seems odd to go insulation then membrane, they make insulation sheets with thin ply adhered to them

I would be concerned about future puncture injuries if there was nothing strong under the membrane, green roofs can work either way insulation wet or dry.

I would be very nervous of water storage = fails and big fun time! and the additional constant weight could cause deflection if not engineered in.

Membrane on top of insulation is quite common. Don't see why puncture is any more likely with strong/weak material below the membrane. Arguably easier to make a hole if something more solid below membrane.

Phil, I've not come across PIR over EPS, but if you are putting 300mm soil on top then it should be fine. For an exposed membrane roof it would be an issue because you'd have to find compatible adhesives, or go for through-fixed membrane (to hold it down). But with a load of soil holding it down that shouldn't matter. You will find that for the membrane guarantee to apply, the whole build-up has to be approved by the manufacturers/installers. So you can do whatever you like, but if it's not one of their 'systems' then they'll refuse to give you the membrane guarantee (or at least that what they told me when I tried to have mixed insulation materials in the build-up).

Thanks for info guys.

I'll go back and talk to the membrane guys about what is a approved solutions in terms of insulation.

Robin, the idea of the plywood seems a good one as long as the membrane guys approve it, especially in one area where we have patio on stilts sitting on the membrane. I was concerned about the point load of the stilts. Did you simply lay the plywood sheet down without any adhesive on the backs or the sides, I would have thought edges may have been an issue with possible lifting between the boards if they were not glued together? Also what kind of ply did you use, I guess it shouldn't get wet one hopes so anything would do?

The problem with a ply layer right under the EPDM is that the system is not fail-safe. That timber is in the condensation zone right on the cold side of the roof, so if any vapour gets into the roof construction (from below as well as above) it will end up condensing there and rotting the timber. This will be somewhat improved under 300mm of soil, but probably not enough to remove the risk.

The EPDM on PIR surface is plenty strong enough to walk on for maintainance, but you need to spread loads like ladder feet. This is OK for a domestic roof.

Ply (or OSB) in roofs has to be roofing grade. That's OSB3 - not sure what the ply designation is. Your BC ought to require this (although my builder tried hard to put OSB2 in until I insisted he do it right - this was clearly his normal procedure).

The joists should be on the warm side (or near it) so will be fine. Few BCOs understand this stuff :-)

And in practice it's a common-enough design, so it obviously doesn't go wrong too often, but with much higher levels of insulation and airtightness you need to worry more about this stuff. I still don't like unventilated timber on the wrong side of a strong vapour barrier.