Green Roof systems appear complicated due to the number of roofing components used in the roof. However, the roof is really divided into two parts: a protected membrane type roof and the “vegetative” portion above it.
Protected membrane roof components:
- First there is a deck; if it’s a concrete deck, a primer will have to be applied for the membrane to stick.
- Generally there is a membrane fluid applied to the deck. Many manufacturers recommend fluid applied hot rubberized asphalt, but that isn’t the only option.
- After one layer of fluid applied is adhered, a reinforcing fabric is put down and a second layer of fluid applied is put on top. This second layer in a hot applied system will melt part of the first layer and laminate the reinforcement in the system. Some manufacturers say this reinforcement is optional, but it is highly recommended to protect against failure.
- After the membrane is dry, a water resistant insulation board is put on top of the membrane. If the project calls for a protected membrane roof, this insulation board would have to be ballasted (have weight on top of it) to prevent blow off.
- In the green roof example, the vegetative portion of the roof is a large portion of the ballast.
- Also, for a green roof, a root blocking fabric is put on top of the membrane before the insulation is placed down. This is to stop any roots that manage to get through all of the other components of the vegetative system from boring into the membrane.
Vegetative roof components:
- Ballast: There are two types of ballast that make up the walkways and perimeters of the vegetative roof. Ballast is needed at the perimeters primarily because wind conditions are more intense at the edges of a roof and plants will not be able to grow there. Ballast can be either pavers or gravel; however, in areas prone to high winds, gravel is not recommended, as it has the potential to create several small projectiles in a storm event.
- Edging: Regardless of type of ballast, a metal edge is put down at the transition from ballast to growing areas.
- Drainage Panels: these panels kind of look like egg cartons. They have a few purposes. They can be filled with lightweight aggregate to add more ballast to the roof, they hold excess water that the plants don’t use right away but could use in the drier times between rain events, and they allow excess water that cannot be held in the cups to drain away in large rain events.
- Filter fabric: this fabric filters the water coming out from above to prevent dirt from being washed away in large rain events.
- Growth media: this is the “dirt”. It’s really not dirt, but a mix of different material that has been tested to support this type of installation. The thickness of growth media is dependent on region and plant type. An “extensive” roof has between 4-6” thick growth media and can hold small plants that will probably not grow more than 8” tall. An “intensive” roof has much thicker media and can hold larger plants.
- Plants: manufacturers have pre-prepared lists of available plants per city and/or region. While it is possible to go outside of these prepared lists, it is not recommended because the lists are full of plants that are able to withstand the abuse that a roof garden is subjected to in that region and are more or less proven to thrive and expand to cover the roof within a couple of years. Most plants are in the sedum and succulent categories.
- There are several ways that these plants can be provided: mats, plugs, and cuttings. In high wind regions, plugs are highly recommended. The mats currently on the market are not developed for the particular issues that we face in the South (tropical winds, higher temperatures, etc.), and cuttings are not recommended for high wind. This means that at installation the roof will probably look quite bare, and it is important to include a “coverage and thrive warranty” with the other warranties in the roof system.
Green roofs do not need to be confusing, even though there are a lot of individual components. At its most basic, it is a protected membrane roof with walkways and planting over top of it.
For Part 1 of this green roof series, “Green Roof: Pros and Cons”, click here.