6X6 Wood Strengths

Updated February 21, 2017

When using lumber as framing material in construction projects, you should always check the strength ratings for the size of lumber you intend to use to ensure that it is strong enough for the intended purpose. You can find span tables online that will give you load bearing ratings for common sizes of lumber, depending on certain variables. These are known as span tables.

Strength Measurements

Lumber 6-by-6 posts can be rated for strength in two basic categories when it comes to load bearing. First is the amount of weight a post can hold on its top when positioned vertically as a support. This is useful when calculating deck post, or other support column sizing. The second measurement is how much weight the lumber will hold when positioned horizontally as a beam, or joist. In the case of a 6-by-6, it is typically used as a beam to support joists running perpendicular on top of it.

Spans and Spacing

When calculating a beam's horizontal load bearing capacity, you will need to consult a span table that will give you the load bearing rating of the timber in question based on its size, how close together your beams will be, and how far it is between the supports that are holding up the beam. How close together the beams are run is known as spacing, and is typically measured from the centre of one board to the centre of the next, or "on centre." For joists, this is typically no more than 24 inches. For beams it can be considerably further. The distance between support columns is known as the span. The greater the span, the less weight will be supported safely by the beam.

Horizontal Strength

When used as a beam horizontally, a 6-by-6 cut from standard southern pine, which is typical of most lumber, will support a great deal of weight. Here is the pounds per foot broken down by the span between supports. Over an 8 foot span it will hold 108 Kilogram per lineal foot. Over a 10 foot span, the same 6-by-6 will hold up to 67.6 Kilogram per lineal foot. Over 12 feet, which is the maximum allowable span between supports, a 6-by-6 beam will hold up to 46.3 Kilogram per lineal foot. You will need to consult a live load chart to estimate the weight you are intending to apply dependent on the type of construction that will be put on top of the beam.

Vertical Strength

A 6-by-6 is a very sturdy structural support and is rated to be used without lateral support up to 20 feet high, provided the proper footing and attachment is used. The measurement in question is the compression strength per square inch of a particular wood species. When calculating actual building projects, other factors will be involved that may put lateral, side-to-side stress on the post, lowering the rating. In general, a 6-by-6 cut from Douglas fir can support just over 11793 Kilogram. The weakest rated lumber is the Southern Spruce or Pine, which will support just over 7.71 Kilogram. Most other lumber will fall between these parameters.

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About the Author

Mark Morris started writing professionally in 1995. He has published a novel and stage plays with SEEDS studio. Morris specializes in many topics and has 15 years of professional carpentry experience. He is a voice, acting and film teacher. He also teaches stage craft and lectures on playwriting for Oklahoma Christian University.