The GREEN ROOF and/or roof garden is one of those fields of application, in which the designer, the client and then the contracting companies must simultaneously consider problems of a structural nature, construction technique, landscape and agronomic. The green roof fact must be able to ensure engraftment, development and the duration in time of the vegetable essences choices and for this it is necessary to reconstruct the natural biological conditions in an artificial environment.
This objective meets almost always obstacles of a structural nature (inability to overload the floor with the ground…) and logistics (difficulty of transporting materials on the roof…), so it becomes crucial the choice of suitable materials both from the point of agronomically both from a point of view of availability and workability.
Surfaces green roofs must submit all of the following characteristics:
Others aspects to be reckoned with in the choice of materials are theirs AVAILABILITY, WORKABILITY and MACHINING. Almost always it is necessary to lift the materials at high altitude with the crane or move them in areas not accessible to mechanical means. To do this you need to transport materials in big bags or tanks with pneumatic discharge. Not all materials are available with these conditions of carriage.
In consideration of the analyzed in the previous lines, when making a green roof, it is necessary to use materials that allow drainage simultaneously balanced, lift-durability, lightness and fertility and that they are available and workable with ease.
To help engineers in the design and clients to become aware of the choices made has been prepared with the UNI 11235 ”Instructions for the planning, execution, control and maintenance of green roofs” This rule, among other things, identifies the requirements that need to have the materials used to create the different layers of cultivation: eg. drainage and fertile layer.
Among the various materials mentioned in the standard, they certainly have a leading role the VOLCANIC AGGREGATES as PUMICE and LAPILLUS, thanks to their special porous structure that has given us the nature.
PUMICE and LAPILLUS are porous volcanic scoria lapilli, which the honeycomb structure is the result of the cooling of magma rich gas internally. The magma that originated the pumice was constituted by the chemical composition of minerals such as to allow the dissolved gases inside expand very rapidly, so as to form a kind of foam. The subsequent rapid cooling of the lava has determined the sudden solidification of the liquid around the gas bubbles generating the appearance of foam glass of pumice. The interior of the rock in fact consists of an enormous amount of canaliculi of average diameter very small intercommunicating with each other and with the outside. The volcanic lapillus is formed from a magma with lower content of silica. The lower viscosity and slower cooled lava have facilitated the release of a certain amount of gas present in the magma. Were formed so the minerals characterized by emptiness of average diameter greater than the number of pumice but decidedly less. The different physical structures of pumice and lapilli mean different agronomic properties and the various fields of application.
PUMICE has a honeycomb structure where the spaces of solid occupy on average only 30% of the volume of the rock. The rest of the space is constituted by approximately 35% of capillary porosity, occupied for the most part by water, and the rest of pores of diameter greater in which it is able to circulate the air.
In conclusion we can say that the pumice is characterized by:
• higher specific surface
• Increased cation exchange capacity CSC (which represents the amount of cations, expressed in milligram equivalents per 100 grams of substrate, which can absorb a substrate by ion exchange at pH 7)
• high water retention
• a greater lightness
• natural and ecological
LAPILLUS has denser structure in which the total porosity does not exceed 50% of the volume and is characterized by the prevalence of alveoli of greater dimension. Lapillus is equipped with:
• greater macro porosity crucial for air circulation
• good mechanical resistance
• presence of trace elements available to the plant
Due to their characteristics, the VOLCANIC AGGREGATES (Pumice and Lapillus), used alone or in mixture, with simple and adequate construction systems may confer to the surface simultaneously: