The bigger footprint is apparently when a thermostat failed(pre having the reset) and was forcing boiling water out of the vent pipe, the CWSC was not fully supported and distorted and the scalding water came through the ceiling
I've now found the 150mm in Document G of the building regs (though I still wonder whether extensive research resulted in the 150mm figure - or whether someone who wanted to cover his rear end thought 150mm sounded like a good figure).
In any case, while the F & E cistern in the photo is probably okay, it certainly isn't right in this respect. So I take back what I said in previous posts.
WRAS claims that at least 15mm thick marine ply should be used according to BS 4213:2004 (so my earlier comment that ply is probably a matter of opinion may, then, be wrong) but can anyone tell me the legal status of this BS? Document G refers back to it, but not specifically to the marine ply clause. Quite how marine ply is acceptable but T&G boards, or a steel plate, are not accepable, is beyond me. After all, if I put a cistern on a first floor floorboards or on a structural concrete hearth, I would not insist on marine ply between the cistern and floor level.
The irony is that floats for ballcocks on any cistern that has the potential to become warm must comply with BS 2456 (as stated in Water Regs - therefore compulsary), and neither Travis, Plumbfix, Graham, or Center seemed to stock compliant floats as a loose item. I had to order a box of them from an independent merchant in Lampeter. So to expect the average plumber's work to be 100% compliant in every respect is optimistic.
