On Sunday, 15 May 2016 21:15:05 UTC+1, Adam Funk wrote:
I've had to do some exploratory & other surgery in the kitchen ceiling
(also bathroom floor) as a result of a loose connection in an old
junction box (kitchen & bathroom lights went off sometimes but came
back on if you stepped on the right area on the bathroom floor).
That's all fine now (Wago box; new ceiling light; big mess cleaned
up), but I wonder about the weird situation I found in the
ceiling/floor.

= bathroom floor
- kitchen ceiling
X joist


================================
XX XX
XX XX XX XX
XX XX XX XX
XX XX
--------------------------------

That's a cross-section. There seem to be two sets of joists: one set
has a gap above & the (lath & plaster) ceiling attached below; the
other has a gap below & the floorboards attached above. I've only
worked in the middle, so I don't know how the ends of the joists are
attached to the walls.

Any explanation for this weird situation?

Thanks,
Adam

It is likely intended to reduce sound transmission between the rooms.