The old saw about how a good house starts with a good foundation applies to laying tile and resilient flooring. What may be a surprise, though, is that a tongue-and-groove subfloor, the typical base in old-house floors, and even common construction plywood or chipboard is neither smooth enough nor solid enough to be a good foundation on its own. What’s typically needed is a subfloor combined with an intermediate material or underlayment that bridges gaps and joints while presenting a smooth, stable surface for the finishing flooring. In kitchens and bathrooms, where tile and resilient coverings appear most, the right underlayment is critical for the best looks and long life of the floor. The catch is, not only do underlayments vary from material to material, but they also diverge from what you may remember using in the past.
Ceramic and Stone Tile
Decades ago, the only way to lay tile—especially in wet areas like bathrooms—was on a flat, smooth, 2"- thick slab of troweled cement mortar, or the once-ubiquitous “mud job” of yore. Mud jobs are still the way to go for poured concrete floors and for high-end work, but for most bathroom and kitchen construction, a mud job’s cost and weight are overkill (assuming you can even find someone with the skills to do it).
Instead, for the past 25 years or so, the standard method for setting tile floors has been over a modern-day simulacrum of a mud job generically called cement backer board (e.g., Durock Brand Cement Board, HardieBacker, Unifix PermaBase, Wonderboard). These materials are cement mortar and aggregate formed with a fiberglass mesh (for structural integrity) into heavy, ½"- and 5⁄8"-thick sheets that vary in size from 32" x 48" up to a massive 4' x 8'.
Whether it’s stone, glass, or ceramic, tile is a brittle material that requires setting on a rigid base to avoid cracking. The cement backer board needs to lie on a very stable substrate. In new construction, a heavy plywood subfloor typically does the job, but in older houses, the slight deflection and unevenness inherent in ¾" board subfloors may be on the hairy edge of acceptable, so some manufacturers advocate attaching a layer of ½" plywood to beef up the subfloor to 1¼" thickness. The plywood is spread with thin-set adhesive using a ¼"-tooth trowel. Next, the cement boards are pressed into place, usually smooth side down, and secured every 8" or so with appropriate screws or nails set just below the surface. Finally, the joints are taped using fiberglass tape and thin-set mortar (or whatever is recommended by the manufacturer).
Like the princess by the pea, the various types of resilient flooring—from vinyl and linoleum to rubber—are quickly bruised by anything except a smooth, solid surface. The slightest dip or raised area underneath can eventually “telegraph” to the surface in the form of a conspicuous wear spot. This means that laying resilients directly on any kind of board subfloor is out—because even if the boards are tongue-and-groove and sanded smooth, they will move with moisture cycles, opening gaps that can show up in the flooring. The fix is to cover the subfloor with a wood underlayment.
Individual manufacturers vary on what can and cannot be used under specific products, but generally most recommend an APA (American Plywood Association)-rated, underlayment-grade plywood, a minimum of ¼" thick (½" thick if the boards are wider than 3" or not tongue-and-groove). Wood underlayment is high-quality plywood with a solid, smooth veneer surface and a specially constructed inner core that resists punctures and indentations. The multi-ply core also makes underlayment more dimensionally stable than ordinary construction plywood, such as A-C exterior plywood. For installations where there might be high moisture levels, underlayment plywoods are made in exterior grades.
The superiority of underlayment plywood becomes clear when compared with what doesn’t work. After dealing with the repercussions of moisture swelling or bumps around nails, flooring manufacturers have moved away from particleboard and chipboard. Lauan board and other common lumberyard sheet goods are nixed as well, because their cores may contain hidden voids, and the surfaces are patched with adhesives that can bleed stains or telegraph through the flooring.
Now, a nice, clean, tight subfloor is ideal, but in a kitchen or bathroom the chances are there’s already some type of flooring in place, which raises the question: Do you take it off or go over it? Removing all old resilient flooring down to the subfloor involves labor and can raise the specter of asbestos removal. Some resilient flooring products from decades ago were made with asbestos fibers; more important, so were some flooring adhesives and liners. In short, these materials must be removed carefully, never by grinding or breaking them up, which can release fibers.
Going over the existing flooring dodges the asbestos issue but adds to the floor thickness and is possible only if the old material is smooth, clean, in good condition, and one layer thick. Each resilient flooring manufacturer has its own conditions but, in general, covering over cushioned flooring, self-adhesive tile, flooring below grade, or old rubber tile is not recommended. The value of a high-quality underlayment is for naught, however, if you don’t install it properly. Each flooring manufacturer has its own specs, but generally the idea is to attach the sheets to the floor, smooth face up, every 4" to 6", using staples, screws, or ring-shank nails that won’t back their way out and dent the flooring. Construction adhesive is verboten because of the staining risk.