I was wondering if anyone had already used a roof garden including the perlite.

Perlite is a volcanic rock effusive varying in color between gray and pink, whose chemical composition is similar to that of dacites and rhyolites. It has the ability to expand its volume up to 20 times when compared to the original flow rate.
The peculiarities of the system including the perlite is due to the limited thickness of the package that allows engraftment and development of plant species provided in 11 cm thickness, bringing the storage value total water up to 45 l / m2 (perlite's saturation value)

I think this roof system can be included into WEc1 calculation by reducing the Ks factor since this is not a typical cistern or pond retention system but it releases water directly at the roots without use of any irrigation system.

Any suggestion?