I don't really like cutting into someone's floor to figure out a problem, especially in a finished house. Often I will talk the builder or floor installer into cutting it up. In this case, I talked the homeowner into it. (Does it ever disturb you how anxious some people are to take a saw to their house? I guess they get so frustrated that a little destruction is a good frustration release, especially when someone else recommends it.)
He had a hump and soft spot in his floor just before you entered the hall from the great room. Nice steel framed house, including steel floor joists, and poured foundation walls. From the basement, everything looked fine. There was no floor insulation obstructing a view of the subfloor. So we cut through the hardwood and started peeling it back. The first thing I noticed was concrete, where I had expected wood subflooring. Hmmm. Next thing I noticed was a bunch of floor adhesive, like almost an inch worth.
It turns out that either the foundation was laid out wrong or the upstairs walls were laid out wrong. The result was that the foundation and upstairs walls didn't line up. The living space of this house looked like the state of Utah, with a garage in the northeast corner and front entry on the east side. The foundation wall at the back of the garage extended under an inside hallway along the back of the garage. The hallway was tiled.
The foundation at the inside corner of the garage stuck out into the footprint of the great room. The builder ran the joists and subfloor up to the edge of the foundation wall. The floor installer then nailed the flooring to the subfloor where he could, but built up the foundation wall and slab with floor adhesive. Then he tried to use that to continue the flooring right across the slab.
In this case, some careful measurements would have revealed the misalignment situation before we cut into the floor. I sure couldn't see it from eyeballing things. But even then, it would not have explained what was wrong because the misalignment wasn't the problem. It was how the flooring installer handled the substrate change that was the problem. The homeowner added this to his list of issues with his builder, but was happy to know what needed to be done to fix his floor.
