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Plywood strength

By Abhijit Phadke

Plywood is a strong material, and to understand why, we need to take a look at the way it is manufactured.

It is an engineered wood product, made by sticking together several thin layers of wood over each other. These individual layers of wood (plies) are called veneers, and they are produced from timber logs by using 'rotary cut' or 'slice cut' methods. The rotary cut method which is more common involves rotating the wood log on its longitudinal axis using machines, and peeling layers from the log wood using cutting tools.

Every sheet of plywood has at least a minimum of 3 such wood veneers glued together.

What makes plywood so strong is the way in which each of these layers are arranged over one another. Each individual layer (veneer) has a grain pattern, such that it is easy to tear the veneer along the grain but not across it. This peculiarity is used to maximum advantage while making a plywood sheet. The layers are arranged over one another in such a way that the grain patterns alternate and are always perpendicular to each other.

The image shown below should help in making this adequately clear.

A damaged sheet of plywood
A piece of plywood that was damaged during the Mumbai floods in the year 2005. Notice the individual layers of ply that are seen coming apart. Also notice the tear lines seen on the individual sheets, and how these tear lines (grain patterns) alternate and are perpendicular to each other in each successive layer of the plywood.

Quality of wood used

The next most crucial aspect that affects plywood strength is the type of wood used in its making. Hardwood (for example teak wood or gurjan wood) is better in quality and much stronger than softwood (for example pine wood or mango wood).

So plywood sheets that are made using good quality hardwood are stronger than those made using softwood.

Thickness of the sheet

Plywood is manufactured in a wide range of sizes and thickness. The general rule of thumb is that the thicker the sheet is, the stronger it will be. Needless to say the thicker sheets also cost more.

Quality checks for strengths

The Indian standards for plywood (IS:303 for general use plywood) lists specifications of several types of tests that should be carried out on ready plywood sheets to check their toughness, and durability.

Some of the quality tests carried out are mentioned below.

Moisture content (percentage) The moisture content of plywood can affect its strength. If the content is too low, the plywood sheet will be too dry and hence will be weak. On the other hand if the moisture content is very high the board will be susceptible to rotting and decay. Should be between 5% to 15%
Glue shear strength Measures the tenacity with which the glue or bonding material holds the individual layers/plies together.
Mycological test To check the resistance against micro-organisms such as fungi.
Water resistance test Checks resistance to water. For BWR grades, samples are submerged in boiling water for upto 72 hours.
Modulus of elasticity Is measured across the face grain as well as along the face grain
Modulus of rupture Is measured along and across the grain
Screw and Nail Holding strength Ability of the plywood to hold on to nails and screws

Several other tests on the plywood are carried out, such as measuring specific gravity, tensile and comprehensive strength, water absorption etc.

Comparison with other wood products

Compared to Particle Board, LDF and MDF

Plywood is much stronger than other engineered woods such as Particle Board (made from wood chips/flakes) or LDF and MDF (Low and Medium Density Fiberboard, which is made from wood fibers).

If you have ever closely observed Particle board or MDF furniture, you must have noticed that instead of nails, the fittings are all done using screws, and this is because the nail holding strength of such woods is very low. Plywood is a tougher material and plywood furniture is likely to last many years longer than Particle board or MDF furniture.

Compared to Block boards

Good quality hardwood plywood is also stronger when compared to block boards or flush doors. (Block boards have a core made from blocks of softwood and plies are glued on either side. Solid core flush doors are also very similar).

However in certain cases such as in the case of long bookshelves or any application where pieces longer than 6 or 7 feet have to be used, block boards are preferred over plywood. This is because plywood has a tendency to bend or sag in the middle, when long pieces of it are used.

Compared to solid wood

The general consensus amongst carpenters is that solid wood is better because it is a homogeneous material and hence unlike plywood there is no question of layers coming apart. So solid wood is generally considered tougher than plywood, however it largely depends on the quality of the wood and the intended use.

Another advantage of solid natural woods over plywood is that its more natural and contains no chemical resins (adhesives). Solid wood (obtained from good quality hardwood such as Indian teak wood - called Saagwan) is usually preferred over plywood although in its pure form it is much costlier.