attic renovation solution

huangyhg

attic renovation solution
i going to finish off an unfinished attic in a single story, wood framed california home.  i need to come up with a couple solutions before i get too deep into the planning of this project.  here are the problems and contributing factors.  i want to disturb the first floor as little as possible.
1) current attic floor is rough 2x4 df spanning anywhere from 12' to 20' (it sags a little!)
2) rafters are rough 2x4, limiting the space at the top plate/rafter connection (e.g. can't fit a wood i beam in there)
my thought is to leave the existing attic floor joists intact and build a new 2x10 floor.  because of the layout, i need to only add two beams to carry the new floor in two different parts of the attic (the other ends of the joists will rest on a load bearing interior walls).  i would rather not cut the existing floor joists as they resist the outward thrust of the roof, and if i chop them, i'll have to put in straps, or some other allthread/bracket set-up.
has anyone designed such a system?  i envision a glulam with topflange hangers.  the glulam would sit on blocks to raise the bottom of the glulam to the top of the existing floor joists(glulam is perpendicular to the existing floor joists).  the top flange hangers would allow the new floor joists to hang down to so the bottoms of the old and new joists are in the same plane.  
typically, what type of attachment does one use to secure the glulam to the blocks (which will rest on a top plate, supported by a post underneath down to a new footing).
the other option is to cut the old joists, drop in a glulam, and hang new joists off it, with straps across the old joists to resist roof thrust.  this is a less preferred option in terms of labor and complexity...
(btw- i am having this professionally engineered, i just want some ideas before i go there)
thanks

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well...
first of all, there is no such thing as an "attic floor". in keeping with the ubc, these two terms just don't belong together. is this an attic with storage load, or is this an actual floor?
secondly, you have to decide if you are actually designing a floor space in this existing attic space. the question may be simply that of accessiblity - i. e., is it accessible?
third, think about how you intend to insulate this space. my experience tells me that the usual type of insulation requires at least the space of a 2 x 10 joist/ rafter.
fourth, does this space require natural lighting, ventilation, heating or cooling? are you considering new windows, dormers, skylights? if so, you are talking about a considerable additional cost due to design, demo and construction.
fifth, how do you intend to pick up the sagging ceiling joists? added floor joists placed inbetween the existing members will not be connected to the sagging elements - that is, unless you indicate how this is to be done.
sixth, once you have answered the above questions, let the engineer work out the structural details. that is what he/ she is paid to do.