Saturday, February 12, 2011

The real size of a 2 x 4?

Lumber used in building Bridge #4 will be "rough sawn" and it is sold by volume and the unit measure for wood is the Board Foot (Bd ft). In most text books a Bd ft is described as a piece 1" thick, 12" wide and a foot long.  A simple formula is  Bd ft = (T" x W" x L' ) / 12.

So a rough sawn   2" x 4"  - 16'  board would = 10.7 Bd ft
Hopefully some readers were saying the question was poorly written because I did not specify if the 2x4 in question was finished dimensioned lumber or rough sawn lumber. What would its green dead weight be?

When I first started teaching the third time, in 2001, I went to the different skilled trades people in the area and asked them to give me the top three specific knowledge/skill things they would want a new, entry level, co-worker to know. The owner of the lumber yard said, "The real size of a 2 x 4." He was serious and I was stunned, but he was right on! To build Bridge #4 we are going to have to know a few things about the materials we use. How is knowing the nominal and actual sizes of wood, how much it weighs, how it is sold, etc. going to be of any use for things other than building bridges?

In the picture are examples of three different systems related to wood that need to be learned and  understood to successfully design and build our bridge. On the left your standard, finished 2 x 4. In the middle a rough sawn 2 x 4 and on the right a piece representing one Board Foot (Bd ft) of wood.

Puzzler - What circumference pine tree would be required to obtain a 12" square Cant?


  1. Oops, I mean 33.94 inches. 16.97 would have been the diameter. That is if I understand the question right.

  2. Right on Luke. I used the circumference as it is easier to measure than the diameter of standing timber. There is however a Doyle Log Rule for standing timber that is held out and sighted over to determine the number of certain length logs and the quantity of board feet each log will yield. For sure the logging people are specialists at this and there is much more to learn in this area.

    MC Hammer - Cant touch this ... is in the next post. Enjoy

  3. The size of a 2x4 at Menards is 1.5" by 3.5". Why, you ask? Well mass production and modern home building with manufactured standard size materials like plywood and drywall requires uniform thickness and width boards. Rough cut a green 2" by 4" piece, dry it and then surface it smooth and you are left with 1.5" x 3.5". What most people call a 2x4.

    At Menards a 2x6 is really 1.5" by 5.5"
    and a 2x8 is really 1.5" by 5.25" What the...!
    I warn you this is confusing but there is a good reason for this and a lot to learn and apply. Too much?
    Ponder this, just how long is a 8 foot 2x4???
    I am hoping for them to put questions like this on the ACT test soon.