BOULDER - When you hear the term "green roof," you may think it simply means using biodegradable shingles and organic construction materials. But today's green roofs are, quite literally, green.

More than 40 years ago European homeowners started the trend of covering their residential rooftops with soil and plants. The idea spread to the commercial sector as well, as businesses discovered the benefits - both economic and ecologic - of living below a virtual open-air greenhouse.

But will green roofing ever really become widespread in the United States? And what exactly are the benefits of having your own green-planted roof?

According to Andy Creath, owner of Green Roofs of Colorado, a Boulder-based green roof design installation and maintenance company, green roofs are not only here to stay - they may become the next big green wave in building construction.

"In major metro areas it's becoming more part of the normal building conversation, both in commercial and residential buildings," he said. "And with the advance of the new green building codes, they're going to be more and more part of the landscape here."

To construct a green roof, contractors must make sure a building is structurally sound enough to bear its substantial weight. Once a roof passes the strength test and is deemed to be an appropriate shape, construction begins by building up carefully designed layers.

The first, a standard roof waterproofing layer, ensures that the structure will be completely watertight and safe from leaks. On top of this is put a root-repellent substance made of organic materials. The next layer, a drainage layer, is built to retain water long enough so that plants can access the water.

"You want to maintain drainage on the rooftop," Creath said. "You don't want standing water to stay on the roof - just enough water so the plants can survive. The rest is filtered out through normal building drainage."

A filter fabric is put on top of this, which is then covered with a growth medium made up of engineered soil, an expanded rock, shale or clay material, and high-quality compost. This soil compound is layered at a minimum 4 inches in depth. After that, the planting begins.

The best plants to use, Creath said, are those that are indigenous to the area. Popular rooftop plants include sedums - a night-opening plant similar to cactus which holds water in its leaves - as well as native grasses.

"With our dry climate we use plants that are specifically adapted to this type of climate and are less water dependent," Creath said.

Plants to avoid, Creath said, include any aggressively rooted species, such as bamboo, which has roots that can dig into a waterproofing membrane layer and actually puncture it.

Given Colorado's high summertime temperatures, fall or late summer are the best times to plant. And with organic waste coming naturally from the plants, adding extra fertilizer is only necessary every couple of years.

As for maintenance, Creath said a good contractor will provide maintenance service, which includes checking on drainage and plant life, as well as testing soil quality and checking for insect damage, which is relatively rare.

"Just to ensure that the plants are doing well and that the drainage system is wide open, in the first year I may visit six or eight times, and then the next year I'll visit four times," he said. "And a lot of times homeowners like to get on the roof and do these checks themselves on a seasonal basis."

In spite of Colorado's diverse climate, Creath said that green roof construction does not have to be limited to lower elevation areas.

"We're installed these in Steamboat and Aspen and Silverthorne and Vail - we haven't yet found a place where we can't do it," he said. "It just takes different plant choices and soils. And it's better to have snow then to have cold weather and no snow."

Typical installation for a midsize roof of fewer than 10,000 square feet can take no more than a five-day week. As for cost, depending on the design, green roof installation can begin from $12 to $20 per square foot, and with more elaborate designs can cost more than $40 per square foot.

According to Creath, when he started his company, he was the only roofing contractor who specialized only in green roofs. A year and a half later, the same still holds true - although other roofing companies are beginning to add green construction to their services.

"I realized that no one was doing this here, and I wanted to build this industry in Colorado," said Creath, who became interested in green roofs while working as a landscaper for the Environmental Protection Agency during a time when the organization was constructing a green roof for its facility in Denver.

"In the last five years (there has been) a much bigger push for these," Creath said. "We've installed two already, and eight more are on the books in the next several months."

Aside from aesthetic appeal, what exactly are the ecological and economic benefits of the green roofing phenomenon?

"Green roofing doubles or triples the amount of time that your standard waterproofing membrane lasts. Usually it lasts between 15 to 20 years, but with a green roof it lasts 50 years at least," Creath said. "It protects your roof from heat and UV radiation. That's why there are 90-year-old green roofs in Switzerland."

Another huge benefit, Creath said, is the reduction of what is known as the "heat island effect" - a phenomenon that occurs when traditional dark-colored, asphalt roofs pick up sunlight and emit that heat into the atmosphere, creating increased temperatures throughout the surrounding atmosphere. This is particularly common in concrete-covered metropolitan areas.

"A green roof will limit the amount of heat that is held in the atmosphere," Creath said. "This balances out those weather extremes we have here in the summer so you don't have to run your air conditioner as much. A normal roof can go up to 150 degrees on a hot summer day, but a green roof keeps everything significantly closer to a median temperature."

In addition, a green roof will also control storm-water runoff by absorbing and holding the water for extended periods of time. This water goes into the plants and is slower to enter into the stream system.

In urban areas gutted by concrete, green roofs also create an all-important natural habitat for wildlife.

According to Boulder roofing contractor David Coddington, owner of 3R Roofing LLC, traditional roofers such as himself are becoming more interested in learning green roof construction. For the last several years his company has focused on using green materials, and he plans to expand into green-planted roof construction in the coming months.

"We owe a lot to the people in Europe who perfected green roofs," he said. "In the United States Chicago was ground zero for green roofs. Colorado is still very new to this, but it's a growing industry that will become more and more popular."

Coddington said that, from a roofing perspective, green roofing adds insulation value to a building. Commercial buildings with a green roof installation can gain a considerable number of LEED points as well.

But, Coddington added, one potential downside is how long the roof can endure. As with any other aspect of construction, longevity all depends on the quality of the materials and construction, as well as the quality of the roof and building itself.

Another potential problem, Coddington said, could have to do with standing water and runoff if it's not regulated properly.

"That's why it's key for the water to be able to taper off. If there's a collection of too much water there will be moisture buildup and mold," he said.

As a maintenance tool, Coddington recommends using an anti-lead grid detector to find potential leaks.

When Andy Creath isn't designing green roofs, he's talking about them. Creath will be participating in a symposium on green roofs, which will be held at the Denver Botanical Gardens on June 18. One topic of discussion will be a push for tax incentives for green roofs in Colorado - similar to incentives already available in other cities such as Chicago, New York and Portland, Ore.

"People are hungry for information about green roofs. That's why I'm out there talking a lot," Creath said.