Of all the types of plywood available on the market, CDX grade plywood is the most commonly available in our country’s lumberyards and building centers. This softwood plywood is the least expensive grade of multi-layered veneer plywood, manufactured in small factories around the country for local use, predominantly in construction.
CDX plywood is manufactured, like other types of softwood plywood, of opposing layers of wood veneers, laid at right angles to each other and glued together with resinous glue. There are always an odd number of layers, balancing the sheet, to prevent warping of the panels. Actually, the odd number of veneer layers allow for an even number of glue layers. An odd number of glue layers would cause the glue to pull towards one direction, causing warping of the panel.
The veneers are rotary cut from a log. This means that sections of logs are placed in a large lathe, which makes a continuous spiral cut, peeling off one long, continuous sheet of veneer, which is then later cut up for use. While this process does not produce attractive grain patterns, the larger veneer sheets make it much easier for construction plywood out of. As CDX plywood is not intended for visible applications, where appearance is important, the lower cost of rotary cutting is advantageous.
Softwood Plywood Grading System
The letters “CDX” refer to the grade of plywood panel. Most plywood is graded based upon the finish of the exterior layers, specifically the amount and type of defects that are allowed in these layers.
|A||Face veneer is practically free from all defects. Smooth, paintable surface. Limited number of patches.|
|B||Solid, more knots than A grade, possible minor splits.|
|C||Limited splits, discoloration or sanding defects that don’t affect the plywood’s strength.|
|C plugged||As its name implies, the splits and defects have been plugged.|
|D||Unplugged knots and knotholes up to 3 inches, splits and other defects allowed.|
Although available, it is rare to find plywood where both sides are the same grade. This is essentially limited to hardwood plywood that is made specifically for cabinetmaking. Most plywood, other than hardwood cabinet grade plywood, is graded with two letters, showing the face and reverse side grading of the plywood.
Based upon this grading system, we can see that CDX plywood has one side that is “C” grade and one that is “D” grade. The “C” graded side is the face side, used where it is more visible, and the “D” graded side is the reverse side, usually mounted in a way that hides it.
This understanding of the face side even applies in situations where the plywood will later be covered up. When CDX plywood is used as roof sheathing, the C side of the plywood is placed up, where it will be covered by roofing felt and then shingles. This leaves the D side facing down, into the attic, where it will most likely be seen. But in this case, the criteria isn’t appearance, as it is utility. The smoother C side is better for putting roofing felt onto, as splits and holes can cause wear in the felt, leading to failure.
Please note that this grading system has nothing to do with the quality of the inner veneers, just the face veneers. The veneers used in the core layers will also have permitted voids and splits. The only exception is plywood which is stated as “having no voids,” such as marine grade plywood. While the voids and splits do affect the structural strength of the plywood somewhat, that is calculated into the permissible load for the specific plywood thickness.
Softwood plywood materials
Softwood plywood, which includes CDX plywood, is either made of pine or fir. In both cases, we’re talking about conifers, which have needles that they do not lose in the fall. They grow quicker than hardwoods, which have leaves that they lose every fall. Therefore, the wood itself is not as dense as it is with hardwoods.
When we compare these two different softwoods, we find that fir has less soft grain than pine. This soft wood is what absorbs moisture, causing the wood to warp and buckle. Therefore, in applications where the stability of the plywood is important, it is better to use fir than pine.
Interior Versus Exterior Plywood
The difference between interior and exterior plywood is in the resin glue which is used to hold the various layers of veneer together. Exterior grade plywood, marked with an “X” after the surface grade (CDX plywood means face side is grade C, back side is grade D, and the plywood is intended for exterior use) uses a resin that is more water resistant than interior grade plywood.
Even within the exterior classification we can find differences, based upon the rosin used to bond the veneers together. A-Bond plywood is made using phenol formaldehyde resin. B-Bond plywood is made using melamine-urea-formaldehyde. Of the two, A-Bond is more weather resistant.
This type of plywood is also referred to as “structural plywood;” a designation which means that it is intended for structural applications. Roof and wall sheathing is such an application. Interior grade plywood, made using resins that can’t withstand water and weather as well, are not considered to be structural grade plywood.
