If its galvanized steel or copper ~ change it for plastic.
If its flexible (plastic) ~ a continuous platform support is needed over the whole base area plus a little bit. Marine plywood at least 20mm minimum. Supported to avoid distortion.
To top of the cistern must be a minimum of 350mm from the roof.
Fit an isolating valve (service valves, usually a wheelhead gate valve) to the main inlet pipe just before the cistern and the outlet pipe. (near to the cistern)
Fit a new float operated valve (Portsmouth/ Diaphragm Ball valve) the best Â£4 you will spend.
Ensure the overflow pipe is fitted with a screened device (to stop bugs getting in).
It has a tight fitting cover to exclude light, dirt/dust or insects. Fitted with a screened air vent
It is insulated to prevent heat loss and undue warning. However, is not insulated underneath.
The mains inlet pipe is top and the outlet (distribution pipe) is on the opposite side as low as possible (to ensure good circulation). May be fitted on the bottom of the cistern.
It is accessible for maintenance and cleaning.
Do NOT cut holes in the plastic with a heated copper pipe, use a proper hole cutter!!
Do not seal fittings with mastic sealer (silicone) use the plastic washers and PTFE tape, if it leaks you have not done it right!! Mastic will hold for a time, however WILL leak later.
Overflow/warning pipe ~ warning pipe connected 25mm above the water level, and the overflow pipe 50mm . Must fall continuously to point of VISIBLE discharge. Up to 100litres cisterns required only a warning pipe, >1000 should be fitter with an overflow & warning pipes.
Backflow prevention ~ Overflow/warning pipes must be capable of removing excess water without the â€˜inletâ€™ becoming submerged!!
i would build a timber frame using 4"x2". make sure that it is sited so that its well supported by over the joists either by fixing the uprights of the frame directly to them or running the 4x2 perpendicular to them then fixing onto that. crossbrace the frame for extra strength and to hold it square. top it with one inch WBP plywood which you can easily drill through to run the pipes etc. quick, easy and you can raise it as high as you like (roof permitting of course) just make sure that your frame is nice and level, rigid and secured to the joists.