Julie asks: What is the square foot cost of a green roof in Montgomery County, Maryland?
Julie, you ask a loaded question. And unfortunately I’m not a green roof expert. As I’m sure you know, design of a green roof (vegetated roof) requires a collaborative effort on the part of several parties. The project architect, structural engineer and landscape designer all must be involved in addition to the mechanical and electrical engineers.
In addition, several high level design decisions must be made early on… whether or not the green roof will be accessible to building occupants, whether or not it will incorporate lighting, drainage, or irrigation and finally (perhaps obviously) the types of plants to be included and what type of nourishment will be made available to them.
Green roofs are divided into two categories: extensive green roofs, which are typically six inches or shallower and intensive green roofs, which are much deeper (and less common). According to an Extensive Green Roofs article by Charlie Miller, PE, for Whole Building Design Guide, “The most common vegetated roof cover in temperate climates is a single un-irrigated 3- to 4-inch layer of lightweight growth media vegetated with succulent plants and herbs.”
Green roofs have been shown to provide a variety of benefits from ecological and economic to aesthetic and psychological. But since your question is so direct, let’s focus on the economic benefits. According to Oak Ridge National Laboratory, most roofing systems fail eventually from leaks and the average roof needs to be replaced up to four times over the life of a building. As we all know, vegetated roofs heat up less than exposed roofs and this reduction in exposure to extreme temperatures can increase roof life as well. And of course there is the additional economic benefit of energy savings.
Unfortunately now I’ll get to the shortest part of this answer, which is the specifics I’ve been able to dig up on costs (no pun intended). Extensive green roofs rule of thumb costs can range from $14-25/ft2 for 3” of growing media, including roofing membranes. Intensive green roofs typically cost $25-40/ft2 and up. Cost will vary depending on location, experience of the design team and contractor, and project size. Green roofs on the scale of 500,000 ft2 have been shown to cost as little as $4/ft2.
Plenty more information is available at the website for the nonprofit organization Green Roofs for Health Cities. This industry association consists of individuals and public and private organizations committed to developing a market for green roof infrastructure products and services across North America. Another good resource is Penn State’s Center for Green Roof Research, which strives to promote the use of green roofs in North America through Research, Teaching and Outreach.
You can also search the Green Roofs Database for projects in your area here.