Several factors affect the weight of a plywood sheet. By understanding what characteristics to look for in very light plywood, you should be able to find a product that will work when keeping your project lightweight is of the utmost importance. Always ask your lumber salesperson for a recommendation though, as he will be able to recommend the most lightweight product that fits any other requirements you might need met for your project.
Any plywood under 1/4 inch in thickness will be quite light. Some stores carry specialty plywood as thin as 1/10 inch or thinner. These products are so light that they provide barely any structural strength. Sometimes wood this thin is also referred to as "skin-ply." This lightweight wood is bent more easily than some types of wood, making it well suited to providing a wooden veneer where curves are necessary.
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The type of wood used in the manufacturing of the plywood is also very important in determining its weight. While balsa wood isn't technically speaking the lightest wood you can find, it is the lightest used in plywood as the other, lighter varieties are non-practical for most building applications. Balsa is used widely in the construction of model aircraft, as the wood is light enough that it allows builders to create products that require less loft force to stay in the air.
Plywood is made in different ways, including pressing larger chips of wood together or making a board out of finely ground wood particles. Veneer core hardwood plywood is normally the lightest type of plywood you can buy. The core of this material is made from fir, which is a very light wood, and then a wood-grain veneer is placed on one side of the board. You'll make sacrifices for the light weight however, as this type of wood often has many inconsistencies and does not cut smoothly or easily.
Plywood Vs. Particle Boards
Particle boards of various types are sold as one alternative to plywood. However, true plywood is usually lighter than a similarly sized piece of particle board. This is because particle board such as MDF is much denser than plywood. A 1/2-inch thick 4-by-8 sheet of MDF could weigh as much as 85 pounds. A regular sheet of plywood in comparison will only weigh up to around 55 pounds.