As it appears you’ve never done this before, I urge you hire someone or bring someone along who has done this before as you really don’t want to find out what happens when you do it incorrectly.
That said there are two ways to do it. The engineer way and the contractor way.
The engineer way involves calculations based upon the present situation. Many engineers involved with construction can do them so can many architects, as well as some draftsmen and contractors. You’ll be told the final sizes of the materials involved and what the final installation will look like. You would build a temporary support similar to the final version until you cut out and place the final supports.
The contractor way involves rule of thumb methods adjusted with past experience.
It goes something like this for wood walls:
- Check that the joists above the wall don’t go further than 16′ to either side of the wall.
- For every 1′ of horizontal opening you need 1″ thickness on a 4x header for a one story wall, or for every 8″ of horizontal opening you need 1″ thickness on a 4x header for a two story wall. Each level of floor or roof joists counts as a floor in this case. So, a 4′ opening requires a 4×4 on a one floor wall, but a 4×6 on a two floor wall.
Keep in mind that this method doesn’t work well as the floor length grows, the opening gets a lot bigger, or you have additional floor levels.
Also, as they don’t have odd sizes for most lumber (ei 4×5) go to the next larger size.
MKDesign 