Past the scale of joists, the spacing concerning them should also be taken into account. Regular joist spacing for the set up of attic flooring materials is 16 inches on Centre (OC), indicating the gap from the middle of one joist to the center of another joist is 16 inches. https://kameronhptyb.iamthewiki.com/8158801/detailed_notes_on_garage_attic_lift