You can add sister joists, you would have to knotch out where the wires run through. Make sure you install new joists in the sill plate above the concrete wall.

Your pictures are too small to see, but consider using steel.

3/4tr plywood is stronger,easier to work with and cost's about the same as 2x10 lumber. The first picture look's like wire run's near center of ceiling. If that's true,I would center plywood on hole,use hole saw to cut smallest hole that accominsates wire,notch plywood for wire,put yellow glue between plywood and existing joists before nailing. Try and find 10' long sheets and I don't believe it will be necessary to sister ends with a 10' strip at center. If using 8' strips,center those on holes then bounce kitchen floor to decide if ends must be reinforced. I doubt wire holes near wall in second picture is having much effect on support. Someone mentioned steel and I agree that 20ga steel sandwiched between add's tremendous strength if you want to take it a step farther. Another extra step involve's checking for "sag" in 2x10s before starting the repair. Simply stretch a string from wall to wall beneath joists to see if they are sagging. If desired,a jack is set beneath each joist while it's being reinforced.

If I was Anna looking for advice reading these 2 comments I would be more lost then when i started....

Adding a metal beams would only make sense of you run it in the center perpendicular acting as a load bearing beam under the joists. Have fun notching out steal for electrical wires running thoses through the existing joists, or parking your car around support columns.

Adding plywood would only work if you lay it over the existing subflooring. Plywood vertically does nothing.
Plywood and a 2x10 are completely different animals. Plywood deflects movement..... Joists supports weight. I can post plywood defection ratings if we know the size of existing subfloor.

That might need a cross beam. Probably needs a cross beam. Rooms over garages are very lightly built, usually for storage only. They often do not meet codes for habitable spaces without beefing up the structural.

Get in a good framer and see what he suggests. You may need a structural engineer’s sign off on some steel with multiple cross pieces in order to not have a center post for a structural beam. Either way, it’s not really a DIY job.

The clueless designers would just draw a retractable post mounted to the garage door opener. Framing "dilemmas" always provide the best entertainment here. They weren't taught prescriptive code in their kitchen design classes, so must defer to an engineer for simple framing solutions..

Your island may have exceeded the recommended loading for that garage ceiling construction. Did it have engineering approval?

Regardless, a framer should be able to construct wooden trusses in place to augment the joists.

Mr. Memes, I am more confused than when I started lol. I have talked to 4 other people, read comments online and posted to this site. One of the guys held a board to the ceiling to see if it was bowing, but it was hard to tell from the old drywall nails on there. I will try the string or a level. They wanted to do two 2x6’s under the island and two 2x4’s. I assume they would notch out the 2x6’s and yes, they were going to run them over the sills. I believe the other 2x4’s were for the other side of the garage (under the dining room) to keep a level surface for the drywall to be installed. They said the boards may have to hang down an inch depending on the wires. The vibration could also be due to a poor subfloor...i had hardwood installed.

Randy427, I had a structural engineer do my kitchen since it involved taking down a load bearing wall. The garage ceiling was exposed at that time so he saw it.

The best solution is to have an electrician pull the wires then reconnect them. It looks like the service is on the garage wall. It might take them all of 2 hours to complete the task. Then you can double up the joists and stand firm.

Without knowing the spans (it looks like a 1 car garage, so about 12'?), size of existing notch, etc., all anyone can do is guess...

Anne,
I truly think it's just a subfloor issue. You probably only have minimum for subfloor. 5/8" is minimal for hardwood floors. The picture is bare bones but if you added plywood you practiclly can put a car in your kitchen and it would hold.

Why make your life so difficult? Just pull the wires out, sister the joists (if that is indeed the right answer), and put the wires back. It will cost you perhaps 30 minutes and the cost of a couple junction boxes and wire nuts if you need to cut and splice the wires. (remember that j-boxes need to be accessible, no problem with open joists, but plan ahead for an enclosed ceiling)

Chances are your island requires an electrical outlet, so you'll be doing wiring anyway whether you can DIY or your jurisdiction requires an electrician. it may be possible to re-route the wires so it can be done in one visit if an electrician is required.

