How to Install Cement Backer Board on a Wood Subfloor

Why this matters

Cement backer board gives tile a dimensionally stable surface that thinset bonds to, over a wood subfloor that thinset bonds to badly. It does that job only when it is fully bedded, fully fastened and laid so its joints do not sit on the subfloor's joints. What it does not do is stiffen a floor: a 1/4 inch board over a bouncy subfloor is a bouncy floor with a cement face, and the tile cracks in a line over the joist that is moving. The board is the last step of the substrate, not a cure for the first.

Start state: what has to be true before the first sheet

• The floor is stiff enough for the tile. Joist deflection and the minimum subfloor thickness for ceramic versus natural stone are owned by Tile Assemblies Over Wood and Concrete. Check them there; if the floor fails, the fix is in the framing or the subfloor, and no backer board changes the answer.
• The subfloor is sound and flat. Every panel fastened to the joists, squeaks fixed (see related: How to Stop Subfloor Squeaks Before New Flooring), and flatness to the tile tolerance (see related: Floor Flatness Tolerances by Floor Type). Backer board follows the subfloor; it does not flatten it.
• The board maker's installation sheet in hand. It sets the mortar bed, the trowel, the fastener type and spacing, the edge distances and the gaps. The figures in the worked job below are what one board's sheet stated; yours governs.
• Tools and materials: a carbide scoring knife, straightedge, the notched trowel the sheet names, a drill with a clutch, corrosion-resistant backer board screws (or the nails the sheet allows), alkali-resistant fiberglass mesh tape, and thinset.

The layout comes first

The layout is drawn before any board is cut, because it is the step that keeps backer joints off subfloor joints and stops four board corners meeting at one point, the two places a floor concentrates movement.

The job: a laundry room 120 inches east to west by 72 inches north to south. The subfloor is 3/4 inch plywood in 4 by 8 sheets running east to west: a full 48 inch course along the north wall with an end joint at 96 inches, and a ripped 24 inch course along the south wall with its end joint at 48 inches. The backer is 1/4 inch board in 3 by 5 ft sheets.

 PLAN, 120 in E-W by 72 in N-S, about 5 in per char
 +------------------------+  N wall
 |            |      :    |  course 1, 36 in wide
 |            |      :    |
 |            |      :    |
 +========================+  backer seam at 36 in
 |     |           | :    |  course 2, 36 in wide
 |........................|  subfloor seam at 48 in
 |     |    :      |      |
 |     |    :      |      |
 +------------------------+
  |  backer end joints: 60 in; 24 and 84 in
  :  subfloor end joints: 96 in; 48 in

Checks against the drawing: the backer's long seam at 36 inches sits 12 inches from the subfloor's long seam at 48. The backer end joints at 24 and 84 inches in course 2 sit 24 inches and 36 inches from the subfloor joint at 48 in the south subfloor course, and 84 sits 12 inches from the 96 inch joint in the north subfloor course. Course 1's joint at 60 sits 24 inches from course 2's joint at 84 and 36 inches from its joint at 24, so no four corners meet.

Cut list and fasteners, with the gaps this board's sheet stated, 1/8 inch between boards and 1/4 inch at walls:

 CUT LIST                   laundry, 120 x 72 in
 N-S: 1/4 + 36 + 1/8 + 35-3/8 + 1/4 = 72
 course 1: 1/4 + 60 + 1/8 + 59-3/8 + 1/4 = 120
   B1 60 x 36 full    B2 59-3/8 x 36
 course 2: 1/4 + 23-3/4 + 1/8 + 60 + 1/8
           + 35-1/2 + 1/4 = 120
   B3 23-3/4 x 35-3/8   B4 60 x 35-3/8
   B5 35-1/2 x 35-3/8
 sheets: B1, B2, B4 one each; B3 and B5 from one
   (23-3/4 + 35-1/2 = 59-1/4, under 60): 4 sheets
 SCREWS, one full sheet, 8 in max, 1/2 in from edges
   across 36: 35 in / 5 spaces = 7 in, 6 rows
   along 60: 59 in / 8 spaces = 7-3/8 in, 9 columns
   6 x 9 = 54 per full sheet
 room: 4 sheets of area x 54 = 216
   cut pieces add edge rows: plan on about 250

Four sheets exactly cover the room with nothing spare; one more goes on the truck for a bad cut or a broken corner.

The install, step by step

1. Cut by scoring wherever you can. Score the face along a straightedge with the carbide knife several passes deep, snap the board over the score, and cut the mesh on the back. Scoring makes almost no dust. Power cutting cement and fiber-cement board releases respirable crystalline silica: where a saw is unavoidable, cut outdoors with a saw fitted with dust collection, under the controls in Respirable Crystalline Silica and the Construction Standard (29 CFR 1926.1153, whose Table 1 lists handheld saws cutting fiber-cement board as an outdoor task with dust collection). Holes for a floor drain or a pipe go in with a hole saw or by drilling a ring of holes and knocking the center out, again with dust collection.

