So here’s the background story: The house was built in 1975, and the garage has its own trusses not common to the living space. The drywall ceiling started to come loose, with one sheet hitting the floor. But missed the cats. The drywall was originally nailed in place directly to the trusses, and 45 years of uninsulated expansion and contraction has taken its toll.

At this time 2/3 of the drywall ceiling has been removed, furring strips are being installed perpendicular to the trusses with coated wood screws, and wainscoting is being installed and completed with the first 8’ x 20’ section.

The sagging problem was discovered when installing the second run of furring strips. Trusses are 2”x4” pine with old style round hole truss plates at the joints. The trusses run in the 20’ measurement, and at 14’ there is a splice that is sagging. Above the splice is a 1’ long board across the splice.

Correction: the trusses run in the 24’ measurement.

Researching on how to repair I have come across multiple methods. One is to bolt 1/2” x 4” angle, four foot long on either side of the splice attached with bolts. Second method is 2”x4” board four foot long attached on either side with either screws or bolts every 6 inches, and banded in four places along the brace.

There are eight trusses that need strengthened, which will require leapfrogging with two jacks to bring the splices back in alignment. Two others were installed 180 out from the rest, and those splices are already covered by new ceiling.

My concern is the horizontal drilling of the existing trusses as nowhere have I found what diameter screws or bolts to use. I do not want to weaken the existing truss by drilling/removing wood.

So has anyone here had the experience of mitigating sagging trusses?

is it an actual truss with vertical/ angled webs as well?

the roofline itself isnt sagging just the ceiling?

Correct. All the triangle bracing, but no visible sagging from outside. But enough that cracked the ceiling drywall seams, and using a straight edge along the bottom there is a 1/2” gap two feet from the splice. Nothing currently stored in the attic, just ceiling and roof weight.

Want to get ahead of it before it gets worse, and right now the new wainscoting won’t fit up right.

lags would be fine, but spax/ GRKs etc will be easier to install, lag bolts aren't used that much anymore for the simple convenience of modern structural fasteners.
I would just sister a 2x6 or something full length not a short 4' section. Angle iron will be more expensive.

20 foot is a long span for a 2x4... Hence the sag.. I had the same issue in my kitchen.. I ran some 2x6 perpendicular to the joists, and jacked up the ceiling and took the belly out... installed a bunch of 2x6s in the attic that ran with the joists and fastened the old 2x4s to that to support the load once the jacks were removed.. If you plan on leaving that splice in place, use plywood instead of regular lumber.. Ply wood will have alternating layers and will resist bending or sagging... Span the joint by 4 feet in either side of the splice with a few layers of 3/4 ply and call it good... You could, make an entire plywood layered beam and sister that to your 2x4 that has your splice in if you have the room to work... Is the space above finished? could you drop a full length 2x4 ply beam in place from above?

I have repaired many trusses and the general rule is the 2x4 that you sister to the bottom cord needs to be 6' long minimum and it can not be the cheap SPF lumber. It needs to be Doug Fir or equivalent. Usually, we would buy 12' 2x4 and cut them in half because most lumber stores don't carry the 8' studs in Doug Fir, they are usually SPF.

Structural screws are available at Home Depot and Lowes if there is one near you. Otherwise, look online and then see if your local store has similar. Structural screws are less brittle than deck screws or long drywall screws, they are made of a steel similar to nails. You can use nails instead of screws, the nails need to be 3" spacing in pairs. Screws allow you to extend the spacing to 4". The fasteners need to go the full length of the board you are attaching.

Lag bolts are not approved for structural repairs or truss repairs.
Most Spax and GRK's are not approved either. Read the box label and if it doesn't specifically say "Structural" on the box, then they are not approved fasteners.

Isnt it code to have drywall on the ceiling in an attached garage or at least a smart thing to do anyways.

The code is either 1/2” drywall on the ceiling, or equivalent barrier ( being installed) between the garage and main living area. To act as a fire stop.

OP, your problem sounds very similar to one my dad had.  He opted for overkill and screwed/bolted a 2x4 metal "C" channel on one side of each truss after straightening them with a jack.
He did this many years ago and has had no more issues.

I have a 24' wide mechanics garage. 16' opening with 4' on each side. Great for bikes and such.

The middle was showing a bit of sag and crack at drywall seam. I used a 4x4 and 2x4's to level it.
Then, got up in the attic and added some bracing and cables at three locations. Details below.

Total DIY, didn't research... just went for it.

Attached File

I'd do as the others said and sister a doug fir 2x4 (or plywood...not OSB...boxing).  This method seems like it'll fail catastrophically over the long term as it's adding more stress to the truss...or am I wrong?

Hopefully, I'm wrong...

A machine thread screw eye long enough to pass completely
through the truss element is much better than a screw thread eye bolt.
Tapered screw threads have a very reduced load in withdrawal.

Well fuck... Now I'm going to drill through the three horizontal truss members, add a fucking 3 x 6 plate and bolt up... I love a project
Of course I'll paint the plates and nut to match the ceiling.

Maybe shear the truss like a gable. Or just the center 4’ .