Strength of Lumber Sizes


This article is meant to provide some basic facts on the strength of various lumber sizes as well as summarize some typical “ rules of thumb “ that are commonly known to an experienced carpenter.

Questions regarding the strength of various wood elements used in construction and readily available at your local home improvement store are common, especially if you do not have any experience with these specific wood products.

Individuals involved in the construction of residential homes, have a “seat of their pants” knowledge and awareness of the strength of various sizes of lumber used in wood construction. Structural engineers understand the analytical loading and strength of various size members and will engineer their structures accordingly. The average homeowner, in most instances, has no real knowledge of the various strengths associated with various species of wood or the size of the lumber pieces.

There are some general thoughts and rules of thumb that can be applied, to better understand the actual strength of lumber, and the proper application for their use. It is important that a professional builder and or a professional engineer be consulted, if there is any significant concern or indecision, when sizing and utilizing lumber in a renovation or a new build.

General Parameters:

Vertical vs Horizontal
Dimensional lumber is always stronger in a vertical as opposed to a horizontal position when used to span an opening or support a load. This can be easily understood by using a standard 12 inch wood ruler and placing it between two books that are separated by 8 inches. This placement would allow a 2 inch bearing, or overhang, at each end of the ruler. If the ruler is placed flat on the books, and a weight is placed in the middle of the span, the ruler will bend if not break, which in structural language is identified as yield. If the ruler is placed on its edge in the same position, the ruler will not bend nor yield.

Douglas vs Spruce
The two basic species of lumber currently being used in construction are douglas fir and spruce. Douglas fir is normally much stronger than spruce, however, spruce is being used more commonly in residential construction due to its ease of constructibility as well as it’s more economical value. Other woods such as redwood, cedar and southern pine are also being used depending on the availability and the location of the construction.

Moisture in Wood
All wood has moisture within the makeup of the wood. Wood originated as a living plant that was nourished by moisture from the ground. Therefore, when cut into lumber, this naturally grown material is full of water. This wood must be seasoned and conditioned to remove much of the moisture that is inherent within the fibers of the wood. There are various methods of moisture removal that lumber manufacturers use. The wood can be merely seasoned, by stacking the lumber in piles with spacers between the individual pieces of lumber allowing air to flow through the piles of lumber and therefore dry out the lumber. This drying procedure must be evenly performed, otherwise the lumber will twist and deform. Another method used to dry out the lumber is to kiln dry the lumber by artificial heat within an enclosed environment. Kiln drying is the normal method of drying used for most commercially available lumber. Uneven drying by either method will result in a warped and twisted material. It is essential that the wood dry out evenly on all surfaces. If one surface remains wet and the other quickly dries out, the wet side will not shrink as quickly as the side that has dried out and therefore the lumber will bend and twist to accommodate the differential in size.

The determination of actual lumber strength, based upon size and material is best determined by load charts published on the internet. These charts will identify the various types of wood, as well as their intended use, and will identify the sizes of the material that will satisfy the loading requirements of a particular situation.

How to Determine What Size Wood Beam to Use

However, experienced builders have a number of common “ rules of thumb” that they will use to determine the size of the lumber to be used. A few of the most common include;

Exterior Walls
Exterior bearing walls, the normal framing lumber would be a 2X4 spaced at 16 inches on center. This means that to form an exterior wall, a 2X4 is installed vertically every 16 inches on center along the wall. In recent construction, due to the need for additional room in the exterior walls for insulation, a 2X6 has replaced the traditional 2X4. However, if saving lumber is desired and the budget is tight, the spacing can go as far as 24 inches on center due to the larger size lumber. This added spacing is based upon the structural ability of the 2X6 to support more load than the 2X4.

Interior Walls
The most common framing lumber is a 2X4 spaced 16 inches on center similar to the exterior walls. For non bearing interior walls, 2X3’s can be used and the spacing could be as wide as 24 inches on center. If the walls are deemed to be bearing walls, a minimum size and spacing would be a 2X4 at 16 inches on center.

Joists
Floor joists are normally 2X8 material or larger. The normal spacing is 16 inches on center, however closer spacing is recommended if the loading of the floor is excessively heavy, such as supporting heavy bookshelves, safes, heavy equipment, etc.

Rafter Size
If pre-manufactured trusses are not used, and typical stick framing is the system being constructed, the use of a minimum rafter size is normally 2X8. With the new insulation requirements for roofs, it is not uncommon to see 2 x 12’s being used to accommodate the need for more insulation space as well as venting area.

Headers
Headers, which are lumber assemblies that span windows, doors or other openings, are normally a double wood element, bolted or nailed together. The size relates directly to the span of the opening. If a 6 foot span is needed, then the next size up lumber size, a 2X8 doubled up is recommended. This “ rule of thumb” would continue as the openings become longer or wider. An 8 foot opening would require a doubled up 2X10 and a 10 foot opening, a doubled up 2X12. After this length of span, engineered beams or headers would need to be designed by a professional engineer to ensure safety and adequate structural support.

Horizontal Stacking
It is recommended that due to the makeup of a wood structural element and the fact that wood has moisture and shrinks during time, the stacking of wood horizontally be minimized in construction. The more lumber stacked on top of each other, the more the overall accumulated shrinkage of this assembly. Piling wood in a horizontal manner to accommodate a height requirement will produce a greater amount of overall shrinkage than a vertical piece of wood properly sized.

Vertical Stacking
Vertical placement is strength, anything that deviates from a vertical installation reduces the ability of a wood structural element to withstand a load. If you were to take the 12” ruler that we have already used as an example, and place it vertically, pushed down on the top of the ruler, the resistance would be consistent and strong. If you were to lean the ruler on an angle and load the end of the ruler, the ruler would bend and yield under the pressure. True vertical is the strongest orientation for any structural member meant to support a load. A 90 degree angle between the structural member supporting a load and the surface that the member sits upon, is the best angle for strength.

Diagonal Bracing
Diagonal bracing is the most common means of stabilizing a structure. The diagonal brace will reduce overall lateral stresses on a structure, such as a wall assembly, and provide a substantial improvement to the structural integrity of the assembly.

Panels
The installation of a rigid panel such as plywood on the surface of a wall assembly, further ensures stability, and therefore structural integrity of the system.

There are several facets of wood construction and structural understanding that we will explain in more detail in future articles.

Such topics as; box beams, laminated woods, manufactured wood, modular partitions, trusses, and other topics of interest in the use of wood in construction will be discussed.

Wood is a universal building product,and if understood and treated in a knowledgeable manner will allow homeowners and professionals alike to create strong and sustainable structures.

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