Plywood bases for countertops can vary in thickness.
You can use a variety of types of products as finish materials when building counter tops, ranging from granite slabs and tiles to butcher-block wood, ceramic tile and marble tiles. The type and thickness of plywood you use depend on the weight and type of the finish material. While most contractors use the industry standard as a guideline, differences for use with specific finish materials are acceptable.
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The average thickness of plywood sheets used for backer on counter tops is 3/4-inch. This provides adequate support for a wide range of finish materials. You can use it as a stand-alone product as well as a finish-type plywood if it has a laminate finish that you can paint or finish according to personal preference. This is a guideline, rather than a reference, as different finish materials have different requirements, but 3/4-inch satisfies the most stringent requirements.
Vinyl sheeting requires little in the way of support, as it is an extremely flexible material that does not rely on any sort of structural support beneath the material. Vinyl does not rely on joints and the material cannot crack, so it can rely on a less supportive substrate, which means you can use any thickness of plywood as a backer when installing a vinyl or plastic sheeting type of counter top.
The standard requirement for ceramic tile or natural stone tile installations is 3/4-inch plywood under a 1/4-inch concrete or fiberboard. Tile counter tops require the thickest types of plywood in comparison to all other types of finish materials. You can also use two sheets of 5/8-inch plywood instead of 3/4-inch with 1/4-inch fiberboard, so long as you have at least least one inch of substrate beneath the tile installation.
You can set wood counter tops on top of any thickness of plywood. The actual thickness is more about personal preference than anything else, as it will affect the finish height of the counter top itself. The most common is to use 5/8-inch plywood or the standard 3/4-inch plywood and then stack the wood on top of that to the desired finish height, depending on the type of wood used for the counter top, such as with butcher-block wood.