This premium I-joist floor assembly uses deeper, stiffer and stronger I-joists and thicker floor sheathing. With this floor system, builders use 7/8 Performance Category OSB or plywood glued and nailed to 14-inch-deep I-joists that are spaced at 24 inches on center.
7/8 floor sheathing is typically rated for 32-inch on-center joist spacing, which means that it provides a very stiff floor system when I-joists are spaced at 19.2 or 24 inches on center. The solid, stiff floor imparts a quality feel that is popular with move-up and luxury home buyers.
The wide spacing of the joists makes it easier to install ductwork and plumbing within the interior of the house, making it easier to meet today’s energy codes. Keeping the ductwork and plumbing within the conditioned space of the house eliminates exterior chase ways that would require additional air barrier and air sealing requirements. This may also eliminate the need for duct testing, depending on the pathway chosen for energy code compliance, saving the builder additional expense for testing.
Even though the thicker sheathing and deeper I-joists cost a bit more, those costs are offset by the materials savings, so you get a premium floor system installed for about the same cost as more traditional construction. Depending on the performance expectations of the floor and the loads that must be accounted for in the design, the number of joists can be decreased by more than 1/3 when compared to the typical lumber floor system. This drives a lot of savings, including:
- 1/3 fewer joists to install by the framing crew
- 1/3 fewer fasteners and hangers used to install the joists
- 1/3 less floor construction adhesive
- 1/3 less floor sheathing fasteners
- 1/3 fewer holes to drill for running ductwork, electrical and plumbing.
Additional savings are typically realized as well. The deeper I-joists may permit the designer to use just one joist in places where double joists had previously been called for. The floor system also allows the home to get under roof faster, which means the interior is protected from the elements faster. And it allows quicker returns on builder’s capital by reducing time under construction.