What the board is actually doing
Cement backer board exists because tile is rigid and wood is not. A tiled floor over a subfloor that flexes will crack at the grout lines first and at the tiles second, and no amount of good thinset fixes a substrate that moves. The board adds stiffness and, more importantly, gives the tile a dimensionally stable, cementitious surface to bond to that does not swell when it gets damp.
On a floor the board is bedded in a layer of thinset before it is screwed down. That layer is not adhesive in the usual sense; the screws hold the board, not the mortar. Its job is to fill every void between the board and the subfloor so the board is continuously supported. Skip it and the board bridges the low spots, the assembly flexes into those voids under foot traffic, and the floor telegraphs the framing underneath.
Screw counts run higher than people expect
An eight inch grid across a 3 by 5 foot board is five columns by eight rows, which is forty screws per board. On a 10 by 8 foot bathroom floor that is six boards and about 240 screws, and a small box will not cover it. Backer board screws also snap, especially near an edge or when the driver cams out, so buy more than the count.
| Board | Coverage | Screws at 8" grid | Notes |
|---|---|---|---|
| 3 x 4 ft | 12 sq ft | about 35 | Easiest to handle in a small bathroom |
| 3 x 5 ft | 15 sq ft | about 40 | The common floor size |
| 4 x 8 ft | 32 sq ft | about 91 | Fewer seams, but heavy and hard to move alone |
Use screws made for cement board. Drywall screws corrode, they do not countersink cleanly in a cementitious face, and their heads are the wrong shape to sit flush without tearing the surface.
Seams, tape and the mortar that goes over them
Every joint between two boards gets alkali-resistant mesh tape bedded in thinset, not drywall paper tape and not fibreglass drywall mesh. The alkali resistance matters because cement is alkaline and ordinary glass fibre degrades in it over time. The seam length this calculator reports is derived rather than measured: it takes the total edge length of all the boards, subtracts the room perimeter where boards meet a wall rather than another board, and halves what is left because each interior seam is shared by two boards.
That gives a figure accurate enough to order tape from. It slightly overstates on a room where the boards happen to tile the floor cleanly and understates nothing, which is the direction you want an estimate to err.
Board thickness and where each one belongs
Quarter inch board is the usual floor product, because the floor only needs the surface and the stiffness comes from the subfloor and the joists. Half inch is the usual wall product, because it matches the thickness of the drywall it meets and gives the screws something to bite. Using half inch on a floor raises the finished height by another quarter inch, which matters at doorways and where the tile meets an adjoining floor. Neither thickness turns an inadequate subfloor into an adequate one.
Cement board is heavy. A 4 by 8 sheet of half inch is a two person lift, and 3 by 5 boards exist largely because one person can get them up a staircase. Buying the smaller board costs a little more per square foot and a lot less in aggravation.
What goes above and below
Underneath, the subfloor calculator covers the structural layer and its fasteners. Above, the tile layout calculator settles where the cuts land before you set anything, and the tile calculator works out tile, thinset and grout quantities. The bedding thinset counted here is a separate purchase from the thinset that sets the tile, and running one bag short of the second because you spent it on the first is a common way to lose an afternoon.
Questions people ask
Do I really need thinset under cement board on a floor?
Yes, on a wood subfloor. The board on its own touches the subfloor only where the two happen to be flat against each other, and no plywood floor is flat to that tolerance. Thinset fills the gap so the board is supported everywhere. Without it the assembly has voids in it, the board flexes into them, and the movement concentrates at the seams where the tile is least able to take it. The screws are what hold the board down; the mortar is what stops it from having anywhere to go. On a wall the situation is different because the board is fastened directly to studs and there is nothing to bed it into.
How much thinset goes under the board?
That depends on the notch and the flatness of the subfloor, which is why coverage is an input on this page rather than a fixed figure. A 50 pound bag combed out with a quarter inch square notch commonly covers somewhere in the region of 80 to 100 square feet, but published coverage varies a lot between products and every rough subfloor consumes more than a smooth one. Read the bag you are buying, and if the subfloor has visible dips, order an extra bag rather than trying to stretch the last one thin.
Can I use drywall screws and drywall tape on backer board?
No to both, for different reasons. Drywall screws are not corrosion rated for a cementitious, frequently damp environment, and their bugle heads tear the board face rather than seating flush. Backer board screws have a wider head with a ribbed underside designed to countersink into the board without crumbling it. As for the tape, drywall paper tape has no strength in a mortar bed and will fail, and ordinary fibreglass drywall mesh degrades in the alkaline environment of cement over time. The tape you want is specifically labelled alkali resistant.
Is cement board waterproof?
No, and this catches people out on shower walls. Cement board is water resistant in the sense that it is not damaged by getting wet and does not rot or swell, which is why it belongs in a wet area at all. But water passes straight through it. A shower assembly needs a separate waterproofing layer, either a membrane applied over the face of the board or a vapour barrier behind it, and which of the two is correct depends on the system you are using. Mixing the two approaches can trap water inside the wall. Follow one manufacturer's complete detail rather than combining parts of several.
What about the dust when cutting it?
Cement board cuts release respirable crystalline silica, which causes permanent lung damage that develops slowly and does not reverse. The safest method is scoring with a carbide scriber and snapping the board, which produces very little dust and handles most straight cuts. Curves and holes need a saw or a hole cutter, and those should be done outdoors with a tool that has dust extraction, wearing a properly fitted respirator rather than a loose paper mask. Sweeping the dust afterwards puts it back in the air; a vacuum with a HEPA filter or wet cleanup is the alternative.