50mms, half inch through your board, half inch through the old ceiling and an inch into the joists, going nowwhere. Why would you wanna take off the plaster and board over the lathes? Knowing my luck I'd do that and they'd be lapped joints lol. Nothing worse than a ceiling coming down all in one, lath and plaster.I prefer to take the plaster off first, clear up, then get the laths down.Saying that, I can't remember the last time we took down a lath and plaster ceiling, we just overboard and tell the office we ripped it down, **** 'em