PLYWOOD:

Plywood is a manufactured wood, made by gluing together a number of thin veneers or plies of softwood or hardwood. It is used mostly in commercial sites, purely because it is a strong durable substance. 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. Also, 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.

Types Of Plywood.

A 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 or SPF), and is typically used for construction and industrial purposes.[1] Plywood is one of the most widely used wood products. Plywood is flammable, flexible, cheap, workable, recyclable, and can usually be locally manufactured.
Plywood is basically very thin sheets of wood glued together. The thin layers of wood are peeled from trees. These layers of wood are called veneer. The veneer is then glued together to make plywood. The grain of plywood only runs one direction, so it is very hard to bend it the opposite way of the grain line. Plywood is softwood. 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.
Hardwood plywood is used for some 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.
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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 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-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 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 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: 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.
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[3].
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.[4]