Looks like a plenum

Thanks A Mat. Do you know how I can locate plaster ceiling joists?

You could use a stud finder.

"Why would they use a soft fit for AC vents?"

I have a soffit like that. It runs the entire length of the house and contains a large-diameter duct from which there are downward-facing branches for first-floor registers (like those pictured) and upward-facing branches for the floor above.

@wdccruise I tried using stud finder but having no luck. It detects studs but not joists.

Drill very tiny holes to search for framing.

Thanks Joseph Corlett. Where can I start? When I put a magnet near light fixture, it stuck but no where else, I tried all over the place.

This is close up picture of hole i was trying to drill. It looks like wood - did i hit joist or wood/lathe? I am getting resistance and can't drill further with current bit. Blue color is painters tape.

IMO:

Assuming that lathe consists of thin wood strips, you're going to have to buy a longer bit and drill into the wood. If you don't have to drill deep before the bit goes all the way through, you hit lathe. If you keep drilling for, say, 1" (wrap a piece of tape around the bit at 1") you hit a stud.

IMO.

@wdccruise my issue is resistance now. is there any harm if i put pressure on the bit? i just dont want to cause any unwanted damage; perhaps i am overthinking.

Have you considered opening an access hole.

You picture doesnt help.

You are on site and you do not know.

What are you trying to accomplish?

"my issue is resistance now"

You have to drill into the wood to determine its thickness. The drill bit can't just touch it.

A Mat: "What are you trying to accomplish?"

Good question.

Sorry, I am trying to install a battery operated smoke alarm. Simple task but turning out to be a nightmare.

Have you considered double stick tape?

I do not know of a home smoke detector that requires to be fastened to a joist.

A Mat, I looked on Amazon but couldn't find anything that works with plaster ceiling.

Maybe you should have asked for suggestions re installing battery smoke detector.

Using my Forrest Gump voice: Double backed tape.

A Mat will double sided tape work with plaster ceilings? Any good ones?

Try command strips with Velcro?

• Alligator Masonry Anchors (spec sheet)
• First Alert SA3210 10 Year Lithium Photoelectric and Ionization Smoke Alarm (CR recommended). Note that it ships with anchors but they may only be suitable for drywall ceilings(?)

Thank you @wdccruise. I put a double-sided tape this morning and attached mounting plate to it and waiting 24hrs to let it set. I will attach smoke alarm tomorrow and keep you all posted. Hoping that it works!

"waiting 24hrs to let it set"

Tape doesn't "set" but in the battle between tape and gravity, I predict gravity will win.

:-(

Wish me good luck! Double sided tape it is! Let's defy gravity!

I am wondering why you just didnt do it with a couple of screws and some screw anchors above the doorway on the wall, instead of putting it out on the ceiling? It would have worked just fine, and isnt really a problem installing that way and it looks a lot nicer than having that in the middle of the ceiling, and yes, I suspect, the double sided tape, will not hold for long, and it will come down, and hit someone, or land on floor broken. The screws and the anchors, come in the package right with the smoke alarm, so you have them there available.

@cat_ky I was informed by the occupancy inspector to install it on the ceiling and not on the wall. I was having trouble drilling into the ceiling.

"Mount smoke alarms high on walls or ceilings (remember, smoke rises). Wall-mounted alarms should be installed not more than 12 inches away from the ceiling (to the top of the alarm).

"If you have ceilings that are pitched, install the alarm within 3 feet of the peak but not within the apex of the peak (four inches down from the peak)." -- Installing and maintaining smoke alarms

The linked guide also states where to put them around the house.

In CA, wall mounted less than 4.5” from the ceiling.

Where does the OP live.

who is the occupancy inspector: government, insurance, or ?

thank you both. I live in MO and the occupancy inspector is from the city of KC.

Your location allows wall mounted smoke detectors.

Maybe an inspector recommendation vs a requirement?

Please google it.

A screw into lath is more than plenty to hold up a smoke detector. If you're hanging a ceiling fan or big cut glass chandelier you need to worry about finding the joists. Stud finders won't work because of the distance from the plaster surface to the joist. Either drill multiple small, easily patched holes until you find one, use an existing opening (eg for an electrical box or HVAc) to look. or cut a small hole to look. A cheap boroscope can help.

If you used good DS tape and the plaster was smooth that's probably going to be fine. If it falls off screw it to the lath.