Plywood is a type of engineered wood made from thin sheets of wood, called plies or wood veneers. The layers are glued together so that adjacent plies have their grain at right angles to each other for greater strength. There are usually an odd number of plies, as the symmetry makes the board less prone to warping.
An common reason for using plywood instead of plain wood is its resistance to cracking, shrinkage, twisting/warping, and its general high degree of strength. In addition, plywood can be manufactured in sheets far wider than the trees from which it was made. It has replaced many dimensional lumbers on construction applications for these reasons.
A vast number of varieties of plywood exist for different applications. Softwood plywood is usually made either of Douglas fir or spruce, pine, and fir (collectively known as Spruce-pine-fir), and is typically used for construction and industrial purposes.
Hardwood plywood is used for some demanding end use. Birch plywood is characterised 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, damage and wear resistance. 
Decorative plywood is usually faced with hardwood, including red oak, birch, maple, lauan (Philippine mahogany) and a large number of other hardwoods.
Plywood for indoor use generally uses the less expensive urea-formaldehyde glue which has limited water resistance, while outdoor and marine grade plywood are designed to withstand rot, and use a water resistant phenol-formaldehyde glue to prevent delamination and to retain strength in high humidity.
The most common varieties of softwood plywood come in three, five or seven plies with a metric dimension of 1.2 m × 2.4 m or the slightly larger imperial dimension of 4 feet × 8 feet. Plies vary in thickness from 1/10" through 1/6" depending on the panel thickness. Roofing can use the thinner 5/8-inch plywood. Subfloors are at least 3/4-inch depending on the distance between floor joists. Plywood for flooring applications is often tongue and grooved. The mating edge will have a "groove" notched into it to fit with the adjacent "tongue" that protrudes from the next board. This keeps the boards from slipping past each other providing a solid feeling floor when the joints do not lie over joists. Tongue & groove flooring plywood is typically 1" in thickness.
High-strength plywood, known as aircraft plywood, is made from mahogany and/or birch, and uses adhesives with increased resistance to heat and humidity. It was used for several World War II fighter aircraft, including the British-built Mosquito bomber which was nicknamed the wooden wonder.
Certain plywoods do not have alternating plies. These are designed for a specific purpose. One such plywood is known as "Bendy Board". This is very flexible and is designed for making curved parts.In the UK this is known as "Hatters Ply" as it was used to make gents stovepipe hats in Victorian times. However these may not be termed as plywood in some countries because the basic description of plywood is layers of veneered wood laid on top of each other with the grain perpendicular on each layer.
Marine plywood is specially treated to resist rotting in a high-moisture environment. Marine plywood is frequently used in the construction of docks and boats. It is much more expensive than standard plywood; costs 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.
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
Other types of plywoods are fire retardant, moisture resistant, sign grade, pressure treated, and of course the hardwood and softwood plywoods. Each of these products are designed to fill a need in industry.
The adhesives used in plywood have become a point of concern. Both urea formaldehyde and phenol formaldehyde are carcinogenic in very high concentrations. As a result, many manufacturers are turning to low formaldehyde emitting glue systems, denoted by an "E" rating ("E0" possessing the lowest formaldehyde emissions). Plywood produced to "E0" have effectively zero formaldehyde emissions.
In addition to the glues being brought to the forefront, the wood resources themselves are becoming the focus of manufacturers, due in part to energy conservation, as well as concern for our natural resources. There are several certifications available to manufacturers who participate in these programs. FSC certified, Leeds Certified, FSI certified, and Greenguard certified. Many of these programs offer tax benefits to both the manufacturer, as well as the end user.
Plywood production requires a good log, called a peeler, which is generally straighter and larger in diameter than one required for processing into dimensioned lumber by a sawmill. The log is peeled into sheets of veneer which are then cut to the desired dimensions, dried, patched, glued together and then baked in a press at 140 °C (280 °F) and 19 MPa (2800 psi) to form the plywood panel. The panel can then be patched, resized, sanded or otherwise refinished, depending on the market for which it is intended.
