What Are the Pros and Cons of a Vegetative Commercial Roof?

As you’re cranking up the heat in the winter or AC in the summer, you might be interested in how having a “lawn” on your roof can help with energy efficiency. 

This post is part of our series about the pros and cons of the most popular types of commercial roofs: TPO, EPDM, metal, slate and tile, asphalt, and, this month, vegetative.

As a property manager or owner, the more you know about your particular roof and the options available when re-roofing, the better!

You can also see our Guide To Commercial Roofing Systems: Pros and Cons that covers highlights of each system in a PDF you can view online or download.

Vegetative Commercial Roofs

Groffs Family Funeral Home in Lancaster, PA (photo by Chesapeake Bay Program)

A vegetative roof has a living vegetative (plant) layer on top of a conventional flat or sloping roof. The vegetation is part of a professionally engineered and installed system that also includes other components, such as filters, a drainage system, and, sometimes, an irrigation system. 

• Modular systems and built-in-place systems: Modular vegetative systems include pre-planted trays with vegetation that’s ready to go. The trays can be moved for any needed roof repairs and can be replaced, if necessary. Built-in-place vegetative systems are more economical than modular systems but require more time and attention as the plants take root and begin to grow. It’s also more labor intensive to troubleshoot roof leaks with built-in systems.
• Extensive, intensive, and semi-intensive systems: Built-in systems are either extensive (shallow soil and plantings) or intensive (deep soil and plantings) or somewhere in between. Extensive systems are installed for their practical benefits while intensive systems add features for people to enjoy, like shrubs, trees, and walkways. As you might imagine, intensive systems require irrigation and continual maintenance as does any landscaped area. Semi-intensive systems allow some flexibility with depth and plantings, and require a moderate amount of maintenance depending on the design and planting choices. Intensive systems weigh 80-150 pounds per square foot while extensive systems can weigh as little as 15 pounds per square foot.

Chicago City Hall

Vegetative roofing systems offer substantial economic and aesthetic advantages and some drawbacks as well.

The Pros

• Increases energy efficiency — plants act as a buffer, keeping roofs cooler in summer and warmer in winter, which reduces heating and cooling costs (estimate your savings with this energy calculator) 
  

  Air Force Space Command, Colorado (photo by Thea Skinner)

• Higher R-value (thermal resistance) than conventional roofs, with actual thermal value depending on how much water the plants are retaining at the time
• Improves stormwater management reduces overall runoff, runoff contaminants by up to 85%, and drainage system peak volume-rate requirements  
• Improves the local environment by reducing airborne contaminants and the urban heat island effect
• May require less roof maintenance, depending on the system
  

  Air Force Space Command, Colorado (photo by Thea Skinner)

• Extends roof life-cycle — protects roof membrane from punctures, ultraviolet (UV) rays and extreme temperature fluctuations
• Can contribute to LEED® Certification
• Can be very easy to install, with options including systems with filter, drainage and geotextile in a single roll
• Adds visual interest and attractive qualities — if accessible, vegetative roofs can improve moods and reduces stress

The Cons

• More costly at installation — vegetative roofs can cost twice as much as a conventional roofing systems initially
• Additional installation materials and processes may be needed, such as sacrificial strips, increased flashing height, vapor retarder and, for intensive systems, root barriers
• Landscaping-type maintenance required for some systems, which may require additional outsourcing
• Difficulty finding roof leaks — technicians must remove vegetative layer sections to find and repair leaks (easier to do with a modular tray system)
• Can lead to unwanted wildlife and insects
• Intensive vegetative systems can be heavy

Warranties

Warranties for vegetative roofs vary greatly. It’s a good idea to choose one manufacturer for all of the system components. A single-source warranty will typically include the cost of removing the plantings to find the source of a roof leak. It’s very important to have an experienced designer engineer your vegetative system and a qualified roofing contractor install it. Warranty coverage limits also vary from manufacturer to manufacturer (SkyScape systems from Firestone, for example, are eligible for protection under the Firestone Red ShieldTM) so make sure you discuss what’s best for your goals with your roofing professional.

Sentry Roof is a proud to have completed dozens of vegetative roofing projects for clients — in the right circumstances, green roofs can be the ultimate in value and longevity. Our crews have the necessary training and skill sets, and Sentry is a member of Green Roofs for Healthy Cities, a professional association that is also an excellent resource for you for more information about vegetative roofing. If you’re interested in exploring the possibility of a green roof, we’ll be glad to discuss it with you or come take a look at your property.