I am installing some new low voltage downlighters in my kitchen and need to fit a plasterboard false ceiling to hide the new wiring. The existing ceiling has 12.5mm plasterboard attached to joists which are 200mm by 45mm in cross section and spaced 400mm apart. The false ceiling would be attached to softwood struts (25mm by 75mm cross section) which are themselves screw attached to the joists. The room size is 5.5m by 3.4m. My concern is over the ability of the existing ceiling to take the weight of the new plasterboard. Are there any rules of thumb which can be applied or calculations made. I had intended to use 12.5mm plasterboard but now feel that 9mm board may be more suitable. Any advice would be much appreciated.