Hi guys. As an enthusiastic, yet unknowledgible DIYer, can I pick your brains for a bit of advice? I have just take down an old lathe and plaster ceiling at home and am about to put up plasterboard. My question is how often should I screw in my plasterboard screws down in each joist? And how far in off the edge of each board do I have to be before fixing a screw- I was thinking 20mm or so? Thanks for the help!