How is hardware store lumber classified?

[kel_aa]

You know, you hear 2x4, 2x12, 4x4... When I measured them, they aren't that spec in inches. Instead, they seem to correspond pretty well with the width of my fingers. My 4 fingers (about 3 1/4 inches wide) pretty well match up with the width of a 2x4, and 2 fingers for thickness. Is this a throwback to the old days when the people involved didn't use incremented rulers, but instead measured with the various parts of their body? Like if you are buying cloth, you would want to find the merchant with the longest arms?

 

[Cindy Denning]

It's in feet and inches. Like a 2x4 is 2 inches by 4 inches and 6 feet long in most places that sell it. Whatever you are making maybe taking the directions with you may help alot.

 

[ahgar]

my understanding is that back in the day the actual dimensions were 2" x 4". somewhere along the way that changed to the current dimensions. I've been told also that 2 x 4 is the size of the rough lumber before finishing. could be wrong though. later,ahgar

 

[WrayH]

The dimensions in question are the dimensions of the board when first cut - green or wet. The dimensions of the board when you are at he lumber yard or home center are the result of subsequent drying of the green material to standard moisture content and planing or other surface treatment as required.

IIRC the actual dimensions of the 2 X 4 that you measured with your fingers should be 1½" X 3½".

 

[TorzJohnson]

I saw a home improvement show the other day, and they showed a modern 2x4 next to some that were in an old barn, The old one was a good beefy 2x4, the new one was considerably slimmer and scrawnier looking. My only guess is that in the old days every manufacturer in the world was not constantly looking for every possible way to rip off their customers.

 

[Gollnick]

My only guess is that in the old days every manufacturer in the world was not constantly looking for every possible way to rip off their customers.

No.

The correct answer is that of WrayH.

But, there's even more to it. Most dimensional lumber these days is graded lumber. Each piece is invididually non-destructively tested by a machine called a "Continuous Lumber Tester" which is made by a company called Metriguard http://www.metriguard.com/. I actually know the founder of the company who invented the machine and the whole technology.

The machine sprays a dab of paint onto each piece grading it into one of several categories. This is why you can go to a lumber yard and see three different piles of 2x4s at three different prices. The weakest wood is sold cheaply and used for non-structural applications. The second grade is used for structural walls and so forth. And for floor joists and roof trusses, they use the best and most expensive lumber. But even this saves money. If an engineer doesn't know how strong the wood will be, he has to design assuming worst-case and the result is floor and roof systems that have way to much wood in them which makes them very expensive... but also very heavy. That weight means that the walls have to be over-designed... and they also have to be designed assuming the lowest strength wood. Because of Metriguard's wonderful machine, modern structures have less wood in them, but are actually stronger and better engineered. To older folks who are used to seeing the big timbers of yesteryears, modern structures may look poorly built, but they are actually better. They're also less expensive. Why waste high-strength lumber in non-structural applications? And why design structural parts assuming low-strength lumber?

Metriguard estimates that the average new house in America costs about $30,000 less becasue of their machine, and that's not exactly ripping off customers; that's giving value!

 

[Dijos]

Aside from the ridiculous post above, Lumber used to be actually 2x4. The wood was dried, and not cut or sold wet, unlike today. It was also rough finished; finished lumber is milled further to make smaller wood. . As a result, we now have 5/4 wood, and things like it, which will result in true size after milling/planing..

edited to add:
http://www.natlhardwood.org/pdf/Rulebook.pdf

More about wood than you wanted to know, including grading, sizing, retail sales.