We inherited a house which had an addition on it with a badly warped floor. There was no top floor and it appeared to be particle board about 7/8 thick. The 2x6 in the framing could very well be warped also. Do we have to totally rebuild the addition to make it work?

 

It could be just the particle board. As you know particle board is like that Sham WOW commercial. It sucks up moisture like crazy.

 

If there truly is particle board that was used for the subflooring it needs to be removed. Particle board should not be used as subflooring, although it is used for stair treads, usually 1" to 1 1/4" in thickness. Are you sure it is not osb(oriented strand board)? This could be a pretty labor intensive job. If there is no water damage, you may want to cut as close to the walls as possible using a skill saw set at the proper depth so that you barely score the subframing. Then remove all the particle board material. Then you can run a string line over the top of the subframing with a 2x block at each end. You will be able to measure off the top of each joist to the string and note wether you have severely crowned joists. (Prior to this you may want to rent a level(transit) to check the perimeter to see if the foundation is level.) If there are some joists that have a severe upward or downward crown, get some more joist material of the same deminsion with some crown to them and install them next to the bad joists in the opposite direction(up or down). You will have to slide them into position on the sill plate(on top of the foundation) then rotate them beside the bad joist. Then drive a nail in the top of the joist which is lower that the other and use the claw of your hammer aroung the nail and the head of the hammer on the other joist and pry them until they are even then nail them together. This should help straighten the floor. I would also suggest adding an additional 2x to the straight joists, if there are any, for additional floor strength. A 2x6, depending on the span, is not sufficiant for a floor system. If the particle board is rotten between the wall sill plate and the subfloor you will need to remove that as well and replace w/ the same 3/4" osb as you will be using as your new sublfloor. Good luck.