what thickness boards for which jobs?

[whitty]

I am to board over a cracked ceiling and skim it. The old crack goes across the joists not along one. I have had a look in the loft and it all looks ok. I was going to avoid joints over the crack but don't know what thickness boards to use. Is there a general rule of thumb or is every job specific?Thanks.

[Bright_Spark]

I'd first try and find out what caused the crack, if you have movement in your ceiling even if you board over it and skim you will end up with another crack. If you have movement in your ceiling fix that first.