brickeyee Your point about RH of winter air seems well taken. What was missing from the Carson Dunlop explanation, I think, is that attic air in a home is often more humid than air outside the attic due to leakage from the conditioned to unconditioned space. Here's how another P.E. describes truss uplift: "A problem that can occur with this system causes cracks in wallboard over time. In recent years, attic insulation usage has increased, which ends up covering the bottom chord of the truss. This tends to keep the bottom chords warm and dry while the top chords absorb moisture and expand. The top cord expansion causes the top chords to lift, pulling up the bottom chord as shown in Figure 1, hence the term truss uplift. " Charles C. Roberts, Jr., Ph. D., P.E. My untrained guess would be that the lengthwise shrinkage of grain does not relate to the length of the wood but to which portion of the tree the wood was cut from. Here's a longer explanation: "Truss uplift is caused by wood's natural response to moisture exposure. This response occurs when humidity changes differentially in two areas (an attic floor and attic ceiling, for example). A truss's top chords expand as their moisture content increases. Problems resulting from truss uplift are most common with flat-bottomed trusses in homes during winter when a truss's bottom chords, often covered by insulation, are kept warm and dry while its top chords are exposed to moisture from condensation. During winter months, warm, humid air emanates from buildings' interior use and occupants. The uplift movement occurs because, as the truss's upper components resize themselves under the differential moisture conditions, internal tensile stress occurs in the truss's web members, which then pulls up the bottom chords (and the attached ceiling drywall), resulting in a separation along the wall-to-ceiling juncture of the drywall. To put this into perspective, consider a truss with top chords made of two members, each measuring 30 feet in length. The total movement anticipated from winter swelling caused by the truss's moisture content increasing is a 0.72-inch expansion per side, or a 1-inch total rise of ridge above the bottom chords. And once a crack forms inside a home, additional moisture generated by occupants and their activities can escape into the attic, travel to the cold attic ceiling surface, condense on the truss's top chords and cause further truss uplift. " Lucas J. Hamilton manager of building science applications, CertainTeed Corp.'s Insulation Group For interest, I will forward this thread with a request for comments to Carson Dunlop. lzerarc As I said, different places, different practices. I've changed only reluctantly. Seeing those spindly stuck-together 2x2s gives me a visceral reaction and I have to rationalize using roof trusses. However, my engineer says they're much better done now than ever and with wall lengths less than 50 feet uplift is rare. None of this applies to warm climate homes.

Thanks for the response. Specifically, the build is going to be in the Coconut Grove/Coral Gables area of Miami where townhouses and duplexes have become very prevalent over the past 10 years or so. Due to the lot size (typically 50x100) or in our case, slightly larger at 65x113, many developers are buying up the lots, knocking down the 50's/60's era bungalows and building a 2-unit townhouse/duplex on the property. These are selling for anywhere from $750k-$975k depending on the area. We have noticed that a lot of the new builds are either concrete 2nd floor or wood trusses with a layer of concrete over the plywood (at least that's what I think it is). I'm sure it's more prevalent here than in other areas of the country for a variety of reasons, with one being the overall sturdiness of the structure built to withstand hurricanes. A lot of the building codes changed after Hurricane Andrew in 1992 including the elimination of "stick" frame houses and the implementation of cinder-block construction and hurricane-strapped roof trusses. Being in South Florida, it's all slab construction on the first floor considering how high the water table is and the fact that you can't dig much more than 12 inches into the ground before hitting water. I'm going to check on Litedeck. One of the architects we spoke to mentioned it I think and having you mention it by it's name jogged my memory.

Do you have a builder yet? I would call a truss company, preferably the one your builder uses, and ask their engineer what your options could be with trusses. Then ask your builder what your options are with stick framing. I've done both before and had a recent thread on here about trusses and my concern about using them on our current new build. My concern was unwarranted, though. The truss company that my builder uses met us onsite after his initial configurations and we were able to tweak our truss design enough to get plenty of height in our living room and master bedroom. Our living room is 20 ft wide but the house is 32 ft wide so our truss is a combination truss. Had we not been able to get the height we needed, we would've insisted on stick framing, which we did on our first new build 15 years ago. Since it seems you are still in the drafting stage, there are multiple things you could do to get more height if you really need it. One can increase the pitch of your roof, raise the base ceiling height, etc.

@Jon Alan thanks for taking a look! Sorry for the small pics, that's how they came in the report. I zoomed in on some pics here below: Basically a few clues: The inspection report specifically say it is a 2x4 truss system. The gable roof spans over much more than 8' in both directions so it's unlikely (to my knowledge) that 2 x 4 rafters are enough. There are plywood gussets at attachment points instead of overlapping pieces that I thought rafters usually have? There isn't a ridge board but a gusset instead. But ya I think you're right, having someone qualified to inspect again would probably be extra safe!