Green, or sod roofs, are a multilayered system of insulation, waterproofing, root barriers, drainage, lightweight soil, sod and/or plants. Green roofs serve several purposes: absorbing rainwater, providing insulation, creating a habitat for wildlife, and they help to lower air temperatures and combat the heat island effect in urban environments.
There are several universally accepted methods for an earth roof. One is on top of a rubberized asphalt surface, another is on top of a slate roof, another uses a true earthen ceiling. It can be said that the first two methods are most successful in humid environments, and the last method is only really well suited for an arid environment. A local architect can design the perfect roof for your needs.
“Intensive” green roofs
Intensive roof gardens require a 12” depth of soil and require irrigation, feeding and other maintenance. Intensive roofs can grow anything from kitchen herbs to shrubs and small trees and may appear as lawn or garden.
"Extensive" green roofs
Are virtually self-sustaining and require only a minimum of maintenance, perhaps a once-yearly weeding or an application of slow-release fertilizer. These roof types are not suited to constant walking or play. They can be established on a thin layer of specially formulated soil, and can easily support a planting of Sedums and mosses.
Pitched sod roofs can be of nearly flat to slight pitch. Pitching allows for the use of simpler roof base materials due to water shedding.
For the best appearance, use super-tough and drought-tolerant plants, with both coarser and finer textures, plus color bursts.
In the US Southwest, try:
- Echinocactus Grusonii cactus Available in bulk at Egyptian-German Agricultural Company
- Nassella tenuissima grass
- Salvia Clevelandii violet sage
- Sedum Rupestre Angelina groundcover
- Sedum Spurium Burgandy florets
- Sempervium Hen and Chicks
- Agastache rugosa violet hummy flowers
- Helictotrichon sempervirens bluegrass
In the Northwest, where epiphytes – plants that don’t have roots, like moss - grow naturally, a traditional roof can be installed and made into a green roof. Simply collect a sampling of mosses from around the area and spread this “starter kit” on your roof. Lichens and more moss will follow forthwith.
Green Roof Pros – (True green or sod roofs only)
They can be beautiful - planted with fruits, vegetables, wildflowers, herbs, and grasses, and the final result is a vibrant habitat for birds, butterflies, and other small animals.
Provide good insulation and reduce heating and cooling costs
Provide usable outdoor space when installed correctly.
Can be used to process greywater and rainwater, and improve air quality.
If you happen to live in a city, green roofs will dramatically cool the area around your home, and provide a source of gardening!
Adding rooftop ponds provides excellent option for greywater treatment.
Increases roof life span.
Reduces stormwater run off – capturing up to 74% of rainwater.
Filters pollutants and carbon dioxide out of the air.
Insulating for sound: soil helps block lower frequencies and plants block higher frequencies.
Reduce heating loads by adding mass and thermal resistance value.
Reduce cooling loads by evaporative cooling.
Green roofs are expensive – costing up to $24 per square foot, compared with id="mce_marker".25 per square foot for conventional roofs.
Required maintenance cost and time.
Do not always work well in very windy places, unless small wind-resistant plant species are used.
Multilayered = multiple resources, multiple trades, and multiple transportation costs.
Green roofs work best on very low slope applications – somewhat limiting the design.
Are not ideal for rainwater harvesting because the plants need the water – limits the amount of water directed to the cistern.
Use of fertilizers is common.
Require complex drainage systems.
Differentiated growth – not all green roofs work well on north sides of pitched roofs (in northern hemisphere).
Weight – these roofs are very substantial and require larger structures (and therefore more structural design and installation cost) to support them. Not ideal in seismic regions.
WHEN you get a leak, it’s going to cost a LOT to locate and repair it.
Poor design = non-functioning roof. Be sure your architect or builder knows what they are doing. Check references! I learned by working for an architect that implied he knew what he was doing that you cannot ‘fake it til you make it’ on green roof design. The client had to install nearly $50,000 in detailed modifications to get the roof to work correctly. The architect insisted it was not his fault as he was following “industry best practices” and the Owner was left with the burden of reinstallation.