Tile needs a stable, water-resistant base, and in showers, tub surrounds and on wood floors that base is usually cement backer board. Unlike drywall, it does not swell or rot when wet. This calculator estimates how many sheets, screws and rolls of joint tape a job takes.
The calculation
- Sheets = area × (1 + waste) ÷ sheet area, rounded up
- Screws per sheet = (sheet width ÷ stud spacing + 1) × (sheet length ÷ screw spacing + 1), counting whole rows
- Joint tape ≈ sheets × (width + length) ÷ 2
The screw count assumes the sheet runs with its width across the studs and its length along them, with a screw at each crossing of a stud line and a screw row.
Example
A tub surround of 100 sq ft with 3 × 5 ft sheets and 10% waste takes 8 sheets. With studs at 16 in and screws every 8 in, each sheet takes about 24 screws, so roughly 190 in all, plus about 32 ft of mesh tape.
Sheet sizes
| Sheet | Area | Typical use |
|---|---|---|
| 3 × 5 ft | 15 sq ft | showers and tub surrounds, easy to carry |
| 4 × 8 ft | 32 sq ft | large walls and floors |
Use 1/2 in thick board on walls and 1/4 in board on floors over a solid subfloor.
Installation tips
- Use cement board screws or roofing nails rated for the product; ordinary drywall screws corrode.
- Keep fasteners 3/8 to 5/8 in from the edges and 2 in from the corners.
- On floors, set the sheets in a bed of thinset over the subfloor and screw every 8 in across the field.
- Leave a 1/8 in gap between sheets and fill the joints with thinset and alkali-resistant mesh tape.
- Stagger the joints so four corners never meet.
Waterproofing
Cement board is water-resistant but not waterproof: water passes through it. In a shower, add a waterproofing membrane on the face, either a liquid-applied or a sheet product, or a vapor barrier behind the board as the maker specifies. Do not combine a vapor barrier behind with a waterproof membrane on the face.
Limits
Niches, benches and curbs add cuts and waste. Follow the board maker’s fastening schedule, which can differ from the default spacing used here.