Harpo Substrates for Green Roofs
Harpo substrates complies with UNI 11235
HARPO’S EXTENSIVE EXPERIENCE
Harpo’s longstanding expertise in the field of specialized, professional substrates for green roofs and our know-how in geotechnics and naturalistic engineering have enabled us to develop a range of specific substrates, offering reliable results:
- HIGH-QUALITY MIXES
- EFFICIENT AND TESTED
- COMPLIANT WITH UNI 11235
Reliability: Thanks to years of experience in producing professional substrates specifically for green roofs, Harpo produces substrates designed for our climates right here in Italy.
Quality control: To guarantee the high quality of Harpo substrates, rigorous quality controls are carried out in specialized analysis laboratories, primarily on the raw materials used in the mixture, while also verifying compliance with the requirements of the finished product regulations.
Component selection: Harpo selects the components with the best performance on the market for the production of its substrates.
NUMEROUS ESSENTIAL REQUIREMENTS IN REDUCED THICKNESSES
The characteristics that substrates for green roofs must have are very particular, as a garden on a roof slab is subjected to extreme conditions, and a “DIY” approach in these projects risks compromising a reliable and long-lasting result that guarantees suitable living conditions for plants. The growing medium is the layer where plants draw nutrients and is the fundamental element of the system.
Harpo substrate complies with UNI 11235
SUBSTRATES FEATURES FOR GREEN ROOFS
The features of a natural soil are subject to extreme variability, making it difficult to express their performance, even after carrying out numerous and costly analyses.
On the contrary, a substrate for green roofs, being custom-built, offers the technician a series of definite performances and enhances the necessary characteristics of a roof.
Possible problems resulting from the use of natural soils
- Excessive compaction: loss of permeability, root asphyxia, total thickness reduction.
- Loss of agronomic characteristics: poor nutrient retention capacity, variation in pH over time.
- Poor water retention: higher costs in terms of irrigation.
- High weight: oversizing of the structure.
- Greater care: high maintenance costs.
Advantages of Harpo substrates complying with UNI 11235
- Low compressibility over time.
- Ideal water-air ratio for the root system.
- High cation exchange capacity, i.e., the ability to retain nutrients, less fertilization.
- High water retention: irrigation water supply reduction.
- Reduced maintenance costs, while creating optimal conditions for plant growth, without resorting to the removal of natural soils and using reduced thicknesses.
- For the same thickness, UNI-compliant roofs are generally much lighter than those using so-called “cultivated” natural soil.
The average weight of Harpo substrate under maximum saturation conditions is approximately 1200/1400 kg/m³ (the weight under natural humidity conditions ranges from 1000 to 1200 kg/m³), while that of natural soil is 1800-2000 kg/m³, a substantial difference when working on a slab.
Requirements for a suitable substrate for a green roof | DIY risks |
compliance with appropriate particle size curves | ● lead to imbalances in particle size distribution ● can lead to clogging of the substrate and due to the excessive presence of particle size fractions with ø < to 0.02 mm lead to clogging of filters ● formation of silts |
correct balance between mineral and organic part depending on the type of greening | ● imbalances in nutrient availability; too much or too little is supplied ● development of weed species in stratigraphies where low maintenance is required instead |
reduced weight at maximum water saturation | ● green roof on a new project: unnecessary permanent overload on the slab resulting in additional cost for structural reinforcement; ● green roof on an existing project: substantial overload where the available loads are often limited instead |
permeability | ● water stagnation |
air content | ● lack of ventilation |
balanced air/water ratio at maximum water saturation | ● development of root system pathologies |
good water retention capacity | ● rapid rainwater disposal that is instead valuable for the water supply of plant species ● improper water retention capacity negatively affects the agronomic habitability characteristics of the substrate |
stable physical and chemical structure | ● rapid nutrient leaching ● poor resistance to wind erosion |
excellent frost resistance | ● thermal shock crushing of the mineral part ● loss of permeability, aeration, drainage characteristics |
reduced compaction over time | over time the substrate compacts forming a real “plug” over the filter cloth resulting in: ● unbalanced air/water ratio ● insufficient drainage capacity at maximum saturation ● insufficient conditions for adequate vegetation development |
absence of weed seeds | ● rapid release of nutrients with negative consequences referred to agronomic habitability of the substrate |
In Harpo substrate production, special importance is given to chemical characteristics such as salinity, pH , adsorption capacity and buffering power. | |
IMPORTANCE OF HARPO SUBSTRATE PHYSICAL PARAMETERS
DENSITY (weight)
Green roof systems are designed and built to minimize loads in order to save in support structures and to be able to work on slabs with low available loads.
