Green roofs have become the rage. Politicians even seem to like green roofs.
But these roofs, often added to a building for either energy or sustainability, add a challenge to the plumbing engineer when it comes to designing the roof drainage system.
Not everyone, though, is a big fan of green roofs. I recently read an article by my good friend Dr. Joe Lstiburek titled, “Seeing Red Over Green Roofs.” The one thing I like about Joe is he’s always direct. He tells it like it is.
Some of Joe’s comments on green roofs were direct. “It is dumb to do a green roof to save energy,” he wrote. “If dirt were energy efficient, we would call it insulation and put it in walls. It is just dirt.”
Joe does go on to praise the beauty of green roofs and discusses the impact on the roof membrane. If you want to read the entire article, it can be found at: www.buildingscience.com. Just type in the article headline I mentioned above in the search engine.
With the issuance of the 2012 ICC International Codes, green roofs now are addressed directly in the codes. If you are involved with a green roof design, it is best to become familiar with the green roof requirements. Many of the code officials discussing and voting on these requirements made derogatory comments about green roofs. The final requirements reflect some of those issues.
Roof Nuts and BoltsUnfortunately, nothing on green roofs appears in the International Plumbing Code. I say that because many nonengineers (without saying architects) think you can cut back on the roof drainage requirements if you have a green roof. They assume the green roof absorbs the rain and uses it to grow the vegetation. Some further assume the remaining water evaporates.
This couldn’t be further from the truth. If you have a green roof, you still must provide storm drainage for a storm of one hour in duration and 100-year return period. No landscaping is going to absorb a rainfall of this intensity.
Another mistake being made is putting all the roof drainage under the roof landscaping. Roof drains should be located both on top of and underneath the landscaping. Furthermore, there should be no reduction in the size of the roof drain. You still need a full-size drain for a green roof. Secondary roof drainage also is still required when there is a green roof.
The International Building Code added a few requirements regarding roof loads for green roofs. When the space is unoccupied, the landscaping is assigned a live load of 20 psf. The landscaping material is added to the dead load for the roof.
Fire Code RequirementsThe remaining requirements for green roofs are found in the International Fire Code, which is probably the last place you would think to search for green roof requirements. The concern expressed is the possibility of a fire on a green roof and the fighting of such fire. Believe it or not, there could be a brush fire on a green roof. Fire officials don’t want a brush fire to spread to the inside of the building.
If the building is required to have a standpipe for fire protection, the standpipe must extend to the roof level when a green roof is installed. Typically, the standpipe terminates at the top floor level. This, of course, provides a water supply to fight a brush fire in the landscaping.
The fire code restricts the size of a landscape roof area to 15,625 square feet, with the maximum dimension in length or width being 125 feet. If you have multiple landscape areas, each landscape area must be separated by 6 feet of a Class A-rated roofing system.
In addition to the barrier between landscape areas, a 6-foot barrier also is required around roof structures, penthouses, mechanical rooms, mechanical equipment, roof vents, solar panels, antenna supports and building service equipment. I have seen pretty photos of mechanical units in the middle of a green roof area. That would not be permitted in the 2012 Fire Code.
Supplemental irrigation is another requirement. In other words, you need a lawn sprinkler system for a green roof. That doesn’t sound very green when you have to water the roof. However, the fire folks are concerned the grass or vegetation will die and become a fire hazard.
Interestingly, there is no requirement as to how often you have to water the roof. The requirement calls for maintaining a level of hydration necessary to keep the green roof plants alive.
Fire officials are permitted to require a maintenance plan for the landscaping on the roof. The fire code requires dead foliage to be removed at regular intervals but no less than two times a year. Don’t you wish the plumbing code would require the roof drains on every roof to be cleaned at least twice a year?
Finally, if you have grass growing on the roof, someone has to cut it. When a gasoline-powered lawnmower is used, the fire code requires the lawnmower be properly stored. If the building is fully sprinklered and there is less than 10 gallons of fuel, you can store the lawnmower inside. If the building is not sprinklered, the code requires the lawnmower to be stored in a room that complies with the building code requirements for mixed use. That could mean a one-hour fire-resistance-rated assembly and/or sprinklers.
What this all adds up to is additional requirements for installing a green roof. If the architect is unfamiliar with the requirements, it would be a good idea, for starters, to identify Section 317 of the 2012 Fire Code. This section is written in such a way that the requirements also can apply to existing buildings.
If the green roof is intended for energy purposes, it would be easier to add more insulation and install a reflective roof membrane. If it is for aesthetic reasons, you have to deal with these new code requirements.