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My home was built in 1921. all of the floor joists are 2x6 oak, 16" o.c. covering a 14' (!) span running north to south. It's a Craftsman Cottage style home, and the upstairs is smaller than the downstairs. The north and south walls upstairs are about 3' in from the downstairs exterior walls and this has put increased stress on the joists. One of our rooms has a measurable sag of 3" from this horrible design. also, our top plates are 1.25x4 oak boards that will be replaced with doubled up 2x4's

My questions are; would doubled up 16" o.c. southern yellow pine work to replace them? And, if I wanted to open up a structural wall to a 10' or 12' opening; what size beam would I need overhead to carry the load properly? I've already looked at online calculators and they didn't really help me.
 

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my recommendation would be to enlist the aid of a professional engineer that specializes in residential construction. Instead of guessing or taking guesses from online forums you'd get the correct answers to your questions from the start. You can try multiple ways and still not find one that works so you're actually saving time and money by getting it right to start with.

Good luck!
 

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There's just not going to be a cheap easy fix for this one.
Many times I've had to tear down the whole ceiling and or removed the floors above also had to cut out some of the old plaster walls to fix floors just like that.
See if you can picture this one. Knocked down all the old plaster ceiling and lath, We had 2 X 6 floor joist running 15' so we snapped a line on the wall and cut away the old plaster with a 4-1/2" right angle grinder with a diamond blade on both sides of the room.
We installed 2 X 10 blocking between every floor joist.
Then we installed double joist hangers on one side made for LVL's.
One at a time you lift the lvl up into place enough to get it in to the joist hanger and lift it until it hit the lowest spot in the sagging floor, the other end will be sitting below the floor but that's fine for now.
Nail the LVL into the hanger and through bolt 1/2 carage bolts any place the LVL touches the old floor, that becomes your pivit point. (I predrill clearance holes in the LVL's before installing)
Once that's done build a doubled up 2 X 8 beam to do the lifting of the other end.
We used no less then 4, 20 ton bottle jacks sitting on 2 X 8's and 4 X 4 lifting beams.
Only lift an inch a day.
Once it's all even finish through bolting and add the hangers on the other end.
Once done the floor had 0 bounce or sag.
 
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