Lumber sizes changed in 70 years?

YES absolutely lumber has changed in the 70 years!

For one all boards that are sold standard any more are planes smooth and cut to a specific size for framing. OK I.E. a 8foot 2x4 "stud" measures 1 5/8 x 3 5/8x 9 5/8 inches. HUH WHAT? BUt I paid for a a 8ft 2x4, the reason is quite simple. Door and window companies as of 1940's were mandated to make the casing for windows and doors before shipping, the casing use to be built on site. now with this new fangled invention called gypsum board (drywall) the lumber yards had to determine a max. allotment for the doors and windows to all fit uniform and true. So they came up with with this weird idea to planes the 2x's uniform. What they also learned was a planed 2x does not absorb as much moisture and so stays straighter. Now the length on studs are cut so the carpenters in the field would not have to stand around and cut boards all day and you could still have a true 8ft or 9 ft wall. OK your boards are all now planed to 1 5/8, a standard wall has three of these boards holding your studs together and connecting your walls to each other. You figure 1 5/8 x 3 and you get 4 and 7/8. Add that to your stud heigth and you get an 96 and 1/8 inch wall. Now this is with allowing either some shrinking from drying out or some expansion ( in width, not length). So between idea from the carpenters doing the work, the designers building the windows and the need to cut labor cost, your homes are basically built on the mill versus the jobsite anymore!

Lumber Sizes

Lumber Dimensions

No lumber sizes have not changed, Back in the 30's houses were built with rough cut lumber. The means a 2" x 4" measured 2" x 4"

Today lumber in dressed to make it smooth. That means the lumber is 1/2 " shorter on the width and height deminsions.

A 2" x 4" is now a 1 1/2" x 3 1/2" measurement. Length does not change.

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Well basically yes....many years ago standard lumber sizes were closer to actual 2 X 4, etc...although I've worked on older homes where the sizes varied a little. They did not have uniform "standard" codes back then but generally tried to stay within a certain range. Any variations could be offset by using shims, lathe and plaster walls which were up to the plasterer to make smooth. Some studs I've seen were 2 X 3 3/4 or 3 1/2 depending on location and not always 2 inches thick since lathe can be attached to thinner ones as well in non-structual areas. Even some of their structural work left something to be desired....no headers or cripples, no jack studs around doors, etc..

When they said rough framing, they weren't kidding.

You'll have to go to 20 foot lengths because you are spanning 15'-8" and the bearing plate must be a minimum of 4" at each end, to meet National Code. Unless the total span is 15'-8" from out to out for the bearing walls, in which case 16' dimension lumber will be enough by cutting it back approximately 4".

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I live in a 75 yr. old house...The 2 x4s are 2" x 4"...the 2 x 12s are 2'' x 12' untreated lumber

I think 2 x 4s today are somewhat thinner