Since you mentioned "spec house", Iím assuming you mean speculation, right?
This phrase is used when a home is built in a development on speculation by the builder. In other words, the builder built the home in advance of selling it, rather than selling from the sample, or model home, then building it.
The point is, since you used this phrase, you must have bought it new since the phrase becomes irrelevant after the first home owner. Therefore, the home is only 6 months old.
Well, then, you should not have to do anythingÖbut call the builder. The home should be under warrantee.
Most likely Itís just from new-home settling, The sagging may be only be an optical illusion. Indeed, if your joist were sagging to the point that they were noticeable, you would also feel in as you walk across the floors, and wall cracks would be everywhere. If that were true, you will be having a serious discussion with your builder since there is a big structural problem.
Contact him soon. He may only offer you a 1-year warrantee.
If my assumption is wrong, let me know more details.