I can't make much sense out of the diagram or the photos because you don't say which way the trusses span, how they are spaced, and the diagram is apparently not to scale (24 ft wall is shown longer than 28 ft wall?) If the trusses are designed to span the 28 ft dimension of the house, then walls near the mid-span would have no load to carry and could simply be removed. Bearing walls under trusses are rare and would occur at about the 1/4 points where the V-shaped intermediate chords meet the bottom chords. The force at the bottom mid-span of this kind of truss is horizontal tension only. If the headers/walls in question were actually carrying the load of the roof, double 2x10's would be needed (or larger in a cold climate). So, it appears to me (although I don't really understand enough about this structure to have a clear opinion) that someone might have tried to remedy the bottom chord sag in overloaded or poorly designed and/or fabricated trusses by reinforced them with plywood gusset plates at the connections and adding walls/headers at the mid-span. However, it does seem odd that vertical or angled chords were not added to the truss to transfer load to the walls/headers. Perhaps there are transfer beams along the bottom truss chords extending out 7 ft or so in each direction from the walls/headers. Of course, they would have to be very large inefficient members. Perhaps they span the entire room. Then again, maybe someone just added some walls and doorways to separate the spaces and used 2x6 headers in case the trusses sagged a bit. Are there sufficient supports below the wall and the jambs of the opening that take loads to the ground? If not, these walls are definitely not structural. A center beam might have been designed to only carry the floor load. This would be obvious to an engineer. You should strip all finishes and then have an engineer tell you how to repair the trusses so they can span the entire space without any walls or headers (if you can find one willing to take responsibility for patching a poorly designed truss). If the exterior walls are leaning out at the top plates, then the connections at those points may have failed, spread, and forced the bottom chords downward. Sistering some chords and proper joint reinforcement should make the existing trusses work properly but if the exterior walls are not plumb, you might have to pull them back in place first. Truss design may look easy but it is a job for a structural engineer. Don't rely on advice about complex structural conditions from a contractor or anyone on the internet. Get an engineer on site. Your son might be wiser than you think.

I agree with #3. You would still need a ridge beam but that would allow a steeper roof with enough rafter depth for proper insulation. The main issue is the possibility if bad weather while the roof is open. I woud build a new roof structure over the old one and then tear the old one out. This post was edited by Renovator8 on Sat, Aug 3, 13 at 10:53

Not quite. It wouldn't void the entire house warranty; just the one year warranty for the structure and the much longer Weyerhaeuser warranty for delamination or manufacturing defects. Of all the possible dangers of home ownership, I find the failure of joists and beams to be far less likely than damage from fire, electrical wiring and falls for which most people take only minimal precautions. The trick is to reduce the added load on the structure, especially on the I-Joists. I would put the rack against the center of the back wall of the garage resting on a horizontal cleat lag bolted to the back wall. I would hang the middle of the front of the rack from the sides of the main girder, then add some hangers at the front corners from the I-Joists. The connection points at the I-Joists should have full-height vertical web blocking and the hanger should be attached to the blocking and the top chord but not the bottom chord. A competent contractor should be able to install the rack so the additional loads are so small they would not increase the risk of structural failure. If you can't find a competent contractor, hire an engineer and build it yourself. Don't conflate warranties with good design. One protects from a failure, the other reimburses you after a failure.

I don't believe it is specifically called out in code, but it is a structural design principle. You are correct, having a gable end truss next to scissor trusses, where you need to tie the bottom chord of the scissor trusses to the end wall for lateral stability, and that point does not match the wall top plate between the gable truss and wall below, so a hinge point exists. It can be alleviated by balloon framing and eliminating the gable truss. Not sure of your specific design with the common wall and studio truss, but be aware of this principle.