Uneven Subfloor

[APZ]

I have a 16' x 12' room which is built upon an area of the house which is over the meeting of 3 different additions.  It's a one story house over a crawlspace.  There's no damage to the joists, but the joists from the different additions don't meet up perfectly to create a flat subfloor.  None of the joists are are longer than 8' and they are 2 x 8's.  The subfloor is a hodgepodge of tongue and groove planks and plywood.  I'm going to rip all of that out.  Once down to the joists can I just sister new boards to the joists to create a level surface for my subfloor?  If so, do they need to rest on the foundation or can they just be attached to the existing joists; do they also need to be 2 x 8s?  I appreciate any input. 

Ken

[Robert Adams]

Just sister 2x4s or 2x6s to the floor joists. Construction adhesive and pop it on with a nail gun or construction screws.

[mike4244]

I have a 16' x 12' room which is built upon an area of the house which is over the meeting of 3 different additions.  It's a one story house over a crawlspace.  There's no damage to the joists, but the joists from the different additions don't meet up perfectly to create a flat subfloor.  None of the joists are are longer than 8' and they are 2 x 8's.  The subfloor is a hodgepodge of tongue and groove planks and plywood.  I'm going to rip all of that out.  Once down to the joists can I just sister new boards to the joists to create a level surface for my subfloor?  If so, do they need to rest on the foundation or can they just be attached to the existing joists; do they also need to be 2 x 8s?  I appreciate any input. 

Ken

When you get down to the joists you may find that shimming a couple of joists will do the trick. If not, do like Robert suggests and add a 2x6 along each joist. Another way that I have seen but not done myself is to pour self leveling gypsum onto the existing floor. I have worked on these floors and had to lay protection down so my shoes did not scuff up the soft gypsum. I imagine carpet is laid over this. Probably the least expensive way but not necessarily the best. I was told recently that new gypsum floor levelers have a hydraulic cement mixed in and are much better than the product I have seen in the past.
mike

[APZ]

Thanks guys.  I already tried self leveling and that didn't work out so well.  I just spent the weekend undoing that mistake and my body didn't appreciate it.  (And I still need to undo the hallway.)  
  I'm better with wood than concrete.  Plug here for the Harbor Freight Bauer rotary hammer. 

2x6 attached to the joists is the current plan.