Plywood is a type of manufactured wood made from thin sheets of wood. The layers are glued together so that adjacent plies have their wood grain at right angles to each other for greater strength. There are usually an odd number of plies, as symmetry makes the board less prone to warping.
A number of varieties of plywood exist for different applications.
Tropical plywood is always made of Meranti or mixed species of tropical wood in the Asian region. Tropical plywood boasts its superiority over 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 UK, Japan, Taiwan, Korea, Dubai, and other major cities worldwide. It is the most preferred choice for construction purposes in many regions.
Softwood anel is usually made either of Douglas fir or spruce, pine, and fir (collectively known as spruce-pine-fir or SPF), and is typically used for construction and industrial purposes.
Plywood is one of the most widely used wood products. It is flammable, flexible, cheap, workable, recyclable, and can usually be locally manufactured. Plywood is made from thin layers owood that are peeled from trees. These layers (or the veneer) are then glued together to make plywood. The grain direction in each layer of veneer is perpendicular to the next layer. There are always odd number of layers so that the sheet is balanced - this prevents warping. Because of the way plywood is bonded (with grains running against one another and with an odd number of composite parts) it is very hard to bend it perpendicular to the grain direction. A common reason for using plywood instead of plain wood is its resistance to cracking, shrinkage, twisting/warping, and its general high degree of strength.
Used for soft demanding end uses. 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, and damage- and wear-resistance. 
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 thick, the thickness 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 prevents one board from moving up or down relative to its neighbour, so 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-builtMosquito bomber which was nicknamed the wooden wonder.
Certain plywood s do not have alternating plies. These are designed for a specific purpose. One such plywood is known as "Bendy Board", ( only in Ohio)Normally called Flexible Plywood. 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 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.
Marine plywood is NOT specially treated to resist rotting in a high-moisture environment. Its construction is such that it can be used in environments where exposed to moisture for long periods. Each wood veneer will 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 WBP glue similar to most exterior plywoods. Marine plywood is frequently used in the construction of docks andboats. 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.
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 include fire-retardant, moisture-resistant, sign-grade, pressure-treated, and of course the hardwood and softwood plywoods. Each of these products is designed to fill a need in industry.
US: 4 ft by 8 ft
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 laid horizontally and rotated about its long axis while a long blade is pressed into it (rather like turning a Swiss Roll against the edge of a ruler), causing a thin layer of wood to peel off. In this way 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.
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" has 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. Forest Stewardship Council (FSC), Leadership in Energy and Environmental Design (LEED), Sustainable Forestry Initiative (SFI), and Greenguard are all certification programs that ensure that production and construction practices are sustainable. Many of these programs offer tax benefits to both the manufacturer and the end user.
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 the strength of wood, optimal performance is achieved in end uses where the wood's moisture content remains relatively low. On the other hand, subzero conditions don't affect plywood's dimensional or strength properties, which makes some special applications possible.
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.
Typical end uses of spruce plywood are:
There are coating solutions available that mask the prominent grain structure of spruce plywood. For these coated plywoods there are some end uses where reasonable strength is needed but the lightness of spruce is a benefit e.g.:
Coated special birch plywood is typically used as a ready-to-install component e.g.:
Birch plywood is used as a structural material in special applications e.g.:
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.:
Tropical plywood is widely available from the South-East Asia region, mainly from Malaysia and Indonesia. Tropical plywood boasts premium quality, and strength. Depending on machinery, 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.