Floor Framing:

Floors are typically built with a 16" on-center layout. When marking for the joists it's important to know where things like toilet/tub drains and HVAC supply and trunk lines are going to be located.

If a floor joist is landing in line of where a plumbing drain or drainline needs to be; then that floor-joist would need to be shifted over one way or the other to make room, thus avoiding having  to cut and header it off later. 

Start your DIY project off the RIGHT WAY with a ROCK-SOLID FRAME JOB!