The notch sets the bed depth and the bed depth sets the coverage. Back-buttering and a wavy floor do the rest — which is why published figures for the same trowel run from 30 square feet to 50.
Back-buttering takes about 20% off coverage. A wavy floor takes another 25% because the low spots swallow mortar the notch never accounted for.
Search for coverage on a ½″ square notch and you will find 30 to 35 square feet a bag on one site and 40 to 50 on another. Both are right. They are describing different jobs.
A trowel combed at 45° lays ridges that occupy about half the floor at roughly seven tenths of the notch depth, so the effective bed under a ½″ notch is close to 3/16″. A 50 lb bag mixes to about 0.65 cubic feet, which spread that thick covers about 44 square feet. That is the manufacturer number, and it assumes a flat floor and no back-buttering.
Back-butter the tile and you are placing mortar twice — roughly 20% more material. Set it on a slab with dips in it and the low spots take another quarter. Do both and 44 square feet becomes 26. That is the low figure other sites publish, and nothing about it is wrong; it is just a different set of conditions with the conditions left unstated.
All three are inputs above, so the number changes when the job does.
| Notch | Effective bed | Flat, no butter |
|---|---|---|
| 3/16″ V | 0.082″ | 95 sq ft |
| 1/4 × 1/4 | 0.088″ | 88 sq ft |
| 1/4 × 3/8 | 0.133″ | 59 sq ft |
| 3/8 × 3/8 | 0.133″ | 59 sq ft |
| 1/2 × 1/2 | 0.177″ | 44 sq ft |
| 3/4 × 3/4 | 0.265″ | 29 sq ft |
Per 50 lb bag. The V-notch figure is empirical; the square-notch figures come out of the ridge geometry and land inside Mapei's published ranges.
Bigger tile needs a deeper notch, because a bigger tile is less flat and has further to travel to make contact. Under 8″ a 1/4 × 1/4 is plenty. From 8″ to 12″ use a 1/4 × 3/8. Past 12″ go to a ½″ square, and at 15″ and above the TCNA wants the tile back-buttered as well as the substrate combed.
The check that matters is not the trowel, it is the back of the tile. ANSI A108 calls for 80% coverage in dry interiors and 95% in wet areas and outside, with the corners supported. Pull one of the first few tiles you set and look. Ridges still standing means the notch is too small, the mortar has skinned over, or you are working too far ahead of yourself.
This is a membrane question, not a preference. Sheet membranes like Ditra and Kerdi specify unmodified, because the polymers in modified thinset need to dry out and a sandwich of two impervious layers will not let them. Over cementboard with a liquid-applied membrane, modified is normally right. Follow the membrane manufacturer, not the tile.
Two 50 lb bags at a 1/4 × 3/8 notch if the floor is genuinely flat. Three on a typical slab once you allow for the dips, and four if you are back-buttering large format as well.
No. Pot life is thirty to forty-five minutes and adding water past that gives you mortar that spreads nicely and bonds badly. Mix what you can place.
Not directly — the bed depth comes from the notch. Thick tile tends to be large tile, and that is what drives you to a bigger trowel.