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I'm in an old home and learned extensively about lath-and-plaster. I'm dealing with a problem like in What methods for installing light fixture in middle of room, with lathe and plaster ceiling?.

However, I don't actually know if my ceiling has laths or not. It's a beautiful coved design from the 1920s. I don't want to start cutting holes in my ceiling if there's any risk to damaging the keys given the age of the structure.

isherwood
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AdamO
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  • It is very difficult to tell as the drywall/gypsum board was invented a few years prior to 1920. However, I think you can distinguish a drywalled ceiling from a plastered ceiling by knocking, the drywalled ceiling will return a hollow sound then. – r13 Nov 19 '21 at 19:10
  • Generally, yes. There were no other common techniques at that time. I've remodeled many older homes and have never encountered a discrepancy between walls and ceilings. It was an age of huge right arms. – isherwood Nov 19 '21 at 19:11
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    AdamO, what are "keys" in this context? – isherwood Nov 19 '21 at 19:21
  • @r13, there were hybrid gypsum/plaster walls up until the 1950s that are probably indistinguishable by that method. You can detect drywall, but not wood lath in particular. – isherwood Nov 19 '21 at 19:21
  • @isherwood The "keys" are the extrusions of plaster between the lathing and that hang over the edge which, once hardened, bind the plaster to the lath. Plaster generally fails once the keys start to detach, leading to the characteristic wall-falling-over phenomenon. – AdamO Nov 19 '21 at 19:28
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    Ah. The squish gobs. I've demolished acres of the stuff and never encountered that term. Thanks. – isherwood Nov 19 '21 at 19:30
  • @isherwood The key is would the plaster ceiling would stand for almost a century without needing repair or remodeling. I had one that was out of your acres of experience - plaster walla and ceiling in the basement, plaster walls in the first and second floors with drywall ceilings. Mine was built in 1918 but remodeled in ??? For an experienced person, it is not that difficult to distinguish the two which have very different characteristics, in both the material and the way of construction. – r13 Nov 19 '21 at 20:08
  • If you are not confident by knocking, drill a small hole, unless hitting a stud, the drywall has very minimal resistance compared to a plastered wall. One small hole won't kill/damage the bond, it can be patched easily. – r13 Nov 19 '21 at 20:14

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I doubt anyone can say better than 'probably'.
Mine do, except the ones that have been re-done over the years - roof/rain fail :\ - which are now plasterboard & skim. Good old moulded coving isn't a good clue, I made sure mine was kept intact whilst removing all the old L&P inside of that.

As a very rough guide… get some horizontal light on it & see if you can see hints of board edges.
Alternatively, if the entire ceiling could be considered 'wavy' or 'drooping' you've got semi-detached lathe & plaster.

If you cannot make that determination by eye, then just push a hole in it with a screwdriver & examine the layering in the hole. If there's paper, you have plasterboard, if you hit a lathe…
You're going to have to knock out at least an inch circle to work through anyway, so you'll quickly find out.

Also consider that if there's floor-boarding above it, then you're going to have to take some up anyway to get the cabling in - at which point you'll be dead certain. If it's a part of the ceiling that's never previously been penetrated, you'll also have to come in from above to put noggin[s] across to be able to precisely place the new fitting securely. You can't fasten into 100-year-old L&P, & I have a mistrust of plasterboard's strength anyway, so I'd fasten to wood even then.

If you absolutely cannot get above it - if you're in a shared property of any sort - then you're going to have to cut sections out of the ceiling itself to get cabling across & noggins in, then re-finish.

Tetsujin
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