Expert: Dan Griffin - 7/23/2008

Question
I have an open loft space that is approximately 25' x 40' with about 15'
ceilings. I would like to divide the space up so that within that space I can
construct (2) rooms on the ground approximately 10'x10' with a third room
on top of that (also about 10'x10'). I do not have a problem with the floor to
ceiling height being less than the standard 8' (actually can be as short as 6.5
feet). The remaining space on the "second" level can be a open space. I 'd like
to know what the "must-do-s" are in terms of structural soundness. each of
the 3 rooms will be small bedrooms, so figure the typical bed, dresser, chair,
person, basic possessions as what the floor would be needing to support.
.Also, any recommendations you can offer with respect to books or guides
that would be helpful as I plan and execute.  Thank you.

Answer
Isobel, the biggest issue is sizing the new floor joists.  Code requires  a minimum of 30 PSF live load and 10 PSF dead load for bedroom flooring with a deflection of L/360.  YOur local code may require 40 PSF.  This site works well for sizing these joists based on these variables:
http://www.awc.org/calculators/span/reversecalc/reversecalc.asp

You will be losing almost a foot of your 15 feet, so a bit over 14' that could be divided equally.  How are you planning to get to the second floor? - a staircase requires a fairly huge amountof floor space above and below.  I hope you have fun on this project.