Flat roof - green roof?

[DanRobertsB]

Hi

I'm wondering whether it's worth* insulating and re-roofing this ground floor flat roof over a bedroom and en-suite, possibly adding a glass balustrade (it could be access via a ladder, which could be permanently attached to the side). It was leaking a little, and some of the board were spongy (hopefully now dried out), but that's been addressed with a flash band repair. Making a green roof would be great, but I doubt it could take the weight. 

Any advice greatly appreciated - anything from what material would be good (bitumen felt, EDPM, single ply uPVC, etc), to who you'd recommend, sequencing (e.g. if considering wall insulation), general tips, whether the "ladder and ballustrade" or green roof idea is nuts, etc. 

* Money no object (alas, it is), I'd just get it done. However, I'm reluctant to spend money unnecessarily, or would prefer to defer it (e.g. we might add external wall insulation in years to come - which, I assume, would be best done before re-roofing). If the current roof really is 0.20 u value, there's less need to insulate it further; the timber frame wall of the bedroom below (seemingly just studwork, plasterboard and timber cladding) is I suspect quite a big limiting factor when it comes to heat retention.

Details:

I have a a c 5.6m x 3.8m (+/- c.10cm) ground floor flat roof, on top of bedroom and en suite bathroom. This was an infill extension, built c. 2006 or 2007. Eaves are vented. 

There has been water ingress (through capillary action) where a seal is poor/has failed (poor original installation, as far as I can tell - the overlapping felt has not been properly sealed; I suspect it was installed by a builder rather than a roofer). I have applied flashband as a temporary fix. However, before the fix at least, the boards (I suspect OSB) under that section (c. 1.5 x 2.5m) were a bit spongey. They may (hopefully!) dry out if the roof is sealed and watertight (as it hopefully now is), but it may be that parts of the OSB need replacing. 

Despite the Building Control insulation specs applicable at the time (u value requirement for building control in 2007 was 0.20; I think it was probably that from 2002 onwards), I suspect the insulation is poor. It may be a cold roof - there are soffit vents. There may be sense in overcladding the whole lot with a VCL and 150mm PIR (I can source at reasonably prices if needed) to create a new (warm) roof on top. I’m not sure what, if anything, needs to be done about the existing roof/ventilation. The gutters could probably benefit from simplification/re-arrangement -  many of them appear to be legacy gutters from before this bedroom infill extension with the flat roof was built.  

It’s possible we might want to install a glass balustrade across most of the open side. 

Depending on cost, we might also be open to a green roof, though I don’t know if the structure will take the weight.