Long story short, I'm replacing the vent fan in our bathroom after the new bath and shower were put in.

The new fan box is about 2" shorter in one dimension and 1" shorter in the other dimension, leaving an L-shaped gap around the fan.  I dry-fitted the plate and it definitely doesn't cover the opening, besides all of the cold air spilling down from the attic.



My current plan is to buy a 24"x24" drywall "patch" and cut it to shape/size (maybe slightly under-sized).  Then screw two pieces of lath/strapping/furring strips inside the attic but through the ceiling as a backer that I can screw the drywall patch to, and then mesh tape and mud the whole kit-n-kaboodle (this is how those "This Old House" guys showed it for square openings, at least).

My main concern is that the current ceiling is 3/8" drywall, wire mesh lath, then 3/8" plaster, for a total thickness ranging from 3/4"-7/8".  I don't know if I need to get 1/2" and 1/4" drywall both, to compensate, or if I should use furring strips to compensate, or what.  I definitely don't want to build up 1/8"-1/4" of joint compound just to make up the thickness.

Thoughts?

Or apply a wide trim with mitered corners.  I cannot tell how much you have to cover from the image.  But if it is about 3” or so, then make the trim as a frame, all glued and ready to put up.  Use construction adhesive. 

If you do a tidy job it will look intentional and not as a patch. 

Patching drywall is not difficult, but the repair extends over 3 or 4 days pretty much like taping drywall does.  My experience is that if you try to condense the process the results are less satisfactory.

I'd want some backer board.  You can probably CA glue it in place using an activator if you don't have room to screw it in.  Then I see a trick where they cut the circle, rectangle in this case, and leave the paper longer than the main drywall section which then gets mudded down and feathered out.  I'm not a pro but seems that's how I'd do it to avoid lines or it being susceptible to cracks.

Thank you all for the suggestions!

I ended up going with the 1x2 backer,



5/8 drywall (what a horrific mess to cut it in 1" and 2" narrow strips),



and regular compound in a tub.




If I had it to do again (or the next time I have to do it), I'll buy the 20-minute or 45-minute mix and mix it myself; I was too nervous and lazy to try mixing my own mud.

Speaking of lazy (or overkill), I wanted to practice using my new DWC 18-4500, so I busted that out for the 7 screws I had to drive.  I think I used more screws practicing than I did on the actual project.  That's also how I found out that my Recon DWC had the wrong bit holder in the Systainer for use with the Depth Stop Chuck, as it left all screws at least 1/4" proud, even at its deepest setting.  I've already submitted that info to the fine folks in Lebanon to rectify...

At the depth I had to fill and with the lower temps in that room, it took the better part of 48 hours to dry all the way through.

First sanding was last night with an ETS125 and CT-MIDI-I, and applied a second coat to level it all out.  The STL-450 came in really handy as a raking light (go figure).  Although, to be honest, the regular Sys-Lite KAL II does a pretty good job all on its own.


Thanks again, everyone!

No tape on the joints?

Were it me I would cut out a larger area of the existing drywall and use a large piece that you can precut to the size of the vent. I am not great at drywall so simplifying that patch works best for me.

Looks like the repair job is coming along well.  I've glued strips of plasterboard inside opening using PU glue and quick clamps.  Then glued more plasterboard to those strips, using PU glue.  A quick sand with the rotex will get everything nice and smooth.

If you repositioned the fan box so it was central in the opening, could you have used PU foam to fill the gap, then skim with plaster.  It sounds as though the fan cover was almost big enough to hide the repair.

Regards
Bob