GRANULOMETRY
An appropriate grain size distribution is necessary to ensure the structural stability of the substrate, so as to avoid compaction phenomena over time that would reduce air and water retention capacity. Particle size is closely related to the choice of filter geotextile, which must not clog over time, so compatibility checks must be carried out with the known filtration formulas of geosynthetics.
PERMEABILITY
Permeability of green roof substrates must be high to avoid water stagnation that could result in an asphyxiated condition for the root system. Permeability must be guaranteed over time.
WATER VOLUME AT pF1
The water present in the substrate at pF1 (i.e., applying pressure to the substrate sample to extract water equal to that exerted by a 10-cm column of water) corresponds to the maximum volume of water that can be retained by the substrate.
AIR VOLUME AT pF1
This is the minimum amount of air that must be present in the substrate under saturated conditions. It ensures plant survival by avoiding conditions of root system anoxia and rot phenomena.
IMPORTANCE OF THE CHEMICAL PARAMETERS OF THE CULTURE SUBSTRATE
PH
Substrate acidity is important for plant life and affects other parameters such as CSC.
CATION EXCHANGE CAPACITY (CSC)
Cation exchange capacity is the amount of cations that a material, called an exchanger, can adsorb and thus retain or release. CSC is expressed in milliequivalents per 100 grams (meq/100g). Cation exchange is critical for retaining and making elements such as Ca, Mg, K, N available to plants and microorganisms, and indicates the potential chemical fertility of the soil.
ELECTRICAL CONDUCTIVITY
The content of soluble salts must be limited within certain values depending on the density of the substrate. The method of determining salinity is based on electrical conductivity, from which the content of salts can be determined with simple charts. Too high a concentration of salts can hinder or prevent the ability of the root system to absorb water and nutrients.
ORGANIC MATTER
Organic matter in green roof substrates should be reduced to the minimum necessary. Too much would result in a decrease in thickness over time and a structural change in the substrate. In extensive green roofs it would make the substrate easily attacked by weed species.
MACROELEMENTS
Represented mainly by K, N and P. Proper balance of these is necessary for the development of healthy and hardy plants.
CHARACTERISTICS OF HARPO SUBSTRATES
Harpo substrates are obtained by mixing in appropriate percentages a mineral component and an organic component that vary on a case-by-case basis.
The elements that make up the mineral component, consist of volcanic materials such as volcanic lapilli, pumice, zeolites, etc…; the organic part has elements such as peat, composted plant residues, etc…
The use of crushed brick is of particular interest, this is granular material produced from selected, crushed and screened bricks from pure production waste, it has undeniable advantages for environmental protection as it helps not to uniquely push the supply of material into quarries and instead offers interesting possibilities for the reuse of a valuable material.
The “secret” of Harpo substrates is the strict quality control of all the raw materials that compose it and the constant verification of the performance that the final mixtures must have in order to guarantee the main requirements, previously described, required by the UNI standard
MINERAL COMPONENT
LAPILLO NO-CRUSH
● high mechanical strength ● strongly vascularized granules that promote the osmotic process ● air presence ● stable structure ● resistance to freeze/thaw thermal cycles ● neutral ph ● lightness | |
PUMICE | ● high water retention ● lightness ● ph control ● fire resistance ● thermal inertia ● particle size compensation |
ZEOLITITES | ● high water retention ● structural effect |
ORGANIC COMPONENT
BALTIC PEAT | ● high water retention ● structural effect ● reduces moss formation |
PLANT COMPOST | ● plant nutrition ● reduces moss formation |
“The organic fraction should be minimal in extensive substrates to hinder the establishment of weed species.
In intensive substrates, the organic matter content may be higher, but still reduced to avoid volume reduction.”
Upon request, Harpo Spa Division of Harpo verdepensile can attest the data with test certificates from the MAC laboratory in Como, including particle size curves and water retention curves up to PF3.5.
CUSTOMIZATION
– Case variability: in addition to the standard range, Harpo spa is able to produce substrates for special situations or needs with customized formulation referring to the planned green project.
– Accurate mixing: all components are carefully mixed avoiding damage to the elements that compose it.
– HARPO laboratories analyses: allow the correct formulation of the suitable substrate responding to the performance required by the specific green project.