Plywood is among the strongest of all building materials. The strength is a result of individual grain layers placed perpendicular to each other bonded with glue under high pressure in the manufacturing process. This makes plywood virtually split-proof. Other properties that can add strength to plywood are type of glue, species and thickness.

Thickness
The strongest of all plywood is used for concrete forms, stair risers and industrial flooring. This 1-inch-thick fir plywood is also available with tongue and groove patterns that lock together for even more strength. Other widely used plywood types include 3/4 inch, which is one of the most widely used of all plywood thicknesses. A 12-by-36-inch piece of 3/4-inch fir plywood can support up to 50 lbs. without problems. Plywood strength drops when thickness diminishes. For example; a 12-by-36-inch piece of 1/4-inch plywood will only support about 5 lbs. before bending.

Species and Glue
Two types of plywood that are commonly used in the building industry are manufactured using two distinct species of wood. The conifers, or softwood plywoods, generally lack the strength of hardwood species. But even though hardwood plywood is much stronger than softwood plywood, manufacturers use exterior glue on some varieties of softwood plywood. This makes it much stronger over time than hardwood plywood since it will not absorb moisture that causes plywood layers to separate and weaken. But when testing equal sizes of hardwood against softwood that have both been manufactured using interior glue, hardwood plywood is significantly stronger.

Flexibiity
Flexibility is an overlooked asset of plywood. For exterior uses such as sheathing typically placed over two-by-four studs on the exterior of a home, flexibility is a definite advantage. Exterior grade 3/4- or even 5/8-inch fir plywood has enough flexibility to resist cracking when applied to uneven frame structures when they are being built, or during settling of the structure. This flexibility -- also known as shear strength -- adds incredible strength to frame structures. Before a frame structure has plywood nailed to it, it is very fragile and can be knocked down easily by a strong wind. But when fir plywood is added as sheathing, the shear strength of the structure improves dramatically, making the structure strong. The shear strength of fir plywood is unparalleled by any other building material.

Layers
Plywood is layered. The number of layers in plywood relates directly to strength. Individual layers can be counted by looking at the edge of any piece of plywood. If there are fewer than four layers in typical 3/4-inch-thick plywood, the plywood has the minimum amount of strength. This is often referred to as shop-grade plywood, the most affordable plywood. It will bend or break easier than almost any other type of plywood. If the plywood has between four and seven layers, the plywood has a moderate amount of strength and can be used for cabinets or almost any other project. If the plywood has more than seven layers, it is the strongest and can be used for special projects such as guitar bodies or router patterns.