fixing 70mm insulated plasterboard

Hi, im Daved of prides plastering. You can get either black phosphate coated drywall screws or zinc coated dry wall screws. Zinc is more expensive but both do the job fine. I suggest you get screws which will go into the joist allowing at least a good 20mm longer than the thickness of the board. You can use wood screws but if your skimming it I recommended drywall screws. Also screw so the head of your fixing is flush with the paper on the board and avoid going deeper as this may compromise the hold. 36 screws per full sized board will be enough and allow atleast half inch space from the edge of the board to the fixing if you can. Hope this helps. Daved

Not a good idea to put insulated boards up in a garage. Especially if it is still being used as one. Garage ceilings and some walls are covered by the fire regs and need fireboard fitting as a minimum of 30 minutes are required to protect the room or adjoining rooms from fire damage allowing egress from the property. Some building control will ask for as much as 60 minutes. Insulate between the rafters then fix the fireboard to the rafters using 45mm drywall fixings. One layer will equate to 30 mins. Double skin will give you up to 60 mins. if double skinning use 60 - 75mm fixings.