Which trowel notch size to use, and why back-buttering matters
By Michał Białas · Last updated
The bigger the tile, the bigger the trowel notch: about 1/4 in for tile up to 12 in, 3/8 in for tile up to 24 in, and 1/2 in plus back-buttering for anything longer. The notch decides how much thinset you spread, so it also decides how many bags you buy.
Why the notch matters
A notched trowel leaves ridges of mortar on the floor. When the tile is pressed down and moved slightly across them, the ridges collapse into a continuous bed. Small ridges are enough for small tiles. Large tiles bridge bigger hollows and need taller ridges, or the back of the tile ends up touching mortar only in strips — and a tile that isn’t supported underneath can crack under load.
Notch size by tile size
| Longest tile edge | Typical notch | Thinset (approx.) |
|---|---|---|
| Up to 4 in (100 mm), mosaic | About 3/16 in (4 mm) | 1.5 kg/m² (0.31 lb/sq ft) |
| Up to 12 in (300 mm) | 1/4 in (6 mm) | 2.5 kg/m² (0.51 lb/sq ft) |
| Up to 18 in (450 mm) | 5/16 in (8 mm) | 3.2 kg/m² (0.66 lb/sq ft) |
| Up to 24 in (600 mm) | 3/8 in (10 mm) | 4.0 kg/m² (0.82 lb/sq ft) |
| Over 24 in (600 mm) | 1/2 in (12 mm) + back-buttering | 5.0 + 1.0 kg/m² (1.23 lb/sq ft) |
These are typical starting points, and the tile adhesive calculator uses them to suggest a notch. Your thinset’s data sheet may recommend a different notch or coverage, and the real check is always the coverage under a lifted tile.
A 12 × 24 in tile is 609.6 mm long — just over the 600 mm line — so the calculator suggests a 1/2 in notch with back-buttering. If you use a 3/8 in notch on a flat floor and your coverage checks pass, switch off the automatic notch and enter your own.
Back-buttering
Back-buttering means spreading a thin, flat coat of thinset on the back of each tile with the flat side of the trowel, in addition to the notched bed on the floor. It fills the shallow pattern on the tile back, wets the whole surface and closes small voids. It is recommended for large-format tile, natural stone, outdoor tile, wet areas and heavily loaded floors. It adds roughly 1 kg/m² (0.2 lb/sq ft) of thinset.
Worked examples
Example 1 (feet): 12 × 10 ft room, 12 × 24 in tile
- Area 120 sq ft = 11.15 m². Longest edge 24 in → 1/2 in notch + back-buttering: 5.0 + 1.0 = 6.0 kg/m².
- Thinset: 11.15 × 6.0 = 66.9 kg; + 10% waste = 73.6 kg = 162 lb.
- Bags: 162 ÷ 50 = 3.2 → 4 bags of 50 lb.
Example 2 (meters): 4.0 × 3.0 m room, 600 × 600 mm tile
- Longest edge 600 mm → 10 mm notch, 4.0 kg/m².
- Thinset: 12 m² × 4.0 × 1.10 = 52.8 kg → 3 bags of 25 kg.
- Same room with 600 × 1200 mm tile: 12 mm notch + back-buttering, 6.0 kg/m² → 79.2 kg → 4 bags.
Example 3 (feet): 12 × 10 ft room, 12 × 12 in tile
- Longest edge 12 in → 1/4 in notch, 2.5 kg/m².
- Thinset: 11.15 × 2.5 × 1.10 = 39.2 kg = 86.5 lb → 2 bags of 50 lb.
Mixing and open time
Mix only as much thinset as you can use within its pot life, usually a couple of hours, and spread only as much as you can tile within its open time, often 20–30 minutes. Touch the ridges now and then: if the thinset no longer sticks to your finger, it has skinned over, and tiles set into it won’t bond. Scrape it off and spread fresh mortar instead of adding water.
How to check coverage
- Comb the thinset in straight lines, all in one direction, not in swirls.
- Set the tile and slide it slightly across the ridges, about 1/4 in, then back.
- Every few tiles, lift a freshly set tile and look at its back.
- Floors generally need at least 80% contact in dry areas and essentially full coverage (95% or more) in wet areas and outdoors, with the corners and edges supported.
- If coverage is short, use a larger notch, back-butter, or fix a floor that is out of flat.
Common mistakes
| Mistake | Result |
|---|---|
| Swirling the trowel | Air pockets trapped under the tile |
| Spreading too much at once | Thinset skins over before tiles are set; poor bond |
| Too small a notch on large tile | Hollow spots, cracked tiles |
| Using the notch to fix an uneven floor | Thick, uneven beds that shrink and crack; level the floor first |
| Cement thinset straight on anhydrite screed | Bond failure; prime first or use a compatible adhesive |
For tile 24 in and larger, also see large-format tile for leveling clips and lippage.
Frequently asked questions
What size trowel for 12 × 24 tile?
Often a 1/2 in square notch with back-buttering, because the 24 in edge is just over 600 mm. A 3/8 in notch can work on a very flat floor if coverage checks pass. Follow the thinset’s data sheet.
How many square feet does a 50 lb bag of thinset cover?
Roughly 95 sq ft with a 1/4 in notch, about 60 sq ft with 3/8 in and about 40 sq ft with 1/2 in plus back-buttering. Check the bag, because products differ.
Is back-buttering always necessary?
Not for small tile on a flat floor. It is recommended for large-format tile, natural stone, wet areas and outdoors.
Square or U-notch?
Both are common. Square notches leave more mortar per pass; U-notches collapse more easily. Choose by coverage: if lifted tiles show full contact, the trowel is right.
Can I use a bigger notch to make up for an uneven floor?
Only for small deviations. Standard thinset is not made for thick beds; it shrinks and can crack. Level the floor first, or use a medium-bed mortar made for thicker layers.