Fir plywood is a type of composite building material that is typically produced in sheets. It is comprised of thin layers of wood, also known as veneers or plies, hence the name plywood. To be considered fir plywood, at least a portion of the plies must come from a fir tree. This often leads to plywoods with fir centers, or cores, covered in birch or another wood, but which are still known as fir plywood. The veneers in a plywood sheet are glued together and placed so that the wood grain of each sheet is at a right angle to the sheet adjacent to it, leading to a greater overall strength.
From the scientific family Pinaceae, the fir tree is a type of evergreen conifer, which means that it has needles rather than leaves and stays green all year. There are between 48 and 55 distinct species of firs, including Abies balsamea, which is the Balsam fir, and Abies braseri, the Fraser fir. Although similar in appearance and well known, the Douglas fir is not actually a type of fir, as it belongs to a different genus, Pseudotsuga.
Wood from a fir tree is not considered usable for general building on its own. It is therefore typically combined with other wood types to create a plywood that is both rigid and durable. It is also mainly meant for indoor use, as fir lacks any insect or disease resistant properties once it has been hewed. Fir plywood is often used inside homes and for sheathing in home construction.
Plywood is considered to be engineered wood, since it is generally a composite of several types of wood. This combination usually leads to a stronger finished product. When a batch of plywood is made, it generally receives a grade based on the amount of knots, splits, and other defects in the finished product. Each side of a sheet of plywood can have a different grade — with A being the highest quality, and D being the lowest.
A typical size and grade of plywood can be referred to as 3/4 AC, which means that it is three-quarters of an inch (approximately 19 mm) thick, and one side is of high quality, while the other is of mid- to low quality. Plywood with a grade D will also sometimes be followed by the letter X. This generally means that it is for use on the exterior of a home or building, and is meant to be glued on.
Specialized types of fir plywood can include marine plywood, which is used in the construction of boats. This type is coated with a resin and is of higher quality than that used in homes. Marine fir plywood often suffers from the problem of checking, however, which occurs when small, hair-line cracks or lines appear in the wood due to weather exposure and the natural expansion and contraction of the wood. For this reason, other woods, such as Meranti or Okume, are often favored over fir in boat building.