CDX Plywood for Subfloors and Roof Sheathing

CDX plywood is the rough construction plywood you buy for roof sheathing, wall sheathing and subfloors that will be covered. Bonded wood veneers give you the structural base, while the rough C/D faces keep finish-grade appearance out of a job that will disappear under roofing, siding or flooring. Put your attention on thickness, span rating and treatment, because those are the choices that matter after the sheet is covered.

Standard CDX Plywood: 1/2, 3/4, 3/8 and 5/8 Inch

What does CDX mean on plywood? C identifies the face veneer, D identifies the rougher back, and X refers to the exterior-type adhesive bond. Expect knots, patches and repairs on the C face and more imperfections on the D back. That is a good match for hidden structural work. A clean-looking face is no substitute for the right structural rating.

The 1/2-inch 4x8 CDX four-ply panel is the pick for covered sheathing work specified at that thickness. Its four-ply construction identifies the panel you are buying; the framing spacing and required loads still govern its suitability. For a roof repair, match the required panel thickness and rating rather than choosing by appearance.

The 3/4-inch 4x8 CDX plywood is the choice for a covered subfloor specified at 3/4 inch. CDX can provide the load-bearing base beneath carpet, hardwood or tile. That makes it useful during Montana’s winter interior renovation season, when replacing the structural floor is part of the work. Choose the structural panel and the finish-floor assembly together so the new flooring starts with the base it requires.

The 3/8-inch 4x8 CDX belongs in work calling for that thickness, including rough utility projects where the panel’s rating and support suit the load. CDX works for storage enclosures, utility shelves and workbenches where appearance matters less. Keep this thinner sheet tied to the job specification rather than substituting it for a thicker roof or floor panel.

The 5/8-inch 4x8 CDX four-ply and five-ply panels give you two constructions at the same nominal thickness. For roof sheathing specified at 5/8 inch, choose the panel whose structural rating matches the roof design. An extra ply is not a reason to disregard the required span and load capacity.

Standard CDX item	Nominal thickness	Sheet size
CDX four-ply	1/2 inch	4 × 8 feet
CDX plywood	3/4 inch	4 × 8 feet
CDX	3/8 inch	4 × 8 feet
CDX four-ply	5/8 inch	4 × 8 feet
CDX five-ply	5/8 inch	4 × 8 feet

For CDX plywood for roof sheathing, thickness is only part of the purchase. Spring repairs after ice-dam damage in the Flathead need a replacement panel that matches the roof’s structural requirements. Snow load, framing spacing and the applicable plans belong in that decision together.

For walls, CDX ties studs together and helps resist racking, the sideways distortion wind puts into a framed wall. That is useful structural work in Great Falls, where winter winds and strong gusts make wall rigidity matter.

Is CDX Plywood Waterproof or Pressure Treated?

Standard CDX is water-resistant, not waterproof. Its exterior-type glue tolerates short-term construction moisture; the wood still needs protection. Buy it for work that will be covered, and schedule the covering along with the sheathing. During Montana’s fall rush to close in before freeze, that sequence matters more than the word “exterior” in the glue description.

CDX does not mean pressure-treated. The letters identify veneer grades and the glue bond. Fire-retardant treatment is a separate treatment, and protection against rot or insects is a separate purpose again. Store your sheets flat and dry so they reach the job ready to use.

Fire-Treated CDX Plywood: 1/2 and 5/8 Inch

The 1/2-inch 4x8 CDX fire-treated panel is the choice when the specified construction calls for fire-retardant-treated plywood at that thickness. Fire treatment pressure-impregnates wood with chemicals formulated to slow flame spread and reduce smoke development. That is the reason to buy the treated panel when the assembly requires it.

Choose the 5/8-inch 4x8 CDX fire-treated panel when the assembly calls for 5/8-inch fire-treated sheathing. Match both requirements. Standard 5/8-inch CDX supplies the thickness but does not supply the fire treatment.

Interior and exterior fire treatments have different exposure limits. Most fire-retardant-treated wood is intended for dry interior conditions, so choose the treatment classification for the actual moisture exposure as well as the structural use. On a Montana fall build, protection from precipitation belongs in the schedule for these panels too.

Fire-retardant plywood remains wood, and fire treatment does not make it fireproof. A one- or two-hour fire rating belongs to a tested assembly. Keep the specified plywood, thickness and fastening details together when buying for that assembly.

How to Choose CDX Plywood for Your Job

Start with the required thickness, framing layout and design loads. Then match the panel’s grade stamp and span rating to those requirements. On a two-number span rating such as 32/16, the first number refers to roof framing spacing and the second to floor framing spacing, in inches. The rating works with its loading and support conditions; it is not blanket approval for every roof or floor at those spacings. Fire-treated structural panels also need the treatment’s approved spans and load limits to match the job.

For plywood grades, CDX versus ACX or BCX is partly a question of where the face will end up. CDX has a C face and D back intended for rough work. BCX offers a smoother face for painted or partly visible work. When the panel will be covered and CDX meets the structural requirements, buy CDX and put the finish-grade face where you will actually see it.

Choose sheets that lie flat, with clean, straight edges. Knots and patches belong to the grade; significant bow or twist makes the work harder. Thickness and structural rating deserve more attention than a tidy patch.

The Pick

Buy 3/4-inch 4x8 CDX for a covered subfloor specified at that thickness. For a roof calling for 5/8-inch CDX, choose the four-ply or five-ply panel that matches the required structural rating. Buy 1/2-inch four-ply or 3/8-inch CDX where the plans call for those thicknesses. When the assembly requires fire treatment, choose the 1/2-inch or 5/8-inch fire-treated CDX that matches its thickness, structural and exposure requirements.

Frequently Asked Questions

What is CDX plywood?

CDX plywood is construction-grade plywood made from softwood veneers bonded with exterior-grade adhesive, with the grain running across the layer below. The C identifies the face grade, and the D identifies the rougher back; the X refers to the glue bond. Expect knots, patches and repairs, and judge a sheet for structural work by its rating and flatness rather than how tidy its face looks.

Is CDX plywood waterproof?

No. CDX plywood tolerates short-term construction moisture, but waterproof glue doesn’t make the wood waterproof. Plan to cover and protect it after installation, and pay particular attention to exposed edges, where moisture can get into the wood. Paint or sealant isn’t a substitute for that protection.

What is CDX plywood used for?

CDX plywood provides the structural layer for covered roof decking, wall sheathing and subfloors. It’s also useful for utility shelves, workbenches and storage enclosures where a rough face is acceptable. For structural work, match the panel’s span rating to the framing spacing and required loads; the veneer grade alone doesn’t establish what it can carry.

Is CDX plywood pressure treated?

The CDX plywood designation doesn’t mean it’s pressure treated; it identifies the veneer grades and glue bond. Fire-retardant versions are pressure-treated to slow flame spread and reduce smoke development, which is a different job from protection against rot or insects. Match the treatment label to the intended use, including moisture exposure, and keep dry-interior fire treatments protected from repeated wetting.