If a hardwood floor is nailed from above, the nail holes can ruin the finish. Still, squeaking floors are not a concern when it comes to the structural integrity of the house.
If the floors in a section of the house are finished hardwood and the remaining floors – except the ones in the bathrooms and kitchen – are covered with wall-to-wall carpet, there likely is no hardwood floor below the carpeting. It is relatively common for a builder to give the option of hardwood floors or carpeting. When the buyer chooses carpeting, it is usually laid over a plywood floor.
Moreover, even if the floors look like hardwood, they may not be – not in the classic sense at least, of solid wood from top to bottom. Instead, it may be engineered hardwood floor or a laminate floor. An engineered hardwood floor is made up of three to ten thin layers of wood with a core of hardwood, plywood or high-density fiber. The top layer is hardwood veneer glued on the top surface of the core, available in nearly all hardwood species. It costs less than hardwood, and due to its multi-ply structure it is much more stable than solid wood, and also less susceptible to shrinking and expanding with changes in temperature and humidity. 
Engineered wood can be installed on any grade level, directly over concrete, as well as below ground level, while solid hardwood should only be installed above grade. Moisture and extreme temperature changes can cause solid wood to shrink and expand, which may cause gaps between boards during colder or drier seasons. If there is excessive moisture on the underside of the solid hardwood floor, the top surface will cup, i.e. form a concave surface with the edges raised.
Between hardwood and engineered flooring, laminate flooring is the least expensive. It looks just like wood flooring, however there is no solid wood used in its construction. Laminate flooring is made up of several materials bonded together under high pressure. Generally, laminate floors consist of a moisture-resistant layer under a layer of high-density fiberboard (HDF). This is then topped with a high resolution photographic image of natural hardwood flooring, accurately reproducing the grain and color of natural hardwood. To protect the laminate, it is finished with an extremely hard, durable clear coating made of special resin. Laminate flooring can be installed either above or below grade, and over virtually any other floor surface. It is not nailed or glued to the subfloor, but usually laid as a floating floor.
Bamboo floor is another type of hardwood floor, typically made by slicing bamboo poles into strips. These strips are boiled and sent to a drying kiln to reduce moisture levels and prevent potential mold, fungus or insect problems. Then, the strips are glued and assembled under extremely high pressure and heat, either face up for horizontal flooring or side by side for vertical flooring. The latter results in a smooth uniform appearance, however horizontal flooring has a more natural look, and the pressure and heat applied assure a strong, stable bond.
After milling, the bamboo is coated with aluminum oxide to increase its strength and endurance, and also to protect it from stains. Bamboo flooring must be regularly maintained, kept free of dust, dirt and grit. Even the smallest particles of dirt can cause minute scratches on its surface, and just as any wood floor, it is damaged by dents, scratches or high heels.
Upon a floor inspection you may notice that the floor is not level, a condition usually caused by past shrinkage, warpage and settlement of the wood framing. This condition is of no concern, but if the floor is sagging in some section, it might be an indication that it is or was improperly supported. In this case, go down to the level below to see whether the ceiling is sagging as well. If it is, have a professional evaluate the condition. Sometimes, this occurs in a kitchen floor when the refrigerator is placed in a location other than that intended by the builder.
The lower level of bilevels has a concrete slab floor. Depending on the quality of construction, the concrete slab could settle to a point where there is a large open joint between the floor and the walls. If the ground below the floor slab is not properly compacted during construction the slab might eventually settle, resulting in open joints. However, the foundation walls all have support footings and are normally independent of the floor slab, so this settlement usually does not indicate a problem with the main structure. The wall above the open joined should be shimmed nonetheless, otherwise the floor above might sag. Of there are also cracked and settled sections of the foundation wall along with the settlement of the concrete slab, a severe problem does exist. Have a professional evaluate it.
Lastly, during your floor inspection, look for any raised wood floor that might be covered with carpeting or resilient floor tiling. Test for soft or spongy sections, a condition sometimes caused by inadequate spacing of the wood framing. However, it is generally caused by rotting wood. If water penetrates the area between the wood floor and the concrete slab, it eventually causes the framing to rot and the subflooring to delaminate. In this case, a certified professional should conduct a floor covering inspection and determine necessary measures for maintenance, cleaning and restoration.