Re Tracing ceiling joists

I previously asked for advice on fixing a crack in the kitchen ceiling and it seems it would be best to screw the plaster board up into the joist somewhere above it.
However I know that the bathroom above has plastic tubing running through the joists and I am worried about drilling into one of these.
How can I detect where the wooden joists are (The floor above has a tiled floor).
Thanks
Dee
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It should be already?

shouldn't need to screw more than half an inch into the joist bottom. I think you just need to pray the tubes are not that near the joist bottoms (they shouldn't be, but doesn't mean they aren't). You don't need to drill first.

You can usually find them with a magnet, to find the screw or nailheads fixing up the boards. That's why I don't understand why you think you need to put more screws in?
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Andrew Gabriel
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writes:

Knocking.
The ceiling will sound hollow away from the joists and flat/solid where the joist is - and it helps if you already know which way the joists run. If you have a strip light that will be screwed to one single joist which will give you a good starting point. Knock with you knuckle as you move away from the light going perhaps an inch at a time.
Generally speaking the joists will be 16" apart in an older house, or perhaps as little as 12" on a modern house, the breakover time being 70's-ish.
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Woody

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