Reinforcing Truss-based Garage Ceiling

Question

I'm currently in the process of insulating/drywalling my attached garage. It is about 25' x 20' with Fink style trusses. The truss bottom chords are spliced with plywood sheets on either side. Rather than metal plates to attach web members to chords, there is plywood sandwiched on either side, since this house was built in the early 70s, I assume this is time of install appropriate?

The trusses run perpendicular to the garage door and there is a center beam installed over the bottom chords of each truss spanning the entire garage from wall to wall running in parallel to the garage door. My question comes in here... this center "beam" is just two 12' 2x4's overlapped, and I'm noticing the center of the trusses sagging.

To my understanding, a truss should not sag like this (obviously). My only thought is the splice is weakening in the trusses or excessive weight was in the attic space at some point. I'm assuming this makeshift beam was added after by a previous homeowner to try to put more storage in this attic space...

Either way, I would like to essentially replace the 2x4 "beam" solution with 2-ply 1-3/4" x 7-1/4" LVL 22' beams and then tie in the bottom chords of the trusses with some sort of strongtie with multiple beefy nails rather than just the toenail approach they used originally.

I've attached a picture of the construction and a close-up image of where the bottom chords are separating from this beam. Additionally, its shown that this beam is not resting on either wall at all and has no support on either end essentially making this whole thing useless.

So in summary, my thought is beef up the top plates on either wall where the beam will sit atop to make up the gap shown in image (since bottom chords sit on top of the top plate). Then sandwich together 2 of these 1-3/4" x 7-1/4" 22' LVL beams together to stretch the span and take the sag out of the bottom chords so I can insulate and drywall. I'd probably avoid putting much, if any storage up there in the future since it clearly wasn't designed with that in mind. I know the depth/width of the beam for this span would normally be much bigger if it weren't for the trusses, but since there are trusses taking most of the load, I feel like this beam doesn't have to be all that large. My goal here is to ensure this isn't going to worsen when I install insulation and drywall and possibly be able to store light weight holiday decor in the attic space since I don't have a basement (crawlspace).

Answer 1

In a typical roof truss design as you are showing, the bottom chord is under TENSION not compression. So any sagging is coming from some other factor than the roof load.

The 2x4 "beam" that you have is NOT carrying the roof load or any other load for that matter. It's not clear why that was added to the structure other than to add some rigidity along the truss bottom chords.

As far as the sagging itself, it's hard to tell from the photos but it could be that something is applying a compression load to the walls and causing the bottom chord(s) to bow. It's not clear that only the one of these is bowed or others as well. Perhaps the wood has just warped over time.

Before you go making significant structural changes in the blind, you will do well to consult a structural engineer to inspect and analyze what's going on here and make specific recommendations to accomplish your goals for the structure. You're likely going to need that anyway before your plans will be approved and issued a building permit.