2. Dry-fit every piece. Lay out B1 through B5 with the gaps, mark each one's position and face, and check the joints against the drawing before any thinset is mixed. Finding a 3/8 inch error now costs a trim; finding it in a mortar bed costs the bed.

3. Bed each board in thinset. Most board makers require a full bed of thinset between the subfloor and the board. The bed is not there to glue the board down; it fills the small voids between two surfaces that are never perfectly flat, so the board is supported everywhere instead of bridging. Mix the thinset the board sheet names, with the bag opened low and away from your face, since the dry powder carries silica too. Spread it on the subfloor with the trowel the sheet names, one board's area at a time, and set the board with the side the sheet specifies facing up.

4. Fasten while the bed is wet. Drive the full pattern before the thinset starts to set, working from the center of the board out toward the edges so trapped mortar moves out rather than lifting an edge. Seat every screw head flush with the board's face. An overdriven head breaks through the face and holds nothing; an underdriven one stands proud and telegraphs through thin tile. Set the drill's clutch on a scrap so it stops at flush. Keep the edge distance and corner distance the sheet calls for; a screw too close to an edge breaks the edge out.

What breaks if skipped: a board fastened on half its pattern, or fastened after the bed has skinned, sits on lumps of hard thinset with gaps between them, and tile cracks over the gaps.

5. Tape and fill the seams. Fill the 1/8 inch gaps between boards with thinset, embed alkali-resistant mesh tape centered over each seam, and skim it flush. Use the same thinset you will set tile with, or the one the sheet names. Ordinary drywall tape breaks down in the alkaline mortar. Do not tape or fill the 1/4 inch gap at the walls; that is the perimeter movement space the tile installation needs (see related: Movement Joints in Tile Floors).

6. Clean up before it sets. Scrape off squeeze-out at the joints and any ridges of mortar on the face, so the tile setter starts on a flat plane.

What the laundry turned up

Before the board: a straightedge across the subfloor seam at 48 inches found the south plywood course sitting about 1/16 inch below the north course for most of the room's length. The board would have carried that step straight through to the tile as a ridge along a line nobody could explain later. The crew sanded the high edge down with a belt sander under a vacuum shroud until the straightedge rocked on neither side, then vacuumed the floor before any thinset was mixed.

During the board: the drill's clutch had been set on a scrap laid on bare plywood. On B4, set in a fresh bed, the board gave slightly under the screw, and the first row of nine went in with six heads below the face, through the surface layer. Overdriven screws do not hold the board down. The installer backed the clutch off on a scrap set in thinset, the same condition as the floor, and drove a new screw about an inch beside each of the six, leaving the overdriven ones in place and skimming over them when the seams were taped.

After the board: the tap test found a hollow patch about a hand's width in the corner of B3 against the west wall. The bed had been spread short of that corner. Screws do not fill a void under mortar that has already set, so the crew backed out B3's screws, pried the small piece up, scraped the subfloor and the back of the board, rebedded it and refastened it to the full pattern. It cost half an hour because it was found before tile; under tile, the same void is a cracked tile and a cut-out.

How to verify you got this right

• Flat. Lay a straightedge across the floor in several directions, across every seam. The surface meets the tile tolerance in Floor Flatness Tolerances by Floor Type, and no seam stands proud.
• Fully fastened. Count the screws in one full sheet against the pattern: 54 on B1 in the laundry. Run a hand over every head: none proud, none broken through.
• Fully bedded. Tap the board with a screwdriver handle on a loose grid once the bed has set. A solid board sounds dull and even; a hollow spot means the bed missed, and it gets fixed before tile, not after.
• Seams taped, perimeter open. Every board-to-board seam taped and filled; the gap at every wall still open.
• Joints where the drawing says. Measure two of the end joints from the west wall; in the laundry, course 2 read 24 and 84 inches.

What changes the call

• A thicker board on a weaker floor is not a fix; the stiffness question goes back to the framing and subfloor first.
• A shower floor or a wet area adds waterproofing over the board, a separate system with its own seams and corners; the backer alone is not waterproof.
• An uncoupling membrane in place of backer is an alternative over many wood subfloors; see related: How to Install an Uncoupling Membrane for Floor Tile.

References

• The cement board maker's installation instructions (mortar bed, trowel, fasteners, spacing, gaps)
• 29 CFR 1926.1153, respirable crystalline silica in construction, including Table 1
• See related: Tile Assemblies Over Wood and Concrete
• See related: Respirable Crystalline Silica and the Construction Standard
• See related: Movement Joints in Tile Floors