Green Roofs, What are green roofs, Why use green roofs

Green roofs speaks to what are green roofs and why use green roofs.

They are also referred to as eco roofs or vegetated roofs are roofing systems covered with vegetation and additional materials needed to support plant growth.

Like protected membrane roofs, thes roofs extend the life of the roof membrane by shielding it from UV radiation and extremes of temperature. Thes roofs also reduce heating and cooling costs by moderating temperature swings in the roof assembly. They reduce storm water runoff and provide habitat for birds. By supporting plant growth and reducing heat island effects, thes roofs improve air quality. They provide aesthetic value and in some cases create, pleasant usable space.

Extensive green type roofs are relatively shallow, with soil depths of 2 to 6 inches (25-150 mm). They are planted with herbs, grasses, mosses, sedums or other drought tolerant plants that do not require irrigation or frequent maintenance.

Intensive roofs may have soils as deep as 30 inches (750 mm) and are designed to support a broader variety of plant types and shrubs. Intensive roofs typically require irrigation and regular maintenance such as weeding, trimming, pest management and fertilization.

Planning for the structural loads of soil and plant materials is an important part of this roof design. Because of their lesser depth, extensive roofs are relatively light in weight, imposing loads, when saturated with water, ranging from 12 to 35 psf (0.57 to 1.7 kPa) on the supporting roof structure. Intensive roofs impose loads of 50 psf (2.4 kPa) or more. While most roofs are essentially flat, with special soil retention measures, extensive roofs with slopes as great as 12:12 (100 percent) are technically feasible.

Considerations:

Typical components of this roof system, top to bottom are:

Plant materials may be selected on the basis of hardiness, climate, depth of soil, maintenance expectations and appearance.

The growth medium or soil must provide long lasting, optimum growing conditions. Particulars of its formulation vary with the depth of soil and the type of vegetation supported. Soil stability, drainage properties, and with extensive roofs, drought resistance are important considerations.

A geotextile filter fabric prevents soil particles from being washed out of the growth medium and clogging the drainage layer below.

Soil restraints made from perforated plastic or metal allow the free flow of water while confining the growth medium at the roof perimeter and around drains or scuppers.

Drainage layer materials such as a molded plastic panel or an entangled plastic filament mat are used to provide efficient drainage and aeration beneath the soil. Some products also provide water retention, benefiting the plants subsoil environment. Crushed rock or other aggregate material may also be used, although at the cost of added weight in comparison to the use of synthetic materials.

Rigid foam insulation boards may be positioned above or below the roof membrane. Where insulation is placed above the membrane extruded polystyrene is used because it retains its insulating value when wet.

Depending on the membrane system, one or more protection layers to prevent root invasion and to relieve physical stress may be laid over the membrane.

Since access to the water proofing membrane becomes difficult once it is covered with the roof components, it should be chosen with consideration of its long term performance in a buried continuously damp environment. Especially for intensive green roofs, robust waterproofing systems are recommended in place of lighter duty conventional roof membranes.

Though many membrane materials may be used hot rubberized asphalt and PVC and multi ply modified bitumen have the record of successful installation. Where modular roof systems are used and the membrane will remain more easily accessible, less expensive conventional roof membrane types may also be suitable.

Prior to being covered green roof membranes should be flood tested, that is, tested for watertightness by placing the membrane under continuously submerged conditions for a period of hours per days, to check for leaks prior to placing the overlying components.

Vapour retarder and air barrier requirements are no different for these roofs than for conventional low slope roof assemblies.

Roof deck and supporting structure must be engineered to carry the additional loads imposed by green roof components.

Modular Green Roof Systems

With modular systems, all the components of the roof system above the membrane are preassembled is easily transported trays or modules. These trays, typically 2 to 4 feet (600-1200 mm) in plan dimension and 2 to 8 inches (50-200 mm) in depth are preplanted and arrive on the construction site ready to be placed directly over the roof membrane.

Modular green roof systems are lightweight, easy to specify, easy to assemble on site, and easy to remove or adjust at a later date.

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