OK normally load bearing walls will have 2 plates on top. But having 2 plates means nothing.
People frame however they learned to or want. Checking door header also means nothing. Some people flip all door headers - doesn't make the wall load bearing. You can never get into the head of the guy who framed your house. I worked for a construction company in college... the owner taught everyone to basically frame every wall like it was load bearing - 2 top plates and headers flipped was a minimum. I asked him one day why we were doing this and he basically said - Well it might cost an extra $10 on some walls but we never have to worry or redo anything.
You told me this wall goes in same direction as attic joists... That is a good sign. You need to get into the attic and see what it is attached to. If it isn't supporting a joist it isn't load bearing. If this wall is just floating between 2 joists - even with a few 2x4s for cross bracing - it is not load bearing.
However if there is structural bracing in the attic it is/could be load bearing. Or if it is directly under a joist.
You have two choices to figure this out. Get up in the attic or tear down enough drywall to figure out what you have. If you are a gambler and want this wall down just tear out the drywall - you will have to remove it to the joist on each side to redrywall anyway.
Each of the two temporary structures (one in each room) will need to mimic the section of wall that is being removed. So if the jack poles are resting on a single bottom-plate and are holding up a double top-plate, then jack poles 16" on center are called for. If something more substantial than the double top plate and single bottom plate are used, then fewer jack poles are required. In the extreme, it'd take only two jack poles placed 6ft apart to hold up their own temporary 4x10 header. The first floor's floor will need similar support in the basement.
I'd build the temporary support structure using lumber that I could use again somewhere else.
All that is assuming the ceiling joists are perpendicular to the wall in question and that the support structures are placed close to the wall in question.
If instead the ceiling joists are parallel to the wall in question, then there must certainly be a 2nd flr wall that is on top of the wall in question, in which case all the temporary support structures will need to be installed up on the second floor.
Personally, though, my choice would be to do it like this, without shoring up ...
0) pick through the pile at the lumber yard for straight 2x10s
1) open the wall on both sides
2) install the king and jack studs
3) notch the top of the studs on one side to let in one 2x10
4) install the that 2x10 onto the jack studs
5) shim between the cut tops of the studs and 2x10
6) cut off the stud ears (produced by the notching) that are on the side
7) nail off the 9" wide rippings of 1/2" plywood filler
8) install the second 2x10 onto the jack studs
9) nail off the header
10) remove the shims
11) remove the old studs
Best Answer
Those are 2x6's? Not "old school" non-dimensioned 2x4's? From my experience, most lath was about 1" wide... and if that lath is 1" it would seem to scale those to 2x4.
Regardless, yes you can confidently shore up that wall by creating a temporary wall adjacent to it. If it is a basement floor it is likely to be sloped, so studs may have to be cut differently. I'd recommend attaching a top plate to the bottom of the joists, and place a "floating" bottom plate directly below on the floor. Cut the studs no more than 1/8" proud of the actual height, align the top under the floor joist, and pound in the bottom. Since it is going in at an angle, the edges will dig in some, but at only an 1/8" long it will be minimal.
You will likely have to repeat the process on the opposite side of the wall, since the joists are "lapped", and the other side is essentially independent. You'll know that you've accomplished your goal when the original stud starts to feel "loose", with only the nails holding it in place. If you have a reciprocating saw (Sawzall) you'll know the weight has been supported when the kerf of a cut through the stud no longer tries to pinch itself shut. Be wary of this method, though, because if there is still substantial pressure on the stud the kerf will begin to close as you pass the halfway point, which can result in kickback.
The shoring wall(s) should be at least as long as the section you're replacing. If it is indeed termite damage of the bottom plate, I would wonder how the wood could have supported any weight. I used to live in Wisconsin, where that type of construction (lath) was very common, as were full basements. Termites were unheard of, however.
Now I live in South Carolina.... Termite heaven. However, basements are very rare here... most homes are built on crawl spaces or slabs. If it is indeed termite damage and not just dry rot, I would highly recommend a thorough investigation of the rest of the house. The bottom plate in the center of a basement likely wouldn't have been a point of entry unless there is a crack in the floor nearby. If it was the point of entry, the termites would likely have began tunneling up the studs themselves as the colony grew and spread.
Look for "mud tunnels" or other very well defined tunnels chewed through the wood. If you find them, expect that they have also munched on other areas of the house. Hopefully it's just dry rot!