A plywood grade such as A-C or C-D names the veneer on the panel's face and back, from N, the highest grade, down to D. Panels commonly called CDX are Exposure 1, not Exterior, and belong only where long-term weather exposure isn't expected.
Face and back letters
When a panel's two outer veneers differ, the face is the better one and the back the lower-quality one. PS 1 grades each veneer by the knots, holes, splits and repairs it allows, written for a 48" x 96" panel and scaled for other sizes. Inner plies have a grade too, and the panel grade sets the lowest one allowed.
Plywood grades chart
Plywood grades chart (PS 1-22)
| Veneer | What PS 1 allows |
|---|---|
| N | For a natural finish; all heartwood or all sapwood, no knots, holes or stain |
| A | Suitable for painting; no knots, holes or open splits, up to 18 neat repairs |
| B | Solid surface; tight knots up to 1", splits up to 1/32" |
| C Plugged | Holes up to 1/4" x 1/2", splits up to 1/8", tight knots up to 1-1/2" |
| C | Knotholes up to 1" (some up to 1-1/2"), tight knots up to 1-1/2" |
| D | Knotholes up to 2-1/2" (some up to 3"); Exposure 1 panels only |
| Panel grade | Face | Back | Inner plies | Surface |
|---|---|---|---|---|
| N-N | N | N | C | Sanded 2 sides |
| N-A | N | A | C | Sanded 2 sides |
| N-B | N | B | C | Sanded 2 sides |
| N-D | N | D | D | Sanded 2 sides |
| A-A | A | A | D | Sanded 2 sides |
| A-B | A | B | D | Sanded 2 sides |
| A-D | A | D | D | Sanded 2 sides |
| B-B | B | B | D | Sanded 2 sides |
| B-D | B | D | D | Sanded 2 sides |
| Underlayment | C Plugged | D | C & D | Touch-sanded |
| C-D Plugged | C Plugged | D | D | Touch-sanded |
| C-D | C | D | D | Unsanded |
| D-D | D | D | D | Unsanded |
Structural I C-D, C-D Plugged and Underlayment are Exposure 1 grades too (see Structural I below). D-D applies only to panels qualified by performance testing.
| Panel grade | Face | Back | Inner plies | Surface |
|---|---|---|---|---|
| A-A | A | A | C | Sanded 2 sides |
| A-B | A | B | C | Sanded 2 sides |
| A-C | A | C | C | Sanded 2 sides |
| B-B | B | B | C | Sanded 2 sides |
| B-C | B | C | C | Sanded 2 sides |
| C-C Plugged | C Plugged | C | C | Touch-sanded |
| C-C | C | C | C | Unsanded |
Table 3 also lists Marine, Special Exterior, concrete form, HDO, MDO and special overlay grades, and Exterior grades can be made as Structural I.
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Exposure 1 and Exterior
Every PS 1 panel carries one of two bond classifications. Exterior is for repeated wetting and redrying, or long-term exposure to weather. Exposure 1 is for uses not permanently exposed to the weather, and those panels are meant to resist the effects of moisture on their structural performance while a building goes up.
C is the lowest veneer grade allowed anywhere in an Exterior panel, on the faces or inside. D veneer appears only in Exposure 1 grades.
CDX
PS 1 lists C-D, a C face over a D back, among its Exposure 1 grades. Exposure 1 panels use the same exterior-type adhesives as Exterior panels, but other panel factors limit how they hold up to long-term exposure.
What the bond class doesn't cover
A bond classification rates how well the glue bond resists moisture in the use it's meant for. It doesn't cover a panel's resistance to erosion, ultraviolet light, mold, fungal decay or insects.
Sanded, touch-sanded and unsanded
The grade tables also give each grade's surface. In both tables, grades with N, A or B faces are sanded on both sides. Touch-sanding is a light sanding to bring a panel to thickness, used on Underlayment and the C Plugged grades. C-C, C-D and D-D are left unsanded, and unsanded panels are the ones used where surface appearance isn't critical.
Sanded panels are held to a tighter thickness range than unsanded ones. The plywood thickness chart lists both ranges for each Performance Category.
Rated Sheathing and span ratings
Some panels are graded by use instead of by face and back, such as APA Rated Sheathing and APA Rated Sturd-I-Floor. Their stamps carry a span rating, an index number PS 1 defines for the recommended maximum spacing of the supports under the panel, with two numbers such as 32/16 for roof and subfloor. Which rating a roof, floor or wall needs comes from your building code, so ask your building department.
Structural I
Structural I panels are designed for engineered uses where properties such as shear and nail bearing are of significant importance. They're made as C-D, C-D Plugged, Underlayment or any Exterior grade, with every ply from Group 1 species, the strongest of the five species groups.
What the panel mark shows
A PS 1 panel is marked with its grade name, its face and back grades or a certification agency's grade mark, its bond classification, the symbol PS 1-22, its Performance Category and its thickness in thousandths of an inch. The mark also gives a species group or class, or a span rating on the grades that carry one.