Grading rules differ according to the country of origin. Most popular standard is the British Standard (BS) and American Standard (ASTM). Joyce (1970), however, list some general indication of grading rules:
APA Sheathing-grade panels are not manufactured with smoothness or appearance in mind, but offer strength suitable for most industrial applications. Common veneer grades used in APA Rated Sheathing are C or D, or a combination of the two. APA Rated Sheathing is also typically available as oriented strand board (OSB).
Panels for industrial applications can be manufactured in a variety of ways – as plywood (cross-laminated wood veneers), as composites (veneer faces bonded to wood strand cores) or as oriented strand board (OSB).
For a full description of veneer grades, consult Panel Design Specificaiton, Form D510.
Bond Durability Classification. For many industrial-type applications, including product and system components, truck trailer liners, crates and pallet decks, Exposure 1 panels are suitable. For uses not permanently exposed to elevated moisture exposure, especially export packaging and refrigeration applications, Exterior panels are recommended for repeated wetting and redrying or long-term exposure to weather or other conditions of similar severity.
Span Ratings. APA Rated Sheathing panels carry numbers in their trademarks called Span Ratings. The Span Rating appears as two numbers separated by a slash, such as 32/16, 48/24, etc. The larger the span rating numbers, the stronger and stiffer the panel regardless of panel thickness.
Species of Wood. Plywood is manufactured from more than 70 species of wood. These species are divided on the basis of strength and stiffness into five groups under Voluntary Product Standard PS 1-09 Structural Plywood. Strongest species are in Group 1; the least strong in Group 5 (see Table 1 in PS 1-09). The group number that appears in the trademark on some APA trademarked panels – primarily sanded grades – is based on the species used for face and back veneers. Where face and back veneers are not from the same species group, the higher group number is used, except for sanded panels 3/8-inch thick or less and decorative panels of any thickness. These are identified by face species because they are chosen primarily for appearance and used in applications where structural strength is not critical. Sanded panels greater than 3/8-inch thick are identified by face species if C- or D-grade backs are at least 1/8-inch thick and are no more than one species group higher in number than the face species group number. Some species are used widely in plywood manufacture; others rarely. Check local availability if a particular species is desired.
Panel Orientation. The strength axis, usually the grain direction of the wood veneers or strands (unless the strength axis is otherwise identified), should run across the stringers for greatest strength.
Unsanded Panels: Designate thickness, APA trademark, grade, Span Rating, bond durability classification, additional information (such as Tongue and Groove, Structural I, edge treatments), dimensions and number of pieces. For example:
15/32″ APA Rated Sheathing,
32/16, Exposure 1,
48″x96″, 100 pcs.
Edge Treatments. A common edge treatment is a tongue and groove edge that is used when engineered wood panels are placed together with their long edges adjacent to each other (one edge has a groove milled into it and the other a tongue). By joining tongue edges into grooved edges, a strong connection is created.
Also, different edge treatments are available for panels through a secondary manufacturing process. Research has shown that a square edge on a panel deck performs as well as a bull-nosed edge and slightly better than a chamfered edge when edges are exposed to certain types of impacts.
Structural I Panels. These panels are especially designed for engineered applications such as structural components where design properties including tension, compression, shear, cross-panel flexural properties and nail holding and bearing are of significant importance. Please see section 5.6.5 Structural Panels in Voluntary Product Standard PS 1-09 Structural Plywood for details on veneer species and grade and bond durability requirements.
Softwood panel is usually made either of cedar, Douglas fir or spruce, pine, and fir (collectively known as spruce-pine-fir or SPF) or redwood and is typically used for construction and industrial purposes.
The most common dimension is 1.2525 m × 2.4 m or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1.4mm to 4.3mm. The amount of plies depends on the thickness and grade of the sheet but at least 3. Roofing can use the thinner 5/8″ (15mm) plywood. Subfloors are at least 3/4″ (18mm) thick, the thickness depending on the distance between floor joists.
Plywood for flooring applications is often tongue and groove; This prevents one board from moving up or down relative to its neighbor, so providing a solid feeling floor when the joints do not lie over joists. T&G plywood is usually found in the 1/2″ to 1″ (12-25mm) range.
Used for demanding end uses. Birch plywood is characterized by its excellent strength, stiffness and resistance to creep. It has a high planar shear strength and impact resistance, which make it especially suitable for heavy-duty floor and wall structures. Oriented plywood construction has a high wheel-carrying capacity. Birch plywood has excellent surface hardness, and damage- and wear-resistance.
Tropical plywood is always made of mixed species of tropical wood in the Asian region. Tropical plywood is superior to softwood plywood due to its density, strength, evenness of layers, and high quality. It is usually sold at a premium in many markets if manufactured with high standards. Tropical plywood is widely used in the UK, Japan, Taiwan, Korea, Dubai, and other countries worldwide. It is the most preferred choice for construction purposes in many regions.
Special Purpose Plywood
Certain plywoods do not have alternating plies. These are designed for specific purposes.
Decorative Plywood (Overlaid Plywood)
Usually faced with hardwood, including ash, oak, red oak, birch, maple, mahogany, Philippine mahogany(often called lauan), rose wood, teak and a large number of other hardwoods. However, Formica, metal and resin-impregnated paper or fabric bonded are also added on top of plywood at both side as a kind of ready for use in the decoration field.
Flexible plywood is very flexible and is designed for making curved parts. In the UK this is sometimes known as “Hatters Ply” as it was used to make stovepipe hats in Victorian times. It is also often referred to as “Bendy Ply” due to its flexibility. However these may not be termed plywood in some countries because the basic description of plywood is layers of veneered wood laid on top of each other with the grain of each layer perpendicular to the grain of the next. In the US, the terms “Bender Board” and “Wiggle Board” are commonly used.
Marine plywood is manufactured from durable face and core veneers, with few defects so it performs longer in humid and wet conditions and resists delaminating and fungal attack. Its construction is such that it can be used in environments where it is exposed to moisture for long periods. Each wood veneer will be from durable tropical hardwoods, have negligible core gap, limiting the chance of trapping water in the plywood and hence providing a solid and stable glue bond. It uses an exterior Water and Boil Proof (WBP) glue similar to most exterior plywoods.
Marine plywood can be graded as being compliant with BS 1088, which is a British Standard for marine plywood. There are few international standards for grading marine plywood and most of the standards are voluntary. Some marine plywood has a Lloyd’s of London stamp that certifies it to be BS 1088 compliant. Some plywood is also labeled based on the wood used to manufacture it. Examples of this are Okoume or Meranti.
Marine plywood is frequently used in the construction of docks and boats. It is much more expensive than standard plywood: the cost for a typical 4-foot by 8-foot 1/2-inch thick board is roughly $75 to $100 US or around $2.5 per square foot, which is about three times as expensive as standard plywood.
Other types of plywoods include fire-retardant, moisture-resistant, sign-grade and pressure-treated. However, the plywood may be treated with various chemicals to improve the plywood’s fireproofing. Each of these products is designed to fill a need in industry.