I'm converting the attic of my single story garage w/ a "A frame" roof to a 2 story building. My ceiling is currently 2x8 lumber spanning 15' 6". I'd like to leave the ceiling(sheetrock/insulation/lights) in place and just double up the 2x8 to become the floor of the second floor. I know the max span for a 2X8 is less than 15', would doubling them be ok? I'm basically going to add a knee wall(6')and change the pitch of the roof(from 7/8 to 4/12). I have to keep the overall height of the building under 20', it's right at 14' now.