The designation of being exterior grade plywood can also mean “Exposure grade 1” which stands for damp-proof plywood used for demanding environments such as underlayments and roof sheeting. This plywood is made with the understanding that it will be exposed to the weather, during a home’s construction. Therefore, it is designed to handle a limited amount of rainfall, without causing any significant damage to the plywood.
CDX plywood is an exterior grade plywood only in the sense of considering the adhesive used for connecting the veneer plys. The wood isn’t pressure treated or treated in any other way to make it waterproof, unless it is marked as being “pressure treated” plywood. These will usually be a distinct color (green or yellow) caused by the resin they were pressure treated in, causing them to stand out from standard exterior grade plywood.
Marine Grade plywood
CDX plywood is not marine grade; therefore, it should not be used for the building of boats, although it theoretically could be. The major difference between them is that marine grade plywood is made of more durable veneers, which will withstand exposure to moisture better than CDX plywood. This is mostly because void free veneers are used in the making of marine grade plywood, whereas some voids are permissible in the manufacture of marine grade plywood.
However, marine grade plywood really isn’t waterproof. It hasn’t been immersed in rosin, like a truly waterproof wood is. It still requires waterproofing, especially on the edges, to protect the end grain. Otherwise, it will absorb water, with all the ensuing problems.
Marine grade plywood also has better face veneers than CDX, which are sanded smooth. This makes it usable in more visible applications, such as the building of furniture, even though it isn’t a cabinet grade plywood. It is usually made from fir or pine, just like CDX.
CDX Plywood Thickness Availability
As the most common plywood product available on the market, CDX plywood is available in a number of thicknesses, ranging from 1/4 to 1-1/4 inch thick and also including 3/8, 1/2, 5/8 and 3/4 inch thicknesses, which are the most common sizes. Please note that these are nominal thicknesses. The actual thickness of these plywood sheets will be 1/32 inch less than the nominal size, due to finish sanding of the sheet. Today, more and more lumberyards are stocking their CDX plywood with more accurate descriptions, referring to:
- 3/4” plywood by its more accurate thickness of 23/32”
- 5/8” plywood by its more accurate thickness of 19/32”
- 1/2” plywood by its more accurate thickness of 15/32”
- 3/8” thick plywood by its more accurate thickness of 11/32”
Common Uses for CDX Plywood
CDX plywood is used for a number of applications, including:
Construction sheathing of walls and roofs
Constructing crates and boxes
Exterior CDX Applications: Walls and Roofs
CDX plywood is often utilized as roof sheathing or for exterior walls due to its water resistance. CDX plywood sheets exposed to the elements can withstand exposure to rain or high moisture conditions, for a limited time, without compromising structural integrity. This plywood type is not waterproof or pressure treated, so it does absorb water. In the process, it expands slightly, but will dry quickly. Once dry, it returns back to its original size.
This resistance to weather and dimensional stability after exposure makes CDX a superior choice for roof and wall sheathing, as opposed to the more economical OSB (Oriented Strand Board). OSB panels delaminate and swell when exposed to the elements and don’t go back to their original shape after it dries. OSB panels used as roofing sheathing exposed to rain or snow can give the subsequent shingles an unattractive wavy appearance, which is most obvious at the edges of the roof.
That said, CDX plywood isn’t immune to the slings and arrows of Mother Nature indefinitely. Repeated or prolonged exposure to rain (or ice and snow) will eventually compromise the integrity of this material. The threshold for this is about four months. But that’s four months of normal rainy weather, not four months of snow and ice buildup. Those need to be cleaned off to protect the plywood from damage.
If you’re in the middle of a project and a summer thunderstorm drenches your roof sheathing or the walls of the garage you’re erecting, relax…your project will be fine. If you’re in the middle of a project and winter sets in…that’s another matter altogether. You might want to give any exposed CDX plywood some protection, especially exposed roof sheathing. Either roof felt or Tyvek house wrap could be used to dry in the exposed wall and roof sheathing. Even roof felt would be enough to (likely) protect it until the weather warms up.
Exterior sheathing is most often ½” thick (nominal dimension; actually 15/32” measured dimension) for both walls and roofs. However, the Universal Building Code states that it is acceptable to use 3/8” thick CDX plywood for sheathing. While this may not be allowed by building inspectors in all jurisdictions, it is in many. It is important, when doing a remodeling job, to understand what is currently installed and what that municipality allows.
