All green roof soil media should comply with published F.L.L. standards. In special conditions, soil specifications may be designed or modified by the landscape Architect or horticulture consultant. Many variables affect the choice of the proper soil mix and these variables should be evaluated for the specifics of each project by the designer.
Greenroof soil components are very different from ordinary fertile topsoil. Greenroof media components are generally classified by their mineral and organic content and blended together in different ratios, typically 85-95% mineral content and the balance organic. Soil considerations include:
- Planting design
- Moisture retention requirements
- Nutrient retention
- Drainage Design
- pH (7, ± .5 generally recommended)
- Compaction properties
- Adequate porosity
- Weight restrictions on structure
- Resistance to fire propagation
- Structure for plant anchorage
Barrett Company has over 30 Approved Engineered Media Suppliers that can supply F.L.L. compliant media, strategically located around the nation.
Plant specifications are determined by the landscape architect or a horticulture consultant. Designs are classified as extensive (low profile), generally with plants from the sedum family which require relatively low maintenance and intensive (high profile), which provides a wide selection of plant choices.
Low profile designs utilize plants with shallow root structure, with a self-sustaining property of regeneration, which are resistant to drought and temperature extremes including frost and extended periods of direct sunlight. Most commonly used plants include Succulents, Phloxes, Sedum, Anntenaria, Sempervivum, Armeria, Creeping Thyme, Aubretia, Allium, Delosperma, Dianthus, Phedimus and Talinum.
High profile systems can include grasses, shrubbery, decorative trees and sod.
Design considerations include:
- Pre-grown, seeding, cuttings or plug planting
- Root system aggressiveness
- Root system anchorage requirements
- Reactivity to heavy direct sunlight
- Drought resistance
- Effect of desirable/undesirable Wildlife Habitat
- Wind resistance
- Regeneration abilities
- Maintenance requirements
- Local climatic conditions
- Modular or built in place
Plant plugs with fully established root systems have an approximate survival rate of 80% as they will quickly spread their roots horizontally to form a dense root vegetation mat within a few growing seasons. Plant cuttings, most often sedums need more time to establish roots to hold the soil and prevent erosion. While the initial cost of cuttings is less, the initial survival rate is less than plugs. Combinations of plugs and plants is generally more efficient and least costly.
Barrett does not sell modular assemblies but we do work cooperatively with Live Roof®, Zinco®, Hanging Gardens® and Conservation Technologies® high quality modular suppliers and can furnish complete system warranties to the building Owner.