Those look like they might be 24" oc. I bet they aren't Select Structural fir either. Probably SPF #2's.

That is going to need spray foam insulating and X drywall after any reinforcement happens. Which means that any splices will need boxes with cover plates accessible from below. Not a big deal.

But reinforcing will also likely lose you head room below to rebuild it properly. That looks very lightly built, and not suitable for a living space above. Hope there was no tile installed.

If if you're getting a bunch of conflicting recommendations in person, time to get in someone whose specialty is the support of buildings. Hire a structural engineer. Best $500 you can spend.

I’m actually freaking out lol. The kitchen is complete so can’t do anything about the plywood. I literally just had the ceiling reinsulated. Spray foam was around the perimeter, pink insulation in between the joists.

Anne,
Don't freak out! your fine go with original plan add your sister joists, make sure they are installed on the sill plates. Knotch out at less as possible for electrical.

I see bridging but there's not enough of it. I would add full height solid blocking at the 1/3 points. Then think about adding 2x4's along the bottom of the joists where the floor feels bouncy.

Were you a plumber in a prior life Mr. Memes? :-) Notching the tops or bottoms of joists pretty well defeats the purpose. That's why they run the wires and pipes through the middle, where they don't disrupt the parts of the board that are in tension or compression. Pulling the wire out, installing solid joists, and re-running the wire is a trivial solution. It would take less time to do it than talk about it, and probably less time than it would take to do violence to the joists with a sawzall.

No, no association with kitchen designers, Houzz, or other Forces of Evil. Just a guy whose idea of fun is rebuilding houses for fun, not profit.

(NB: Plumbers are (in)famous for hacking away structural elements to make it convenient to run their pipes. I've spent many happy hours fixing their work.)

Just because something is rigid enough to not fall down does not mean that the cabinets, counters, and tile above won't become damaged due to the excessive deflection over time.

Unless they kitchen is carpeted, the cabinets are plastic, and the counters are laminate, deflection in a kitchen has to be remedied. Just sistering and blocking when you've got 24" on center is not enough. Get that structural engineer back out there.

Unless you are in FL, fiberglass is not going to provide the needed R value above a garage. Unless the garage is insulated and heated?

If it was my house, I'd add a 2x4 on each side of each joist, below the wires. I'd use a lot of nails, and I wouldn't bother trying to get above the plates, where there is already enough shear strength. You'd be adding tensile strength to the bottom of the span, right where it needs to be to help with deflection. You'd have to remove and reinstall the joist bracing.

Without knowing the span, claiming anything is adequate or not is futile. R30 fiberglass batts would satisfy the insulation code for most of the country (the present insulation install doesn't look adequate).

The house that Houzz built- 1876% over budget. Luckily these pouzzers aren't out there in the real world misleading customers.....

2x10's @ 24" o.c.? Its a bouncer alright. Many threads on the GW have been about this subject but little agreement on solutions.

Mr. Memes, I didn't suggest a structural engineer, that would be overkill in this case. I was just trying to suggest a potentially faster, easier approach that might yield a better result.

Yes, I know that most plumbers aren't hacks. I was making a poor joke based on a common industry meme.

With over 50 years of experience building, rebuilding, fixing my own mistakes and those of others one thing I've learned is that shortcuts are frequently the long way around. In this case not getting the wires out of the way so that you can make sound repair seems like one of those long way around shortcuts. Stiffness is the goal here, notched joists do not provide the best approach to stiffening the floor and it's a trivial amount of work and less than $10 worth of parts. Hardly a budget buster.

Thank you everyone for your input. I have a "friend of a friend" architect coming tomorrow. I don't want to contact the structural engineer who did my kitchen because he was a very difficult personality. I measured my joists under the insulation... they are actually 2x9's (is there such a thing?), 16 inches on center. My pink insulation is R30. I will try the string trick with him tomorrow.

Anna,
Sounds good! Thanks for the update. Good luck

A 2x10 is 1 1/2" x 9 1/4" and 16" o.c. is typical. Did you ever measure the span (length of the joists)?