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I ripped out the subfloor and underlayment in my bathroom. The subfloor plywood was thinner than the underlayment plywood.

Should I make the subfloor thicker than the underlayment when I lay the new floor? (Does it matter? I plan to use the same thicknesses as the old materials, simply reversing the order.)

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  • What were the products? It was common at one time to see half-inch plywood under 5/8" particle board, if I recall correctly. Nowdays you usually see 3/4" (nominal) t&g plywood under 1/4" birch underlayment. Particle board isn't used anymore.
    – isherwood
    Jan 14, 2017 at 20:46
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    It absolutely matters but it depends what you're going to be putting on top of it. What type of material and what type of tile, if it's tile? And what sizes of tile?
    – Joe Phillips
    Jan 14, 2017 at 21:51
  • If your subfloor was 3 feet thick and your underlayment was 4 feet thick, I'd say those are both too thick. Without actually telling us the thickness, you really aren't asking anything useful
    – Joe Phillips
    Jan 14, 2017 at 21:52

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What matters is the total thickness of the floor to support your finish floor. Tile manufacturers require certain thicknesses going under their products as well as some makers of "decouplers" that isolate the movement of wood under a tile floor. Laticrete and Schluter come to mind here. If it is vinyl flooring the surface needs to be smooth. The thickness of either layer does not matter which level it occupies. My preference would be to set the same thickness in the same level as original. Only thing I may do different would be to use a better grade material than the original if possible. Plywood not OSB, not particleboard. The more layers in the makeup of the plywood the better.

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