Do not get the wrong idea. Solid wood lumber in the form of "2x" studies and joists still qualifies as premium framing material for use in house construction. But in the last 20 years or so, engineered lumber has taken about a revolution in building.
The Thrust Joist I Beam
Today, the majority of builders and architects prefer to frame floors with TJI's, or truss-joist I beams made from composite wood. TJI's are essentially wood I-beams with a center "web" made from OSB (oriented strand board) and identically sized top and bottom chords made from plywood or other composite lumber.
Like other engineered wood products, TJI's are more uniform and dimensionally stable than old-fashioned solid wood joists. The solid wood joists used for floor framing in older homes can be weakened by knots and cracks; individual joists can warp, bend and bow. This "real wood" lumber will also shrink and swell in response to changes in humidity. A floor framed with TJI's does not suffer from these defects or dimensional variability.
Wood Girder
In many older homes with crawl space foundations, there will be solid wood floor joists that are supported at mid-span by a wood girder. The girder is often made from multiple 2x joists that are nailed together. Girder ends are supported by the foundation, while wood posts bolster the girder at regular intervals in the middle of the foundation.
The cumulative result of joists weakened by knots, cracks or decay, in addition to joists that have warped or shrunk, is a floor that may sag, tilt, or bounce, making life difficult for building inhabants. Another structural problem has to do with the post-and-girder assembly that provides mid-span support for the floor.
Because of postponed posts, inadequate post footings, or an undersized girder, the overall floor structure canaken and bow downwards. If the girder lifts or sinks, the entire upstairs floor structure is often affected. Wallboard typically cracks, while door and window openings become skewed so that doors and windows become difficult to open and close.
The Alternative
As mentioned above, new construction using engineered lumber for floor framing offers improvements in floor flatness, strength and longevity. But what can be done with a floor which joists and mid-span support have weakened? Fortunately, there's a good one-word answer: Plenty. Foundation repair specialists are familiar with all the above-mentioned problems. What's more, they have access to well-engineered products designed to repair sagging, bouncy floors.
Foundation repair experts can replace undersized, deteriorated wood posts with certain steel posts that can (in many cases) actually jack a sagging floor back to its proper flat, level position. Individual joists that have been weakened by knots or cracks can be strengthened with new "sistered" joists. It's even possible for repair technicians to install an entirely new post-and-girder assembly using steel posts, and a beefed-up girder. This retrofit provides a secure and permanent solution when many of the original floor joists are sagging because they are undersized for their span and load-bearing requirements.