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In respect to this, how thick should a bathroom subfloor be?
Make Sure Your Subfloor is Structurally Stable The installation guide for 1/4 inch HardieBacker recommends having wood subfloors made of 5/8 inch exterior grade plywood or OSB (oriented strand board) that is 23/32 inches thick.
In respect to this, what plywood should I use for bathroom floor?
Tongue-and-groove plywood is manufactured specifically for subfloors. The subfloor rests directly on the floor joists and is the lowest layer of flooring material. Plywood three-quarters inch thick is the most common and the strongest.
The National Tile Contractors Association and the Resilient Floor Covering Institute both recommend plywood for subflooring and underlayment, because it doesn't have the risk of swollen edges that OSB does. Plywood also has a slight advantage in stiffness, which means that subflooring panels need not be quite as thick.