Of all the types of plywood available on the market, CDX-grade plywood is the most commonly available in lumberyards and building centers.
It’s the least expensive grade of multi-layered plywood, and it’s predominantly used for construction. This is mainly because it’s visually unattractive compared with other types of plywood (like cabinet-grade plywood).
Keep reading to learn more about CDX plywood, how it’s made, where to use it, and what type of CDX plywood you need for your woodworking project.
How is CDX Plywood Made?
CDX plywood is manufactured similarly to other types of plywood. Opposing layers of wood veneers are laid at right angles and glued together with resinous glue.
An odd number of layers is laid to ensure a sturdy surface and a balanced sheet. The odd number of veneer layers allows for even glue layers, which prevents plywood from warping and stops the glue from weighing one side.
To make CDX plywood, the veneers are rotary cut from a log. This means that sections of logs are placed in a large lathe to create a continuous spiral cut. The long sheet of veneer is then cut before being glued together.
While this process doesn’t produce attractive grain patterns, it’s a cheap construction method. This is partially why CDX plywood is more affordable than other types of plywood.
Additionally, the lower cost of rotary cutting is advantageous because CDX plywood is not intended for visible applications (unlike cabinet-grade plywood).
How is Softwood Plywood Graded?
The letters “CDX” refer to the grade of the plywood panel. Plywood is graded based on the finish of the exterior layers. Specifically, it’s graded on each layer’s defect amount and type.
Although available, it is rare to find plywood with both sides of the same grade. The only plywood with both sides of the same grade is hardwood, made especially for cabinetmaking.
Other than cabinet-grade plywood, most plywood is graded with two letters. This explains the grade of the face and reverse side of the plywood. The “X” means that the plywood is water-resistant and suitable for outdoor use.
Based upon this grading system, CDX plywood has a “C” grade side and a “D” grade side. The “C” graded side is the face, used where it is more visible. 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 is placed up, where it will be covered by roofing felt and then shingles.
This leaves the “D” side facing down in the attic, where it will likely be seen. But in this case, appearance is not as important as utility. The smoother “C” side is better for fixing roofing felt. The splits and holes on the “D” side can wear the feet, leading to failure.
Plywood Grading System
Remember, the plywood grading system does not affect the quality of the inner veneers, just the face veneers. The veneers 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 affect the structural strength of plywood, they are calculated using the permissible load for the specific plywood thickness.
What is Softwood Plywood?
Softwood plywood, including CDX plywood, is made of pine or fir conifer needles. It is less dense than hardwood, but it’s cheaper. Conifers grow quicker and don’t lose their leaves every fall, making softwood plywood cheaper.
Fir softwood has a slightly harder grain than pine softwood. This makes it better at resisting moisture, unlike pine softwood, which has a softer grain. The softness absorbs moisture quickly, causing plywood to warp and buckle.
Therefore, it’s better to use softwood rather than pine in applications where the plywood’s stability is essential.
Is CDX Plywood Good For Outdoor Use?
The two types of waterproof plywood best for outdoor use are CDX and marine-grade plywood.
The difference between interior and exterior plywood is the resin glue that holds the veneer layers together. Exterior grade plywood marked with an “X” after the surface grade uses a more water-resistant resin than interior grade plywood.
For example, CDX plywood means the face side is grade C, the back side is grade D, and the “X” means it is intended for exterior use.
Even within the exterior classification, differences in the resin used to bond the veneers together impact plywood’s water resistance.
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 (CDX) is called “structural plywood”. This means it is intended for structural applications, like roof and wall sheathing. Interior-grade plywood (made from resins that also can’t withstand water and weather) is not considered structural-grade.
CDX plywood can also be classed as “Exposure grade 1”. This means it is a dampproof plywood used for demanding construction environments, such as underlayment and roof sheeting. Therefore, it can be exposed to rainfall and moisture during construction without causing significant damage to the plywood.
However, it’s important to remember that while CDX plywood is suitable for outdoor use, it is not fully waterproof unless it has been pressure-treated. Fully waterproof plywood, ideal for external use, is marked as “pressure treated.” These sheets are usually a distinct green or yellow color.
What’s the Difference Between CDX Plywood & Marine Grade Plywood?
CDX plywood is not the same as Marine-Grade plywood. Both types of plywood are water-resistant and can be used externally. However, Marine-Grade plywood is made from more durable veneers with minimal voids, which means it withstands exposure to moisture better than CDX plywood.
Because of the differences, CDX plywood is best used for residential construction projects, including shed buildings, subfloors, and roof sheathing. Meanwhile, Marine Grade plywood is best for boat building.
However, most marine-grade plywood isn’t fully waterproof unless immersed in resin. It still requires waterproofing to protect the end grain, especially on the edges. Otherwise, it will absorb water, causing it to bode.
