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What size plywood should be used for a subfloor?

A plywood subfloor generally uses 1/2-inch to 3/4-inch-thick sheets of plywood with one rough side (which faces down) and one smooth side, which faces up. The sheets are typically 4 by 8 or 4 by 12 feet in size.

thespruce.com - OSB vs. Plywood: Best Subfloor Material Compared - The Spruce

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Plywood OSB Subfloor Structural Strength Stronger, more rigid Slightly weaker Moisture Resistance Usually does not swell when wet Edges, in particular, swell if they get wet Suitablity for Flooring Materials Suitable for all flooring Avoid with ceramic or stone tile Cost About $21 per 4 x 8 sheet About $16 per 4 x 8 sheet Nail/Screw Holding Power Better holding power Less holding power Installation Details Nails or screws driven along seams and through the field Nails and screws along seams and through the field; may require more fasteners Sizes 4 x 8 or 4 x 12-foot sheets 4 x 8 or 4 x 12-foot sheets

Appearance

Plywood

Plywood is made from several layers of wood veneer glued together at alternating right angles. The layers will be clearly visible when the sheets are viewed from the edge, but when viewed from above the surface veneer makes the sheet look like solid wood.

OSB

Oriented strand board has many flat wood chips glued up together in a solid sheet. These flat chips are clearly visible when the sheet is viewed from above, and it is clear that OSB is not a sheet of solid wood.

Best for Appearance: Tie

Since the subfloor is entirely hidden, neither material is visible, except from the unfinished space beneath the floor.

Water and Heat Resistance

Plywood

Plywood has a slightly greater capacity for absorbing water, but it also dries out faster. Overall, plywood is more resistant to permanent swelling.

OSB

OSB is slightly more resistant to water, but it tends to absorb and hold it longer once it gets wet. OSB has untreated edges that are somewhat more susceptible to swelling if they get wet. Serious spills and flooding should be remedied as soon as possible.

Best for Water and Heat Resistance: Plywood

Plywood is more likely to resist permanent swelling, which is a good quality for a subfloor material, keeping the flooring even. Both plywood and OSB will burn should a house fire break out.

Care and Cleaning

Plywood

No care and cleaning necessary; this is a hidden building material.

OSB

No care and cleaning necessary; this is a hidden building material.

Best for Care and Cleaning: Tie

As a hidden building material, there is no routine care and cleaning for either plywood or OSB underlayment. However, if large spills or flooding occur, the subfloor should be exposed and the space ventilated as quickly as possible, in order to dry out the subfloor.

Durability and Maintenance

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Plywood

A well-installed subfloor should last as long as the house itself. Because plywood is inherently a stiffer material with greater strength, it is a better choice as a subfloor under ceramic or stone tile.

OSB

A well-installed subfloor should last as long as the house itself, but OSB is slightly more flexible than plywood, so it is not the best choice under heavy flooring like ceramic or stone tile.

Best for Durability and Maintenance: Plywood

As subfloor materials, both plywood and OSB should last as long as the house itself if the subfloor has been properly installed. There is no maintenance required, except to keep the subfloor as dry as possible. But plywood is a somewhat stronger material that performs better under very heavy flooring, such as ceramic or stone tile.

Installation

Both subfloor materials are installed the same way. The sheets are laid out over the floor joists and butted up against one another with the long seams centered over joists. End joints should be offset from row to row so they don't line up. The sheets are anchored to the joists with nails or screws driven 6 inches apart, and they are secured to joists across the "field" of each sheet with screws or nails driven every 8 inches. Many installers apply a bead of construction adhesive across each joist before laying the subfloor sheets down.

Plywood

Construction crews tend to prefer plywood because it is easier to install and makes for a somewhat more secure and stronger subfloor.

OSB

Because OSB has slightly less holding power for screws and nails, some builders like to space fasteners closer than they do with plywood.

Best for Installation: Plywood

OSB is a heavier, more brittle material than plywood, and it has slightly less holding power for nails and screws. Where the panels are glued down, OSB also has slightly less holding power with construction adhesives.

Cost

Plywood

A 4 x 8-foot sheet of 3/4-inch-thick subfloor-grade plywood sells for about $21.50 at home improvement centers.

OSB

OSB costs $3 to $5 less per panel when compared to plywood, so the cost savings when building an entire house is significant. At a major home improvement center, a 4 x 8-foot sheet of 23/32-inch OSB costs about $16.50,

Best for Cost: OSB

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OSB costs significantly less than plywood subfloor sheets.

Lifespan

Plywood

Plywood subfloors should last as long as the house itself.

OSB

Best for Lifespan: Tie

Both OSB and plywood subfloors can be expected to last for the life of the house, if they are properly installed and kept dry.

Sizes

Plywood

Plywood is available in the same sizes as OSB.

OSB

OSB comes in the same sizes as plywood subfloor sheets.

Best for Sizes: Tie

Subfloor sheets for both OSB and plywood come in 4 by 8 or 4 by 12 sheets. Thicknesses for products used for subfloors are generally 23/32 inch (just under 3/4 inch), but thicker sheets are available where building codes call for them.

Comfort and Sound

Plywood

Plywood is roughly 10 percent stiffer over the span between joists, so it may flex less underfoot than OSB. However, very few people are able to recognize this.

OSB

As a slightly more flexible material, OSB may flex underfoot when installed under carpet or sheet vinyl, but it is almost never noticed with ceramic tile or wood flooring.

Comfort and Sound: Tie

There is usually no appreciable difference in the performance characteristics of plywood or OSB subfloors. Under some circumstances, OSB may flex underfoot when installed under carpet or vinyl flooring; this usually occurs if joist spacing is a wide 24 inches rather than the standard 16 inches.

Resale Value

Plywood

Because plywood is the stronger material, very observant home buyers may place a small premium on plywood subfloors.

OSB

Rarely, a prospective homeowner may recognize that OSB is the cheaper subfloor material.

Best for Resale Value: Plywood

Although it's very unlikely that the subfloor material will be noticed, a plywood subfloor may be preferred by a few prospective homeowners.

The Verdict

Costs aside, plywood is still rightly considered a slightly superior material to use as a subfloor, thanks to its superior structural strength. However, the difference is not very significant, and the cost-saving offered by OSB makes it a perfectly acceptable subfloor material in most situations.

thespruce.com - OSB vs. Plywood: Best Subfloor Material Compared - The Spruce