Dimensional Lumber Sizes
There are many standard sizes of wood members used in various aspects of building and residential construction, the most common being the “2 by” 2x4 is the name, as well as the nominal cross-sectional size of the member. This member falls into a category described by American Wood Council’s National Design Specifications (NDS) as Dimension Lumber.
“Hey, this 2x4 board isn’t 2 inches by 4 inches? I think this one was cut down too much on accident, it measures 1.5 inches in breadth and 3.5 inches in depth.” – Everyone holding their first 2x4.
Fun Fact: A 2x4 is actually 1.5” x 3.5”. All of these dimensions, along with many other section properties, can be found in Chapter 3 of the NDS Supplement. Here is an image taken from this chapter of the NDS Supplement showing a variety of sizes and actual dimensions.
Figure 1: Section properties of various board sizes. 2015 NDS Supplement Ch. 3 Pg. 14
As mentioned dimensional lumber comes in many different sizes, here is a clip of a page out of the NDS showing some additional members. Dimensional lumber is milled into its specified sizes and graded through one streamlined process from to tree to a usable wood member at a lumber mill. This is an interesting process to see how the tree is cut into different shapes, but what if you need a very large member to act as a beam carrying roof loads over a large open span. A quick napkin calculation shows the beam needing to be 24 inches deep. If you want to go find the perfect tree to mill down to this size, by all means go ahead, but there is an easier way; enter engineered lumber.
Examples of Engineered Lumber Types
This product has become a rather household name but what is exactly may not be known to some. Plywood is a combination or “layup” of multiple thin sheets of wood veneers oriented in varying directions at each layer. It is then pressed together with adhesives and turned into its final dimensions. A typical plywood sheet is 4 feet wide by 8 ft long, the thickness ranging from 1/8 inch to 1 inch and slightly greater.
Oriented Strand Board: Similar to plywood, OSB is a sheet product which is used for walls and floors. Instead of thin veneers being pressed, small mixed size pieces of thin wood, also called strands, are used in varying orientations to create the OSB sheet. OBS can be purchased in 10 foot lengths, which makes it ideal for use in wall sheathing, since only one vertical sheet is needed to cover the entire height of the wall in standard residential construction.
These are like similar in shape to steel I beams, except constructed with a piece of lumber on top and bottom, oriented flat wise, connected by a tall thing piece of OSB acting as the web of the beam. These are mostly used in floor and roof assemblies.
Glue Laminated Timber: Glulams are not new in the construction industry at all, they have been built and used for many years now. They provide one of the most effective and economical ways to span longer distances with wood products. The “laminations, “lams”, are placed flatwise and pressed together as shown in the figure. Each individual lam is graded and thus the way in which lams are stacked create different uses for the beam or column. An unbalanced layup can be used to efficiently place tension graded lams in the bottom and compression lams in the top for simple beams. Balanced layups are used for large cantilevers or continuous beams with equal stress ratings in the top and bottom to combat the positive and negative moments induced by these types of spans.
Figure 2: Schematic layup of a glulam beam.
Laminated Veneer Lumber: LVLs are similar to glulams in the since that they provide an economic solution to spanning longer distances without using large sizes of dimension lumber. Typically, LVLs are found in residential construction used as headers over doors, especially garage doors. Other types of engineered lumber similar to the LVL include: Laminated Strand Lumber (LSL), Parallel Strand Lumber (PSL), and Oriented Strand Lumber (OSL).