Expert: Dan Griffin - 1/27/2008

Question
I built a new house in 05, had to put it on about 17 12" blocks high cause of flood zone.  I need a new bedroom cause of a new baby we didn't expect.  My house now is 24*42.  I have a closet in my hallway that is above my stairs that go to the basement(great place to put stairs to my new upstairs).  I would like to tear half (well maybe 60%) off my roof to build a master bed/bath.  Could I still leave the bottom of the trusses there and cut the top of the truss off(to keep from tearing the drywall up on my ceiling).  Then just put 2*6 or 8 Floor joists for the upstairs right beside the old truss bottoms. If you need any other info, please let me know

Answer
Shawn,

If these are truly trusses, you need to do the sistering with the floor joists before you cut off the tops.  Trusses are usually installed on 24" centers, joists at 16"  It may be easiest to plan on going to 12" centers to carry the old bottom chords and to get a solid second floor.  You will also need to develop a bearing wall at mid span.  The trusses could clear span from outside wall to outside wall, but normal dimension lumber won't make the 24 foot span.  The slab, crawl, or basement was not designed to carry this load.  They may happen to do it, but you will need a structural engineer to determine if it does and what to do if it does not.  LVL and truss joists can make the span, but your new stairway will kill an awful lot of them, probably better to go with bearing wall(s) and extra joists.

Even with a well organized crew, you will be open to the weather for quite a while.  This will probably be your biggest problem.