Our home has a first story addition in which there is a noticeable grade change at the entry (the addition is a family room that is entered through our dining area. The change in grade could be 3" or more. I am looking to replace old carpeting with hardwood in this room and it appears that the subfloor is "bent" at the entrance, creating a ramp effect into the room. I assume this is due to settling (the addition is at least 20 years old), and am wondering if there is a solution to make the subfloor even with the rest of the house.
Rick, there are floor leveling compounds that can be used for minor height deviations in subfloors when installing hardwood, but 3 inches isn't normally considered a small adjustment. Actually it may be an indication of a major problem. When you describe the addition as having a subfloor, I take that to mean that it's wood and there is a crawl space or basement below the room. It sounds to me like some of the floor framing has failed at the entrance to the room or possibly a section of the foundation.
The only way to know for sure is to go below and take a look. If the floor system is composed of engineered floor trusses, I would suspect that the problem is with the foundation. Engineered trusses usually don't fail unless they are damaged during construction or weight limitations are exceeded - while not impossible, neither situation happens very often.
However, if the floor system is dimensional lumber such as 2 x 10s, cracking or bowing could be causing the problem. A large hole cut in the wrong place of floor framing member can also cause structural failure - this may have been done by an HVAC or plumbing contractor.
The issue should be addressed from below as the floor may continue to settle even if you level up the subfloor. Repairing the foundation or floor framing can both be done, although the framing would more than likely be a little easier than getting concrete into the space. Unless you are a very experienced DIYer, this sounds like a repair for a professional contractor. The floor will need to be jacked up to the proper height and then some difficult framing or concrete work may need to be done.
I believe it would be a good idea to ask a structural engineer to take a look at the addition's settlement. They often have a little more expertise in these matters than many contractors and can provide a detailed drawing on how the repair should be made. The last thing you want is to spend a lot of money to get the floor level, install your hardwood, and have the addition begin settling again.
While no one can guarantee that won't happen, the odds are much more in your favor if you follow an engineer's recommendations. They may even be able to save you some money on the project by devising a simple fix such as jacking up the floor and attaching some lumber to the sides of the existing joists.