US plywood grades
Plywood grades are determined by a veneer quality on the face and back of each panel. The first letter designates quality of face veneer (best side), while the second letter denotes the surface quality of the back of the panel.The letter "X" indicates the panel was manufactured with scrap wood as the center plies, not "exterior" as is commonly thought. The A-D rating is only good for construction (softwood) plywood, not for hardwood plywoods such as oak or maple.
"A": Highest grade quality available. Can be defect free or contain small knots, providing they are replaced with wooden plugs (the fillers having a "boat" or an "American football" shape) or repaired with synthetic patch. This grade may contain occasional surface splits that are repaired with synthetic filler. The surface is always sanded and provides for smooth paintable face quality.
"B": Second highest quality veneer grade. Normally a by-product of downgraded "A" quality veneer. Solid surface, but may contain small diameter knots and narrow surface splits. Normally repaired with wooden plugs or synthetic filler. The surface is normally sanded smooth.
"C": Considered to be a lower end face quality, but a reasonable choice for general construction purposes. May contain tight knots up to 1½ inches diameter, some open knot holes, some face splits, and discoloration. Some manufactures may repair the defects with synthetic filler. Panels are typically not sanded.
"D": Considered to be the lowest quality veneer and often used for the back surface for construction grade panels. Allows for several knots, large and small, as well as open knots up to 2½ inches diameter. Open knots, splits, and discoloration are acceptable. "D" grade veneers are neither repaired nor sanded. This grade is not recommended for permanent exposure to weather elements.
Plywood is used in many applications that need high-quality, high-strength sheet material. Quality in this context means resistance to cracking, breaking, shrinkage, twisting and warping.
Exterior glued plywood is suitable for outdoor use, but because moisture affects on the strength of wood, optimal performance is achieved in end uses where woods moisture content remains relatively low. On the other hand subzero conditions don't affect on plywood's dimensional or strength properties which opens some special application possibilities.
Plywood is also used as an engineering material for stressed-skin applications. It has been used for marine and aviation applications since WWII. Most notable is the British De Havilland Mosquito bomber, which was primarily made out of wood. Plywood is currently successfully used in stressed-skin applications.. The American designers Charles and Ray Eames are famous for their plywood-based furniture, while Phil Bolger is famous for designing a wide range of boats built primarily of plywood.
Softwood plywood applications
Typical end uses of spruce plywood are:
- Floors, walls and roofs in house constructions,
- Wind bracing panels,
- Vehicle internal body work,
- Packages and boxes,
There are coating solutions available that mask the prominent grain structure of spruce plywood. For these coated plywood there are some end uses where reasonable strength is needed but lightness of spruce material is a benefit e.g.:
- Concrete shuttering panels
- Ready to paint surface for constructions
Birch plywood applications
Coated special birch plywood is typically used as a ready to install component e.g.:
- Panels in concrete formwork systems
- Floors, walls and roofs in transport vehicles
- Container floors,
- Floors subjected to heavy wear in various buildings and factories,
- Scaffolding materials
Birch plywood is used as a structural material in special applications e.g.:
- Wind turbine blades
- Isolation boxes for liquefied natural gas (LNG) carriers
Smooth surface and accurate thickness combined with the durability of the material makes birch plywood a favourable material for many special end uses e.g.:
- Die cutting boards
- Supporting structure of parquet
- Playground equipment
- Sign and fences for demanding outdoor advertising
- Musical instruments
- Sport equipments
Tropical Plywood Applications
- Common Plywood
- Concrete Panel
- Floor Base
- Structure Panel
- Container Flooring
- Lamin Board
- Laminated Veneer Lumber (LVL)
Tropical Plywood are widely available from the South East Asian region mainly by Malaysia and Indonesia. Tropical Plywood boasts its premium quality, and strength. Depending on machineries, tropical plywood can be made with high accuracy in thickness, and is a highly preferable choice in America, Japan, Middle East, Korea, and other regions around the world.