PLYWOOD AS A CONSTRUCTION MATERIAL
Plywood as a building material is very widely used due to its many useful properties. It is an economical, factory-produced sheet of wood with precise dimensions that does not warp or crack with changes in atmospheric moisture.
Ply is an engineered wood product made from three or more ‘plies’ or thin sheets of wood. These are glued together to form a thicker, flat sheet. The logs used to make plywood as a building material are prepared by steaming or dipping in hot water. They are then fed into a lathe machine, which peels the log into thin plies of wood. each ply is usually between 1 and 4mm thick.
USES OF PLYWOOD AS A BUILDING MATERIAL
Plywood has a huge range of used within the construction industry. Some of its most common uses are:
To make light partition or external walls
To make formwork, or a mould for wet concrete
To make furniture, especially cupboards, kitchen cabinets, and office tables
As part of flooring systems
To make light doors and shutters
HOW PLY IS MADE
Plywood consists of the face, core, and back. The face is the surface that is visible after installation, while the core lies between the face and back. Thin layers of wood veneers are glued together with a strong adhesive. This is mainly a phenol or urea formaldehyde resin. Each layer is oriented with its grain perpendicular to the adjacent layer. Plywood as a building material is generally formed into large sheets. It may also be curved for use in ceilings, aircraft, or ship building.
WHICH WOOD IS PLY MADE OF?
Plywood is manufactured from softwood, hardwood, or both. The hardwoods used are ash, maple, oak, and mahogany.Douglas fir is the most popular softwood for making plywood, although pine, redwood, and cedar are common. Composite plywood can also be engineered with a core of solid timber pieces or particleboard, with a wood veneer for the face and back. Composite plywood is preferable when thick sheets are required.
Additional materials can be added to the face and back veneers to improve durability. These include plastic, resin-impregnated paper, fabric, Formica, or even metal. These are added as a thin outer layer to resist moisture, abrasion and corrosion. They also facilitate better binding of paint and dyes.
High Strength: Plywood has the structural strength of the wood it is made from. This is in addition to the properties obtained from its laminated design. The grains of each veneer are laid at 90 degree angles to each other. This makes the whole sheet resistant to splitting, especially when nailed at the edges. It also gives the whole sheet uniform strength for increased stability. Furthermore, plywood has a higher strength to weight ratio as compared to cut lumber. This makes it ideal for flooring, webbed beams, and shear walls.
High panel shear: Plywood is made with an odd number of layers, making it tough to bend. The angle at which the veneer grains are laid against each other may be varied from 90 degrees. Each veneer can be laid at a 45 or 30 degree angle to the next one, increasing the plywood’s strength in every direction. This cross lamination increases the panel shear of plywood, important in bracing panels and fabricated beams.
Flexibility: Unlike cut timber, plywood can be manufactured to fit every requirement. The thickness of each veneer can vary from a few millimeters to several inches. The number of veneers used also ranges from three to several, increasing the thickness of the sheet. The extra layers add more strength to the plywood. Thinner veneers are used to increase flexibility for use in ceilings and paneling.
Moisture resistance: The type of adhesive used to bind the veneers makes the plywood resistant to moisture and humidity. A layer of paint or varnish can also increase resistance to water damage. These types of veneers are suitable for exterior use such as cladding, sheds, and in marine construction. They are also suited for holding concrete while it sets. Moisture resistance is important in interior applications as well, including on floors. The cross lamination ensures the veneers do not warp, shrink, or expand when exposed to water and extreme temperature.
Chemical resistance: Plywood treated with preservative does not corrode when exposed to chemicals. This makes it suitable for chemical works and cooling towers.
Impact resistance: Plywood has high tensile strength, derived from the cross lamination of panels. This distributes force over a larger area, reducing tensile stress. Plywood is therefore able to withstand overloading by up to twice its designated load. This is critical during short-term seismic activity or high winds. It is also useful in flooring and concrete formwork.
Fire resistance: Plywood can be treated with a fire resistant chemical coating. More commonly, it is combined with non combustible materials such as plasterboard or fibrous cement. This makes it ideal for use in fire resistant structures.
Insulation: Plywood has high thermal and sound insulation. This makes it a useful insulating material for flooring, ceilings, roofing, and wall cladding. Insulation offered by plywood can greatly reduce heating and cooling costs.
TYPES OF PLYWOOD
Structural plywood: Used in permanent structures where high strength is needed. This includes flooring, beams, formwork, and bracing panels. It can be made from softwood or hardwood.
External plywood: Used on exterior surfaces where a decorative or aesthetic finish is important. It is not used to bear loads or stress, such as on exterior door surfaces, and wall cladding.
Internal plywood: This has a beautiful finish, for non-structural applications like wall paneling, ceilings, and furniture.
Marine plywood: It is specially treated using preservatives, paint, or varnish, to resist water damage. It is used in shipbuilding, resists fungal attacks and does not delaminate.
GRADES OF PLYWOOD
Plywood grades are determined by strength, discolorations, surface defects, and resistance to moisture, among other properties. The quality of surface veneer, type of wood, and strength of adhesive, will then be allocated a particular rating. Each rating will determine the type of application the plywood is suited for.
Plywood grades are N, A, B. C, and D. The D grade has several surface defects such as graining and knotting, while the N grade has few of these. An “interior C-D” rating for example, indicates the plywood has a grade C face, and a grade D back. It also means the adhesive is suited for interior applications.
The unique characteristics of plywood, its cost effectiveness, and ease of use will continue to popularize plywood as a building material.