To understand the strength of plywood and the factors that impact this, it's first essential to comprehend how plywood sheets are made. 
The first step in plywood production is gathering the timber. Plywood can be made from either hardwood or softwood, or a combination. Hardwood plywood may be made of birch, oak, mahogany, or teak, while softwoods such as beech or pine are often used to make softwood plywood. 

After removing branches and bark and soaking the timber in water to make it easier to peel, the logs are cut to size. Then, thin veneers are peeled away from the log using a rotary lathe. These sheets are cut to size and graded based on their quality -- veneers with defects can become the core of the plywood, while high grade veneers are used for the face. 

The next step is to dry the veneers and check them for defects, plugging holes and filling in splits. Then, they are glued and stuck together, with the grain alternating (known as cross-graining or cross-lamination). The glue that is used may vary on the type of plywood being made -- urea-formaldehyde resin is the typical adhesive used for plywood, although exterior or marine plywood that needs to be resistant to moisture often uses a phenol-formaldehyde resin instead.

The plies are pressed together twice, first without heat and then using heat. Finally, the sheets of plywood are trimmed and sanded, ready for quality control, packaging, and use. In some cases, the finished sheet material may be treated with a chemical to make it more fire-resistant. 

As an engineered wood product that is used in applications such as ship and aircraft building, it's clear that plywood has to be very strong. But what makes it more robust than solid timber?

It's all down to the manufacturing process described above. In particular, the cross-grain structure (where the grain of each veneer is alternated) is what gives plywood its strength because each veneer's grain direction is rotated 90 degrees from the ones directly next to it, if one set of grains breaks, the next veneer will prevent the sheet from breaking. 

The cross-laminated structure also has other benefits. It reduces shrinkage, expansion, and bending, which gives plywood the upper hand over real wood. Furthermore, the cross grain makes the panel less likely to split when screwed into. 

The number of plies in the sheet also impacts strength. Plywood is always created with an odd number of plies to keep it balanced. The sheet would be at greater risk of warping without an odd number. 

Finally, another benefit of plywood over natural wood is that real wood can have knots going through the whole panel, creating a weak point. On the other hand, if there are knots in plies of plywood they are filled in and are not all in the same place. This gives plywood's structure more strength.

Thickness

Perhaps the most obvious factor that impacts the strength of a board of plywood is its thickness. The higher the thickness, the more strength the board will have. Here at Cworkshop, for example, we work with thicknesses up to 30mm (1.18 inch), which allows the material to be very strong. Thicker plywood sheets are created by gluing together more plies in order to create the desired size and strength. For example, a five-ply sheet will be stronger than a three-ply sheet. 

Hardwood plywood vs softwood plywood

Another of the factors that impacts plywood strength is the type of wood it is made from. Plywood made from hardwood has a higher density and strength than softwood plywood. So, while hardwood plywood is often used in creating long-lasting furniture, durable flooring, and more, lower density softwood plywood is used in applications such as ceilings and doors. It's also useful to keep in mind that because of the differences in density, the heavier hardwood plywood weight makes it more difficult to work with in large sheets. This is why ordering hardwood plywood cut to size can be a good option for hobbyists working alone. 

Type of adhesive

Finally, the type of adhesive used to glue together the veneers also has an impact on the strength of the material. Plywood is only as strong as the glue that holds it together, after all!