Use one and one quarter inch cement board screws and place them to prevent any movement before tiling.

For a bathroom subfloor install use one and one quarter inch corrosion resistant cement board screws rated for cement board rather than drywall or wood screws. Drive screws so the head sits just below the cement board surface without tearing the paper face which can trap moisture and cause weak spots. Space screws about every eight inches across the field of the board and tighten them to firm the board to the subfloor without compressing the underlayment; at seams double up the fasteners by placing screws on both sides of the joint no more than eight inches apart to eliminate movement. Start by fastening the center of the board and work outward to avoid bowing then check for any rocking and add screws where needed especially near corners and cut edges. Use a countersink bit or the screw head design to set heads below the surface so the thinset can cover them and tiles will sit flat. If your subfloor is wood or plywood confirm it is structurally sound and fasten into framing or use additional screws to tie into joists for long term stability.

Common problems include overdriving screws which crush the cement board surface and create low spots so back the screw out a turn and reset slightly shallower to remedy that and replace any damaged board; missing screws at seams that allow movement so add screws on both sides of the joint and stagger them to pull boards tight; and using the wrong fastener which can corrode or strip and lead to failures so always choose screws labeled for cement board or thinset backer applications and keep a small stash of extras for rework. For each issue check under the board if possible and add blocking or shims for unsupported areas, remove and replace boards with torn faces, and verify screw length and coating match the substrate conditions.

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