Types of Plywood: Veneer Grades, Bond Class and Span Rating

There is no single list of plywood types, because a sheet is described by three independent things: the grade of the face and back veneer, the bond classification of the glue, and the span rating for the job it is meant to do. US Voluntary Product Standard PS 1-19, Structural Plywood, sets all three.

So "CDX" is not a type in isolation - it is a C face, a D back, and an Exposure 1 bond. "AC Exterior" is an A face, a C back, and an Exterior bond. Learning to read the stamp is faster than memorising product names, and the stamp is the only part of the sheet that cannot be marketing.

PS 1-19 defines five veneer grades, best to worst.

• A - firm, smoothly cut, free of knots, pitch pockets and open splits, well joined, sanded with no sander skips. Suitable for painting.
• B - solid and free from open characteristics, slightly rough grain allowed, minor sanding and patching up to 5% of panel area.
• C - the lowest grade permitted in Exterior panels; tight knots up to 1-1/2 inches, knotholes up to 1 inch with a larger one allowed under a critical-section calculation.
• C-plugged - C veneer with the open defects plugged, giving a flatter surface for underlayment.
• D - any number of plugs, patches, shims and borer holes that do not seriously impair strength or serviceability; tight knots to 2-1/2 inches on faces, occasionally to 3 inches.

A panel with different faces is named face first, so A-C has the A veneer showing.

PS 1-19 classifies structural plywood as either Exposure 1 or Exterior, and each classification depends on both veneer grade and adhesive performance.

Exposure 1 panels are suitable for uses not permanently exposed to the weather. They are built to resist the effects of moisture on structural performance as may occur during construction or conditions of similar severity - that is, a roof deck that gets rained on before the shingles go down, not a panel that lives outside.

Exterior panels are suitable for repeated wetting and redrying or long-term exposure to weather. The standard also holds them to tighter repair limits: in Exterior panels, synthetic filler may close splits only up to 1/16 inch wide, against 3/16 inch in Exposure 1.

There is no "Exposure 2" in the current standard, and marine panels are a separate panel grade built from Exterior-bonded veneers.

The span rating is an index number giving the maximum recommended centre-to-centre support spacing in inches. Roof and subfloor ratings are written as a fraction: a span rating of 32/16 means a 32 inch roof span and a 16 inch subfloor span. Single-floor panels carry one number, such as Floor 24 oc, and by convention a rating of 20 designates 19.2 inch spacing. APA notes that these ratings assume the panel's long dimension, the strength axis, runs across three or more supports.

Thickness is labelled as a Performance Category, a designation linked to a thickness range rather than a single measurement, tied to the nominal thicknesses used in the IRC and IBC and abbreviated PERF CAT or CAT on the stamp. That is why a sheet sold as 3/4 inch measures 23/32 inch. The codes specify a minimum 3/8 Performance Category for wall sheathing.

PS 1-19 covers structural plywood. The birch, maple and oak-faced sheets sold for cabinets and furniture are hardwood plywood, graded under their own industry standard and sold by face appearance and core construction - veneer core, MDF core, particleboard core, or the thin multi-ply construction usually sold as Baltic birch.

The regulation that does reach across is formaldehyde. EPA's rule under the Formaldehyde Standards for Composite Wood Products Act of 2010 applies to hardwood plywood, medium-density fiberboard and particleboard, along with finished goods containing them. Since 22 March 2019 these products must be labelled TSCA Title VI compliant. If a cabinet panel has no such label, that is a real question to ask the seller.

• Roof and wall sheathing: rated sheathing, Exposure 1, with a span rating that matches your framing spacing, at or above the 3/8 Performance Category the codes require for walls.
• Subfloor: rated sheathing or Sturd-I-Floor with the right span rating; tongue-and-groove edges can remove the need for blocking under panel edges.
• Painted exterior trim and soffits: an A or B face with an Exterior bond.
• Cabinet carcases and shelves: hardwood plywood, chosen by core; check for the TSCA Title VI label.
• Concrete forms: the standard has a dedicated panel grade for concrete formwork; a general sheathing panel is not it.

If you are trying to identify an unlabelled offcut, count the plies on the edge, look at the face veneer species, and check whether the glue line is dark. Photographing the face grain with WoodLens will name the likely face species, which narrows what kind of panel you are holding.

The two sheets differ in three ways, and PS 1-19 spells out each one.

• Faces - C-D has a C face and a D back, with D-grade inner plies. B-C has a B face and a C back, with C-grade inner plies. B veneer is solid with only minor repairs; D veneer may carry knots up to 2-1/2 inches and any number of plugs and patches.
• Glue and exposure - C-D is listed in the standard's Exposure 1 table (Table 2); the 'X' in CDX is the trade shorthand for that exposure rating. B-C is listed in the Exterior table (Table 3), whose heading says it does not permit D-grade veneer anywhere in the panel.
• Surface - C-D is unsanded; B-C is sanded on both sides.

In practice: CDX is the rough structural sheet for roof and wall sheathing that will be covered; BC is the choice when one face will show or be painted and the panel will live outdoors. For a flatter underlayment, the standard's C-D Plugged grade is touch-sanded with its open defects plugged.

Strength depends on what you are asking the panel to do, but the standard names one classification built for it. Structural I panels are, in PS 1-19's words, especially designed for engineered applications where tension, compression, shear, cross-panel flexural properties and nail bearing matter, and every face, back and inner ply must be a Group 1 species. Structural I is made as C-D, C-D Plugged, Underlayment or any Exterior grade.

Group 1 is the first of the standard's five species groups, which are classified under ASTM D 2555; species that cannot be told apart in veneer form must be classed in the highest-numbered group that could apply. For North American trees it includes American beech, sweet and yellow birch, Douglas-fir grown in Washington, Oregon, California, Idaho, Montana, Wyoming, Alberta and British Columbia, western larch, sugar maple, the southern pines (loblolly, longleaf, shortleaf, slash) and tanoak. Group 5 includes basswood and balsam poplar.

Marine plywood is the other top grade: an Exterior panel made only from Group 1 Douglas-fir or western larch, with every inner ply B grade or better and full length and width, and A faces limited to nine wood repairs per 4x8 sheet. The rules aim at solid, gap-free inner plies and weather resistance. This Old House also points to lumber-core panels, with solid wood strips at the centre, as exceptionally strong with minimal deflection.

'Three types of plywood' is answered three different ways, and each is useful.

• By wood: softwood plywood and hardwood plywood are the two main categories in Columbia Forest Products' guide.
• By core: This Old House lists veneer core (the most common, all wood veneers, strong and light), MDF core (a smooth, uniform centre) and lumber core (solid wood strips, very stiff).
• By use: structural (sheathing and subfloor, graded under PS 1-19), exterior (Exterior bond for weather) and interior or decorative (hardwood plywood for cabinets).

Hardwood plywood uses its own letters: Columbia describes face grades from AA, 'exceptionally high quality and ideal for custom cabinetry', down to E, and back grades from 1 to 4, written face first as in A-1 or C-3. If you are trying to name the species on a hardwood face veneer, WoodLens can identify it from a photo. For how sheathing fits into a house, see wood frame construction.