, separate voids under the tile | Local areas with no paste | Lift from the void edge and relay; do not pour paste into the joint |
| Void only around the perimeter | Edges never pressed | Work a rubber mallet from the centre out to the four edges; the perimeter is pressed at the end |
| Paste feels dry to the finger and has skinned over | Open time exceeded | Scrape off and comb again; never add a second layer on top |
How do I put the adhesive layer into one line for the briefing sheet and the enquiry?
One line for the briefing sheet (wet-fix cases): 10 mm square-notched trowel, combed in one direction all the way through, no back-and-forth and no cross pass; back-butter 3 mm (to fill the saw marks and shallow hollows), lay the tile and press, then lift one tile before the paste skins over and check whether the ridges have flattened. Four items sit in that line: notch depth, back-butter thickness, combing direction and the moment to lift. Leave one out and the site fills it in on its own. Three questions for the supplier: what bed thickness range does the data sheet give for this adhesive; does it state that the adhesive suits low-absorption stone, and on what basis; how long is the open time, and what shortens it. All three should be answered on the data sheet; if they are not, change the adhesive. The grade on the bag only says which class it passed; the usable bed range still comes from the data sheet. Maple Red (Cenxi Red) G562 in 30 mm is a stock item, 100 m² minimum, 5 days to ship. The adhesive and the notch belong to the site; thickness and lead time belong to the factory.