For many years, homes have been built with dimensional lumber, meaning the lumber was milled from trees. This method has proven successful in the past, but as time goes on and trees are becoming scarcer, it has been realized that we cannot keep up with demand. As a result, the industry has come up with some alternatives to the floor joists themselves.
Today, we still sometimes use dimensional lumber for additions and small areas, most commonly the 2×8 or 2×10. This option, while still good, has a tendency to warp and twist, dry out more than the one beside it, and may cause a hump.
The new options are the “I-joist” and the “open web trusses”. The I-joist is a top and bottom cord that is of dimensional lumber, but quite small, with an OSB (oriented strand board) between the two cords. These can span longer distances without the support below, and the moisture content is extremely low, so they maintain a squeak and stable floor system.
Lastly, the open web truss design is the best option, because you can now design them with the ductwork in mind meaning the open webs can accommodate the plenums and all sorts of mechanical work. This, in turn, allows for better headroom in the open areas. These also can span longer distances.
It doesn’t hurt to educate yourself on the different types of floor joists, and to be aware of what your contractor is using. If you have any questions, do not hesitate to ask!