How to calculate backer board
Measure the area you will tile — floor length × width, or each wall's width × the height the tile reaches — and divide by the area of one sheet. Add 10% for cutouts and offcuts and round up. Most cement board comes in 3 × 5 ft sheets (15 sq ft) that one person can carry; foam boards also come in 4 × 8 ft (32 sq ft).
Worked example — a 3 × 5 ft shower tiled 7 ft high: the walls are (3 + 5 + 3) × 7 = 77 sq ft. 77 × 1.10 ÷ 15 = 5.6, so buy 6 sheets of 1/2 in board and about 153 screws.
Board sizes and where they go
| Board | Sheet | Weight | Use |
|---|---|---|---|
| HardieBacker ¼ | 3×5 ft, 1/4 in | 29.5 lb | Floors, countertops |
| HardieBacker 500 | 3×5 ft, 0.42 in | 44.5 lb | Walls, floors |
| Durock ¼ | 3×5 ft, 1/4 in | 25.2 lb | Floors, countertops |
| Durock ½ | 3×5 ft, 1/2 in | 35 lb | Walls, floors |
| Durock ⅝ | 3×5 ft, 5/8 in | 45 lb | Walls, floors |
| GoBoard 3×5 | 3×5 ft, 1/2 in | light | Walls, showers (waterproof) |
| GoBoard 4×8 | 4×8 ft, 1/2 in | light | Walls, showers (waterproof) |
| wedi 3×5 | 3×5 ft, 1/2 in | light | Walls, showers (waterproof) |
| wedi 4×8 | 4×8 ft, 1/2 in | light | Walls, showers (waterproof) |
HardieBacker "1/2 in" is actually 0.42 in thick — plan trim and niche depths accordingly. Durock and HardieBacker 1/4 in boards are for floors and countertops only; walls need the thicker boards.
Cement board or foam board?
Cement board is cheap and stiff: a 3 × 5 sheet of 1/2 in Durock costs about $15.85 at Floor & Decor but weighs 35 lb, and it needs a separate waterproofing layer in a shower. Foam boards such as GoBoard and wedi are far lighter and are waterproof once every seam and fastener is sealed with the maker's products — but a 3 × 5 wedi panel costs about $56.31 at Floor & Decor ($67.91 list). Many installers use cement board on floors and dry walls, and foam board for shower walls where the membrane would otherwise be a second step. Whichever you pick, stay within one system for boards, fasteners and sealants.
How many screws per sheet
- Floors: James Hardie and USG call for fasteners every 8 in over the whole sheet — about 54 per 3 × 5 sheet, kept 3/8–3/4 in from the edges and 2 in from the corners.
- Walls: screws go into studs (16 in on center at most) every 8 in — about 27 per 3 × 5 sheet hung vertically and 52 per 4 × 8 sheet.
- Foam boards: GoBoard uses its own fasteners every 8 in; wedi uses one screw and washer per sq ft. Every seam and screw head gets the maker's sealant.
Thinset under floor board
Cement board on a floor is set in a bed of modified thinset with a 1/4 in square-notch trowel, then screwed down. The thinset fills voids so the board does not flex; it is not glue. At a conservative 75 sq ft per 50-lb bag, a 5 × 8 bathroom floor takes 1 bag. Leave a 1/8 in gap between sheets (1/4 in at the walls for Durock floors), and tape every seam with 2 in alkali-resistant mesh embedded in thinset.
Common mistakes
- Using 1/4 in board on walls — it is too flexible between studs.
- Skipping the thinset under floor board, which leaves hollow spots that crack tile.
- Lining up seams with the subfloor seams. Offset them, and stagger the board joints.
- Assuming cement board is waterproof. It is not; showers need a membrane over it.
Next, count tile with the tile calculator, which also adds membrane for showers, and work out setting mortar with the thinset calculator.