split level sagging girder

[homerookie]

We are looking at the purchase of a split level home, 9 yrs old. It has three bedrooms and a bathroom above a 2 car garage. We noticed a slight unlevel condition in the floor of the front two bedrooms, sloping down towards the front of the house and the center of the garage (low spot is directly over the area between the two garage doors). It appears that the support (girder?, beam that the floor joists rests on) is sagging due to the large span across the front of the garage, the sagging above the garage doors can be noticed from the street (only very slightly, if your looking for it). There are no cracks in the drywall in the bedrooms to indicate major settling. So, I would guess that it may be sagging in the center by an inch or less. My question is : Is this a common problem in this type of home ? The garage has no adjustable steel posts installed as I have seen before, could a post installed between the garage doors fix this ? If so, is the concrete slab of the garage capable of supporting this type of load without breaking ? The home inspector noted it in his report, but didn't seem overly concerned. He said that it was due to "crowning", not sure exactly what this is ? Once again the sagging appears to be minimal but is annoying, and will be a concerned when reselling if not remedied. Should this be a deal-breaker ? I know that if the beam is lifted, there is a great probability of cracking out the drywall above etc... Could a small lift in this beam (3/4") do damage to the roof ?Is correcting the beam on the order of thousands of dollars ?
Or, is it something I can do (I am fairly knowledgeable, and handy, and an engineer to boot, mechanical, not structural though) ? I understand no one here can give me a difinitive answer other than, "having a reputable contractor look at it". I am just looking for someone who has had a similar experience or a general knowledge of what is involved in the repair. I know there are a lot of questions here but any qualified input would be appreciated.

[pushkins]

The "sagging" you refer to isn't all that uncommon, though obviously it shouldn't be there on a home only 9 years young. You say there are no cracks in the drywall anywhere, yet the sag is noticeable both internally and from the street, this would suggest to me that the house was built with the center support (between the garage doors) lower than the walls on either side and the builders and drywall installers worked around the problem. It takes very little movement or settlement for drywall to crack.
"Crowning" is referred to the placement of lumber beams/joists with their highest point along the length facing upward, this could give you your center dip but I doubt it as there would need to be many beams/joists with severe crowning all in the same area.
Your concrete slab most likely is between 3" and 4" thick, this will not hold as a footing for an adjustable post to to lift the sag, you would need to cut out a 12" square section, dig down to code depth for your area (frost line etc...) and fill the hole with concrete then install you jack.
Generally when lifting homes you do it like a tortoise SLOW and STEADY or you WILL crack drywall and have door problems 1/4" lift leave a few days then 1/4" lift....slow and steady.
One thing to look for is how doors are presently closing above the garage, do they close easy?......when closed have a look at the space between door and frame, is it uniform are there points on the door or frame where there have been obvious rubbing, this will help guide you on if it's settlement or if the house was built around the lower center span post on the garage.