Roof sheathing may be supported by H-clips, in addition to being nailed to the rafters. These clips, so named due to their shape, connect the edges of adjacent sheets of plywood, halfway between the rafters, helping to ensure that one panel doesn’t bow one way and the next bow the other way. The use of H-clips is a decision made by either the architect or the structural engineer.
There is a mistaken idea that H-clips are there to provide spacing for expansion between the sheets of sheathing. This is false. The point where the H-clips are, will not have any spacing, but the two sheets of CDX plywood will be butted up against each other. This means that if they expand at all, due to water absorption, they will buckle in some manner.
Interior CDX Applications: Kitchen and Bathroom Subfloors
Although considered an exterior plywood, CDX is often utilized as the subfloor for kitchens and bathrooms— especially for floors destined to be finished with ceramic tiles. When placed across equally spaced beams, joists or rafters, CDX is almost 10% stronger than OSB, the other alternative which is often utilized as a subfloor due to its economy.
This additional strength really helps when using CDX plywood as subfloors. This added strength means less bowing/flexing in the subfloor. If more flexible substrates are used as the subflooring, the extra bowing or flexing allowed by those substrates could lead to the separation of the adhesive mastic from the tile or cause the tile to crack. When it comes to choosing a subfloor for a ceramic tile floor it is always best to choose CDX plywood over OSB— it’s stronger and doesn’t have the problems with moisture that OSB does.
It is not necessary to fill the splits or knotholes in the face veneer of CDX plywood, when used as a substrate for ceramic tile. However, if vinyl floor covering is to be applied directly to the plywood, which is not recommended, fill any holes, knotholes and cracks. Allow the filler to dry and then sand, before applying the flooring. Better yet, install a layer of lauan plywood over the CDX plywood, as an underlayment, providing a smooth surface for the vinyl flooring to be mounted to.
Fire Resistant CDX Plywood
CDX plywood sheets are also available in a fire retardant variant, though those sheets come with a much heftier price tag. This plywood is chemically treated with a fire retardant, making it less likely for the plywood to catch fire. The end result is that it slows the spread of fire, although it cannot stop it. This type of plywood is used in commercial building applications to meet with building code regulations for fire suppression and/or safety concerns.
Constructing Crates and Boxes
Packing crates or shipping large valuables and furniture are often made of CDX plywood, taking advantage of its low cost, high strength and water resistance. Lower cost engineered sheet wood products, such as OSB can much more easily be punctured in these applications, limiting the amount of protection they can provide to the goods stored inside. The use of CDX plywood provides superior protection.
As long as appearance isn’t a concern, CDX plywood can be used to build a wide variety of smaller projects. C and D grade plywood veneers aren’t smooth and don’t have an attractive grain pattern, so they wouldn’t be used for furniture or commercial fixtures where the grain is visible, unless the intent is to provide an industrial appearance.
However, I have built a number of pieces of furniture out of CDX plywood, where the intent was to keep the cost low and the furniture pieces were to be painted. These were mostly shelving units of one sort or another, but I have also built children’s furniture, such as fire truck bunk beds and animal shaped toy boxes.
The secret to using CDX plywood for these applications is to first, design your project in such a way as to leave no exposed edges. This grade of plywood has many voids in it. Even areas that don’t have voids, will have rough end grain that can’t be made smooth. If you have exposed edges, cap them with hardwood. Once assembled, cover the plywood with a thin layer of wood putty, spackling or even drywall mud. Then, once it has dried, sand it smooth, providing you with the surface you need for painting.
You can use CDX plywood for the construction of storage shelving in garages, attics and basements; places where appearance isn’t important, without going through all that trouble. The plywood will provide a stronger shelf than OSB would, with less tendency towards waviness. However, the shelving will need some longitudinal support. Even ¾” thick plywood cannot cover a broad span, supporting weight, without sagging.
For this sort of shelving, it is common to use 1”x 4” or 2”x 4” dimension lumber to build a ladder framework to support the shelf. In most cases, 1”x 4” is enough, as it will support the weight to be put on the shelf. This support keeps the shelf from sagging.
CDX plywood takes glues and adhesives well (better than OSB due to the waxes involved with OSB manufacturing) as well as fasteners. Any such shelving made of OSB requires special construction techniques, so that there is a secure means of fastening and supporting the panels. Therefore, it is much easier to use CDX.