Why are 2x4's actually 1.5x3.5?

They did it because engineers found that at code spacing the smaller sizes still offered enough tensile integrity to the structure, and you could get a lot more “2×4s” out of the same amount of timber than real 2×4s. Also softwood grows a lot quicker, and hardwoods are now prized for the beauty of their grain, so are more likely to be made into veneers.

Lumber just hasn’t been able to keep up with the general expansion of the universe.

I believe 2×4 is the rough cut out of the sawmill, then to clean them up more is taken away, then making the dimensions smaller.

It’s all about $$$$$!$
how many more 2×4’s can they sell if they shave a little off each one. They aren’t using big old growth timber any more so they are getting less out of every tree.

It used to be that the lumber for various markets was produced locally; the lumber used in construction depended on trees locally available, and transportation was a negligable expense. For some time now, that hasn’t been the case. Now, construction lumber is shipped cross-country, or internationally, and a huge portion of the cost of lumber is transportation of the finished product. Reducing the size standards for lumber may get a few more boards/log, but the main savings comes in shipping those smaller boards.

Even before the 60s though, a 2X4 wasn’t 2” X 4”. Standards from the 20s were loosely based on the rough-milled green board dimensions, before surface finishing and drying shrinkage. For decades after that, the standards continued to nibble away at the dimensions. By the current standards, the boards were never 2” X 4”, even when rough-cut.

As for why hardwood isn’t used much in framing these days, there are a few reasons:

Slower growth rate=more expensive

Heavier= greater transportation cost

Easier to get longer, straighter boards from conifers

I am not sure about a few of these answers… I’m not saying that they’re wrong or that there isn’t some bit of truth to them, but I don’t think there is an international conspiracy to cheat users of dimensional lumber.

In reality, the 2×4’s, 4×6’s and other dimensional lumber “sticks” you buy at the lumberyard still come from the same size full sawn members. These full sawn shapes start out with the same nominal and actual dimensions (i.e. a full sawn 2×4 is actually 2“x4”). And these members are still available to purchase from many lumberyards if you want that old-timey rustic feel. Then the mill rips them down to the rough sawn size, which is 1/8 smaller that the nominal—this is done to remove inconsistencies in the outer surface and make a better finished product. The problem is that these members have some disadvantages that were recognized early on. For one, they make bad finish material on anything but those rustic house because the grain is still exposed and can seem unsightly in certain applications. Also, because the grain is exposed, it is much harder to get good paint coverage with a thick exterior grade paint (more likely with full sawn than rough sawn), so there is more likelihood of rot. Even in interior (or interior wall cavity) application, these rough sawn members are more susceptible to rot or mold due to the fact that the pores are all open and therefore more able to harbor moisture and fungus. Finally, by planing the rough members down into the dimensional members we are used to today, lumberyards and mills are able to more accurately guage the quality of the wood and rate it appropriately. There is a significant difference between, say, DF-1 and SS members of the same species.

Therefore, the mills take those initial sticks cut from the logs and saw them down to the rough size. This is what is referred to as S4S, or “sawn four sides”. It is not possible to get a 2×4 dimensional lumber piece without starting with the bigger piece, so there is not savings at this point by the mill. However, most wood these days is some form of engineered lumber anyway. You don’t see as much of this at your local Home Depot, but if you were to observe the construction of many larger homes and smaller commercial buildings, you would see that most of the “plywood” is actually OSB (you can see the strands of wood chips in the material as opposed to continuous plies of wood), many of the floor and roof joists are i-joists with dimensional lumber top and bottom chords and OSB webs (the vertical part of an “i-beam”), and even studs are being made out of OSB type processes. All of this material in the engineered wood products comes from the waste that is cut down from the original full sawn and rough sawn members to make the nominal sized dimensional lumber.

Here is a link to Google that describes the different types of members:
http://bit.ly/v4eHu

@freerangemonkey

Here’s more than you ever wanted to know about the history of lumber dimensioning standards. The following excerpt is from there:

“This varies with the mill, but a representative situation is about as follows: To make dimension lumber 1–½ inches thick dressed at 19 percent maximum moisture content requires a “set-out” of 1–7/8 inches. Subtraction of 3/16 inch for the saw kerf leaves 1–11/16-inch rough green thickness.”

So right off the saw, the green rough-cut “2X4” is only 1–11/16 thick.

“S4S”, by the way, stands for ”surfaced four sides”, in other words planed on all four sides to remove saw marks.

@Harp good catch. Going from memory…and college was a while ago ;)