The second floor system will consist of floor joists, deadwood, bracing and necessary blocking. A floor joist can be made up of regular lumber such as 2 x 12 or a floor truss system.
One consideration before starting a second floor system is to take into account the plumbing and the HVAC ducts. When using regular lumber for the second floor system, it is very important not to cut out to much of the lumber and weaken the floor. Floor joist, such as a web truss will allow a lot of the plumbing and HVAC duct work to pass through without jeopardizing the strength of the truss. Another thing to remember is never to cut a floor truss without consulting with the designing engineer. With a little planning the floor joists can be laid out in a way to allow for the plumbing and HVAC duct.
The floor joist can be laid out either 16 inch or 24 inch centers. Normally with regular lumber, the floor joist will be spaced on 16 inch centers. With a truss floor system the spacing can be 24 inches depending on the sub-floor strength and design. When laying out for joist, it might be necessary to install extra joist where the second-floor walls run parallel to the floor joist. Also around the perimeter of the floor system normally there's a rim joist installed to provide the nailing for the subfloor around the exterior of the floor system.
Once the floor system is installed it will be necessary to pull a string around the exterior of the second floor in order to make sure the rim joist is straight. There also will be need for blocking, deadwood and joist bracing.
Once all the joists are in straightened and nailed off, you will be ready to install the sub-floor. Subfloor is three quarter inch tongue and groove plywood. The plywood or sheathing should be glued and nailed in place. It is best to start at one exterior wall and run the plywood perpendicular to the floor joists. A normal nailing pattern would be 6 inches around the perimeter and 12 inches inside. A good floor adhesive should be used and not allowed to dry before the subfloor is nailed completely off. Once the subfloor is complete, you're ready to pop out the walls and openings as on the first floor.