Marine-grade plywood also has better face veneers than CDX plywood. It is sanded smooth and is usually available in A-A, A-B, or B-B. This makes it usable in more visible applications, such as furniture building, even though it isn’t cabinet-grade plywood. It is usually made from fir or pine, just like CDX.
How Thick is CDX Plywood?
As the most popular plywood product on the market, CDX plywood is available in a wide range of thicknesses. Sheets are available from 1/4 to 1 1/4 inch thick and 3/8, 1/2, 5/8, and 3/4 inch thicknesses.
Remember, these are nominal thicknesses. Due to the finish sanding of the sheet, the actual plywood thickness will be 1/32 inch less than the nominal size.
Due to its water resistance, CDX plywood is often utilized for roof sheathing or exterior walls. CDX plywood can withstand exposure to rain or high moisture conditions for a limited time without compromising structural integrity.
This plywood type is not fully waterproof or pressure-treated, so it does absorb water. When it encounters moisture, it expands slightly but dries quickly. Once dry, it returns to its original size. After heavy water exposure, it can become warped, but you can flatten warped plywood before continuing to use it.
The resistance to weather and dimensional stability after water exposure make CDX a superior choice for roof and wall sheathing. It’s better than using the more economical OSB (Oriented Strand Board).
OSB panels delaminate and swell when exposed to the elements. They also don’t go back to their original shape after drying. After nominal exposure to rain or snow, OSB panels used as roof sheathing can give shingles an unattractive, wavy appearance, especially at the roof’s edges.
Remember, CDX plywood isn’t immune to the elements indefinitely. Repeated or prolonged rain, ice, and snow exposure will eventually compromise its integrity. CDX plywood should last around four months outside. But that’s four months of typical rainy weather, not four months of snow and ice build-up. Those need to be cleaned off immediately to protect plywood from water damage.
For example, a partial roof, shed, or wall construction project made from CDX plywood will survive morning dew and a few showers.
However, prolonged, heavy rainfall, ice, and snow exposure will warp it and damage its structural integrity the wetter it gets. To prevent this, protect your plywood mid-construction using roof felt or Tyvek house wrap.
Using CDX Plywood Internally For Kitchen & Bathroom Subfloors
Although considered an exterior plywood, CDX is often utilized as a subfloor for kitchens and bathrooms. This is especially true for floors destined to be finished with ceramic tiles. When placed across equally spaced beams, joists, or rafters, CDX plywood is almost 10% stronger than OSB (which is often utilized as a subfloor).
The extra strength prevents bowing and 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 choosing a subfloor for a ceramic tile floor, it is always best to choose CDX plywood over OSB. CDX plywood is stronger and more water-resistant than particle board (OSB).
Filling the splits or knotholes in the face veneer of CDX plywood when using it as a substrate for ceramic tile is unnecessary. However, if the vinyl floor covering is to be applied directly to the plywood (which is not recommended), fill any holes, knotholes, and cracks first.
Allow the filler to dry and then sand before applying the flooring. A layer of lauan plywood should also be installed over the CDX plywood as an underlayment. This provides a smooth surface for the vinyl flooring to be mounted to.
For Crate Construction
Packing crates for shipping large valuables and furniture are often made of CDX plywood. This is because of its low cost, high strength, and water resistance. Lower-cost engineered sheet wood products, such as OSB, can also be used. However, these don’t provide as much protection to the contents as CDX plywood.
Plywood For Furniture
If appearance isn’t a concern, CDX plywood can be used to build various smaller projects. Plywood veneers in grades C and D aren’t smooth and don’t have an attractive grain pattern. Therefore, they should not generally be used for furniture or commercial fixtures where the grain is visible unless the intent is to provide an industrial appearance.
However, CDX can sometimes be used to build furniture, provided the construction leaves no exposed edges. This grade of plywood has many voids, but a good design can hide such imperfections. If exposed edges are unavoidable, they can be capped with hardwood for a more attractive appearance.
Once assembled, the plywood can be covered with a thin layer of wood putty, spackling, or drywall mud. When dry, it can be sanded smoothly, providing the surface you need for painting.
CDX plywood can also be used to construct storage shelving in garages, attics, and basements. This type of plywood provides a more substantial shelf than OSB, with less tendency towards waviness. However, the shelving requires some longitudinal support. Even ¾” thick plywood cannot cover a broad span and support weight without sagging.
For this sort of shelving, using 1″ x 4″ or 2″ x 4″ dimension lumber to build a ladder framework to support the shelf is common. In most cases, 1″ x 4″ is enough, as it will help the weight 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) and 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.
Using CDX Plywood For Fire Resistance
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 to catch fire.
The 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 building code regulations for fire suppression and safety concerns.
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