woodsmith
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Firstly have you ascertained that the roof is able to take the weight of the flagstones and people etc? To take flagstones it must not flex at all and unless the roof was specifically designed to take this load I doubt it will be strong enough plus I expect this job would need building regulations approval from the local council before you could do anything.
As for laying flagstones on EDPM rubber, you really need to ask an EDPM roofing specialist but I would think at the minimum you would need to decouple the flagging completely from the EDPM? You could do this by laying pond underlay first, this is a thick polyester material and is extremely resistant to puncture and tearing. Then you may be able to lay the flags onto a thin bed of sharp sand but as I said you need to get specialist advice for this for both the EDPM and also to see if the roof is up to the job.
Thinking about it, this job is looking more and more impractical and expensive as you would also need compliant balustrading all round the roof, which you couldn't fix through the EDPM.
Another option to consider may be to deck the roof, this would be much lighter than flags and would make fitting a balustrade much easier.
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