I am building a single story 1600 sq ft house in the Pacific NW.  6" walls with a truss roof.  The trusses have allowed  for a future "bonus room."  The architect established a 40psi live load and 20psi dead load for the future bonus room.
   Per the SIPs builder's recommendation I hired their recommended structural engineer to review the wall system plans and the point loading of the trusses.    
    At this point, the SIPS supplier has informed me that the second floor loading is not within their scope of work.  And their recommended engineer has made no comment about the second floor loading levels.
   Obviously the SIPs wall system is of no use unless we can be assured that it will support the second floor loading levels.
   How is this issue to be dealt with?  If it were a 3 story house, surely the floor loading levels must be established in order to keep the residents from falling through the floor.
   Your comments please.  My thanks in advance.

lkg907;

It sounds like the SIPs engineer does not want to take responsibility for the floor design itself which is understandable. Certainly the SIP walls are stronger than conventional framing and could be certified to support the residential floor load.
Your truss supplier would normally have their engineer supply signed and seal documents to support their product, separate from the SIPs supplier.

The SIPs engineer does not need to take responsibility for the floor loading or design. All they have to do is confirm that their walls will support the trusses. Seems fairly basic to me. How can the SIPs supplier expect to sell any panels without confirming that the panels will support the roof? This is getting a bit frustrating at this point. I would like to place the order and get started before the snow flies.

I'm not an engineer, nor a code inspector or similar, so take this for what it's worth.

If the SE has plans that describe the truss loading, including the 40PSF live and 20PSF dead and which correctly describe the rest of the structure, including the SIPS and is willing to seal (stamp) those drawing, then you are in the clear. I can understand that the SIP supplier will not do this, but the SE should be willing to do so.

If the SE is not willing to do this, I would say change engineers, but we've spent the last year trying to get a building permit, most of the time spend trying to find an engineer interested in the project. SEs seem to be in short supply, and seem to have recognized the residential structures are more prone to law suits, so tend to avoid them - unless you're a regular customer. This is a gross generalization based on my experience and not universal - the SE we finally used was very happy to help with a residential structure, including the SIPs and the roof trusses.

We don't have "room-in-attic" trusses, but we do have "storage-in-attic" trusses (10PSF dead, 20PSF live) which did not seem to present any particular problem.