Originally, gravel was not meant to be used in roofs. But the Kaluga plant produces 300000m³ cellular glas gravel, which is almost completely used for flat roofs. Typically, the upper layer of green roofs is soil, which is needed for the plants but can also serve as ballast.
In case ballast is present, we have the choice between a warm and an inverted roof. And if an inverted roof is accepted, we can indeed use cellular glass gravel. In case gravel is used under the concrete slab (floor insulation), the gravel is compacted to obtain a large compressive strength. This seems not be necessary for the roof, but this is a wrong statement.
In the roof, the warm side is under the gravel and convection is possible (contrary to the floor). An interesting paper about the thermal conductivity of gravel shows with calculation and experiment that compacted gravel has a lower thermal conductivity than uncompacted gravel.
However, even with compaction, there is always the risk that sudden cold rain water reaches the bottom of the gravel, creating condensation inside the building at the ceiling. This is precisely the effect where the inverted roof got its bad reputation. This can be avoided by installing 40mm GLAPOR cellular glass boards under the water proofing membrane. The following figure shows such a roof, with a thin layer cellular glass boards to avoid any condensation problem.
By using tapered GLAPOR insulation, drainage is also installed. The water proofing membrane is protected against UV, thermal changes , humidity changes and mechanical damage. In our opion, this is the best roof possible in winter and summer for a very sharp price.