Expert: Bruce E. Johnson - 4/15/2009

Question
Hello.
I have a 2 story, 10 yr old home with unfinished attic space.  The Roof Truss design is made up entirely of 2x4's, 24" centers with 'W-style' bracing, where what would be the bottom of the 'w' is approx. 8ft apart. There is also about 6" of blown in insulation (question #1).   There is nice 18ft long section in that 8 ft span that i would like to utilize as storage space, storage consisting of clothing bins mainly, anywhere from 10-15lb containers.  underneath that section of attic is one long wall (perpendicular to chords) that is about 3 ft off center of the 8ft wide space.


1.) Do I/Would you recommend raising the floor by sistering that 8ft span with either 2x6's or 2x8's to allow room for the blown in insulation?
   -If so, do I add the board on one or both sides of bottom chord (spanning past the bottom of the 'W's or further)?  OR
2.) Could I build a 2x4 'frame' perpendicular to the existing joists and lay decking on top of that? OR
3.) Other......
4.) What dimension/type decking material would you recommend?

Thank you for your time,
Mike

Answer
Hi Michael, standard roof trusses have been engineered for a certain amount of live load, this includes the weight of the drywall hanging on the ceiling. This live load can vary from 20-40 pounds a square foot. So structurally you are probably ok for storage.  But as you suggest you need to elevate your attic floor because of the insulation.  A 2x6 or 2x8 sistered to one side is fine.  For simple storage 3/8" plywood is all that you need.  I have that in my attic and it is plenty thick enough..I hope this information helps feel free to write again regarding this or other matters, sincerely, bruce e johnson..bejohnsonconsulting.com