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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.

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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
    Commented Nov 19, 2021 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
    Commented Nov 19, 2021 at 19:11
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    AdamO, what are "keys" in this context?
    – isherwood
    Commented Nov 19, 2021 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
    Commented Nov 19, 2021 at 19:21
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    Ah. The squish gobs. I've demolished acres of the stuff and never encountered that term. Thanks.
    – isherwood
    Commented Nov 19, 2021 at 19:30

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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.

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