You should not install tile directly over standard plywood in a laundry room.

Tile requires a stable, rigid, and moisture resistant substrate to bond to and plywood alone typically fails to meet those needs for a tiled floor. Use a cementitious backer board or an uncoupling membrane over the plywood to create a proper tile substrate. First, confirm the plywood is structurally sound and fasten any loose boards with screws every six inches along edges and eight inches in the field using corrosion resistant, countersunk screws to eliminate movement. Install a layer of thinset mortar to bed the backer board, then screw the board down following the manufacturer recommended pattern and leave a one eighth inch gap between panels that you fill with alkali resistant fiberglass tape and thinset to create a continuous surface. For moisture control in a laundry room apply a thin layer of waterproofing membrane over the seams and penetrations and extend it up the walls at least two inches to protect the floor plane from splashes and washer leaks. When using an uncoupling membrane like a polymeric mat, bond it to the plywood with the correct adhesive and then set tile into thinset over that membrane; this method reduces the chance of tile cracking from subfloor movement.

Common mistakes are assuming plywood alone is sufficient, using the wrong fasteners or spacing, and neglecting moisture detailing around plumbing and appliance areas; each causes failures. If someone tiles over plywood because it looks flat, they risk tile cracking and grout failure, so always install a cement backer or approved membrane to create a stable bonding layer. Using nails or under sized screws will allow movement, so replace them with corrosion resistant screws at the recommended spacing or you will see squeaks and tile stress. Skipping waterproofing at seams and around drains lets moisture migrate under the tile and causes adhesive breakdown, so always seal seams and transitions with an appropriate waterproofing product.

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