Building a crate, a bookcase, framework, door or wooden case requires wood that is both strong and light. Hardwoods have the strength, but they're also heavy. Softwoods are generally light, but not as strong as hardwoods. The good news is that there are specific woods that fit the need for light, but strong wood.
Crates, Plywood and Shelves
Sometimes cardboard just won't cut it. Crates are a perfect example, especially shipping crates, that require strong, but lightweight wood materials. Solid woods represent only a percentage of the woods used, crates are usually made from plywood, which must also be lightweight and strong. Shelving materials offer another example when solid wood might be too heavy.
As the most common wood used for structural lumber, as when framing a house with studs, Douglas fir, technically a softwood, is light, straight and strong. You can rip two-by-fours in half lengthwise to build the framework for a crate, or anything else. Douglas fir can also be used to build lightweight bed frames, struts for shelving or anything that requires a sound structural framework.
Fir plywood is lightweight and one of the strongest structural materials available. Fir plywood is affordable, and if you purchase exterior-grade CDX plywood, anything built with it has a certain amount of resistance to moisture. Premium grades of fir plywood can be sanded and finished for lightweight shelves that can handle the weight.
For the lightest crates, use 1/2-inch-thick fir plywood; it weighs about 40 pounds for a 4-by-8 sheet. For heavy duty crates, use 3/4-inch-thick fir plywood. It averages about 61 pounds per sheet.
Pine or Poplar
Use common-grade, one-by-two 3/4-by-1 1/2-inch pine or poplar to build frames, braces and stretchers for smaller boxes. Create the frame for the inside of the box to give to give fasteners and nails something to grip. Poplar is stronger than pine, but slightly heavier. Neither pine or poplar are as strong as fir, but they both weigh less than fir.
Plywood for Boxes
Use 1/4-inch-thick fir plywood for smaller boxes. A 4-by-8 sheet of 1/4-inch fir plywood weighs about 22 pounds.
Cases and Cabinets
Cases display musical instruments, bicycles, souvenir collections and keepsakes in a well-crafted, attractive enclosure. China display cabinets, for example, keep fine china safe while using it as decor. Display cabinets are freestanding and need to be moved by hand occasionally; they shouldn't be extremely heavy.
Alder and Mahogany
Pound for pound, alder is one of the lightest hardwoods available. Alder's straight grains are the secret to its strength. Mahogany shares the same straight-grain composition with alder, but it's a bit heavier and definitely more expensive. Mahogany is about one-third the weight of oak, for example. Alder is almost half the weight of oak. Use alder and mahogany for quality cabinet face frames and doors; when milled to 1/4 or 1/2-inch-thick, these woods are perfect for display cases.
Use 1/8-inch-thick hardwood plywood for the lightest display cases, cabinet backs or craft items of any type. Sometimes referred to as luann plywood, this plywood consits of a mahogany face finished with stain and lacquer. A 4-by-8 sheet of 1/4-inch-thick luann plywood weighs about 13 pounds.
Doors represent another item that requires using strong, but light woods. Hollow-core doors are just what the name implies; they're designed with materials that are strong and light, with a cavity sandwiched by the lightweight panels.
Use fir for the framework on hollow-core doors, followed by 1/8-inch luann plywood for both faces. Oak plywood is doable, but it adds weight and may not always be available in 1/8-inch thicknesses.
Alder is at the top of the list for an affordable, lightweight solid-wood door. Nothing even comes close to the strength-to-weight ratio offered by alder, and you get the beauty of hardwood.