I'm going to lay 18-inch porcelain tile (mixed with a little travertine) in the hallways, bathroom, and kitchen of my one-story house that was built in 1973. It has a subfloor of plywood 1-1/8-inch or 1-1/4-inch thick over 4-x-6 joists on four-foot centers. I'm going to apply a layer of flexible tiling mortar on the plywood and then screw down 1/4-inch Hardibacker on top. The same mortar will then go top of that and the tile. I've heard stories of tiles popping up or cracking when laid over old floors, but one thing that gives me hope is that a tile floor in the entry is in great shape. Do I need to add more support?
Tile installations run into trouble when there's too much deflection (bending) in the subfloor, a tile setter's term for a floor that sags more than it's supposed to. Tile and grout are rigid building materials, and when the subfloor beneath them flexes too much, you can count on seeing cracks. So a firm foundation for tile is key. The extra work involved in making sure the subfloor is flat, level, and unyielding pays off in a long-lasting installation.
The old standard for deflection was described by a simple formula: l/360. That means the floor should not bend any more than its span divided by 360. For example, if the distance spanned by the floor joists is ten feet, the maximum allowable deflection would be 120 inches divided by 360, which is about 1/3 of an inch. This rigidity can usually be provided by plywood that adds up to a total of 1-1/8 inches. In that respect, your subfloor sounds almost right.
But there's a catch. The Tile Council of America says that while joist manufacturers can meet the l/360 standard even with floor framing 24 inches on-center, deflection between the joists may exceed that. The result? Tile fails. Even though you haven't noticed any problems with the tile in the entry, a span of four feet is a lot.
Preparing the Floor's Frame
For advice on your situation I called Michael Byrne, who's been in the tile business for 40 years and is the author of Setting Tile. Floor framing as you describe isn't unusual, but it's probably not adequate for a long-lasting tile installation.
Without knowing more about the framing -- how the 4-x-6 joists are supported, for example, and what species and grade of wood they are -- Michael isn't comfortable suggesting a specific fix. For that, you should call a structural engineer or an architect who can look at the framing and recommend reinforcements that will work over the long haul.
Your framing and subfloor may be perfectly fine for another type of flooring. If recommended repairs are too costly or too much trouble you might think about changing your plans and going with something else.
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