How do you join the top/bottom of plasterboard on a ceiling?

Hi- just about to plasterboard a ceiling. Can anyone help with these please?

1/. The side joints are thinner to take tape and filler, but how do I join the top/ bottom flat edges so cracks don't appear?
2/. Can I use 9.5 in a kitchen, or is 12.5 plasterboard a regulation?
3/. Going off the plastering subject a bit, but do I have to insulate the joist gap? I've been told that I do in my converted garage (fine) but do I have to do in my existing kitchen?

Many thanks! Richard.

RRRRRR said:

1/. The side joints are thinner to take tape and filler, but how do I join the top/ bottom flat edges so cracks don't appear?

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You must tape the joints or it WILL crack; are you getting it skimmed or are you just going to tape & fill? It’s one of the problems with tape & fill on ceilings & you will have to fill over & try to blend it all in so it doesn’t notice. Very difficult & more trouble than it’s worth IMO, tape the joints & get it plastered. Without wishing to open up the debate again of taper over square edge boards when skimming, you’d do better to board out using square edge; stagger the boards with longest edge across the joists.

RRRRRR said:

2/. Can I use 9.5 in a kitchen, or is 12.5 plasterboard a regulation

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Use 12.5mm; there is no regulation but nobody I know uses 9.5mm any more.

RRRRRR said:

3/. Going off the plastering subject a bit, but do I have to insulate the joist gap? I've been told that I do in my converted garage (fine) but do I have to do in my existing kitchen?

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Assuming it’s not a single story flat roof & has a room above it, no.

Thanks for replying- I might be being a bit thick here, but are you saying if a room IS above, I need to insulate ceiling void? Thanks!

he is saying the opposite

1john said:

he is saying the opposite

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Yes indeed