May 2006

Perlite on Green Roofs
By Jennifer Appel, ASLA, RLA, LI

Publisher's Note:  On May 6 through May 11, 2006 Jennifer Appel was teaching classes at the International Perlite Institute’s Annual Meeting in Buenos Aires, Argentina.  The event included classes on how to design lightweight Perlite green roofs and how to market Perlite to landscape architects, architects and engineers for green roof construction.

 
There was a building whose owner wanted to have a lush green roof - an outdoor “room” including shade structures, benches, lighting, fountains, an outdoor kitchen and waterfalls to rain harvesting structures.  Big dreams but the client did not want to spend the money to reinforce the structure to carry the weight of the grand design.  The landscape architect thought and thought.  How can we reduce the weight of the media in order to achieve the client’s dream?

Why Use Perlite on a Green Roof?
Something that needs to be white and not transfer heat from the roof to the plant roots would be good. Something that is organic, (carbon based), yet takes a very long time to break down.  The product needs to be very light weight yet strong enough to stabilize mature trees without staking them to a structure.  Something that when saturated weighs at least 10 pounds less than any other material.  Is there a product with a proven track record in horticultural growing operations that can be used on a roof?  Ah ha!  The light bulb went on.

What is Perlite?
Perlite is a white, lightweight, organic media that is used to increase air porosity for planting mixes in horticulture.  Perlite is a recyclable material that comes from diamond mines.  It is the gray carbon material that does not turn into diamonds.  Air is blown into the gray material and it turns white and puffs up like popcorn.  Perlite was commonly used for bean bag chairs during the early 70’s.  It does not cause a reaction with waterproofing membranes.

Weight
The weight of a Perlite green roof can vary widely.  We have blended wet medias that weigh as little as 2 pounds per inch of depth to as much as 50 pounds per foot of depth depending on the grade of the Perlite that is being used and the volume of packing peanuts that are incorporated into the mix.  The saturated weight will depend on the design of the drainage system slope and whether the roof drains to penetrations or the outside of the building.  Horticultural grade Perlite when saturated and drained will weigh about 32.5 pounds per cubic foot.  When packing peanuts are added to the mixture as the large aggregate, the media weighs about 24 pounds per cubic foot.  See the chart below for a comparison of some media weights in pounds below:

Design
Perlite does not float or blow away when the green roof is designed correctly.  See detail below of Sarnafil (rick.chappell@sarnafilus.com) waterproofing with wind edge restraint and straps encasing the Perlite pillow.

Detail Courtesy Rick Chappell of Sarnafil.

The trick to using Perlite in any green roof design is the installation.  Complete the waterproofing phase and tape or glue the Bio Barrier, a 25 year warranty filter fabric with chemical nodes that repel plant roots that come within 2” of the fabric, see picture below...:

Proper preparation and installation of Perlite is key.

...and the drain and filter/drainage fabric to the waterproofing protection board, see picture below:

Photos of the Historic Humble Marriott by Jennifer Appel.

Install the irrigation and remember to include a Vitamin Tank for adding Compost Tea.  Lay the Pyramat over the area and make sure there is about 6” extra on each side for “wrapping.”  (Pyramat is commonly used for slope stabilization for geotechnical applications. Contact Randy Wilkins at Storm Water Structures, rwilkins@stormwaterstructures.com).  Tape, glue or zip tie the Pyramat on 3 sides to the drain fabric.  Cut holes for the irrigation heads to be pulled through the Pyramat.  Leave one side open.  Fill the “pillow” with packing peanuts first and then Perlite.  Close the 4th edge of the pillow. I  punch holes in the Pyramat and drain fabric and use zip ties to lace the pieces together.  Zip ties are a simple solution for their ease of use and they are cheap and easy to get at any hardware store.  The zip ties will be covered by the wind lift restraint of the Sarnafil edge so don’t worry about people seeing it.  (They won’t!)

After the pillow is complete, add about 2” of compost on top of the Pyramat.  Add 1 pound of Granular Vitamins per 100 square feet to feed the compost and get it ready to grow plants!  Lay out the plants and run a test of the irrigation system to make sure the plants get enough coverage and that you have not located a plant directly in front of the irrigation head.  Now you are ready to install the plants.  Cut slits in the Pyramat and plant the plants.  Test the irrigation system and water your plants with Compost Tea to get them started.

The installation pictured below is the Marriott Green Roof with 2.5’ of Perlite and packing peanuts with 2” of compost cover.  See the project profile in The Greenroof Projects Database here.

Two and a half feet of Perlite and packing peanuts are covered by 2" of compost cover.

Compaction
Compaction is only an issue if the Perlite is walked upon.  In areas where there are no trampling feet – we noticed only about 2” of settling per every 2.5’ of media over a period of 3 years.

Photos of the Historic Humble Marriott by Jennifer Appel.

Growth
In side by side tests, the plants grown in Perlite established faster than the same plants grown in structural soil.  (Faster vegetation establishment means fewer weeds, less maintenance, less water needs, fewer wind lift issues and happier clients!)  Perlite allows the roots of plants to grow through it faster than in structural soil such that the roots from one plant can intertwine with the roots of the next plant creating a “mat” that reduces wind lift issues.

Planting trees in Perlite
Trees do not need to be staked in Perlite because the density is low enough to allow the roots of one tree to bind quickly with the other tree or plants – thus keeping the tree straight and creating a quickly developed root mass that acts as a stabilizer to reduce wind lifting issues in hurricane prone areas.  In side by side applications on a green roof, 12’ crepe myrtle trees planted in Perlite are standing perfectly erect after 3 years while their counterparts planted in structural soil started leaning within 7 months.

A beautiful way to combat Houston's urban heat islands!  Design and photos by Jennifer Appel.

Watering
Another great aspect of Perlite is that is has a low water holding capacity.  Low water holding capacity means the media does not expand or increase in weight with the addition of water.

Perlite grown plants do not need more water than those that are planted in structural soil if the designer covers the Perlite with 2-4” of finished biological compost.  (Specifications for finished biological compost may be downloaded for free from the www.landscapevitamins.com test results page.)

If drip irrigation is being used: the manufacturer states that drip must operate daily to remain under warranty.  Drip systems are designed to give an inch of water per week. (That is 52” of water per year.)  Most plants used in green roof environments only need about 20 - 30” of water per year.  In humid climates a green roof may not need irrigation as the native plants can “pull” moisture out of the air.  One quick note: in our experience, native plants need about ½ the water of a genetically modified plant.

If the irrigation is set such that the plants receive 52 inches of water per year and a percentage of that water stays on the roof (a perched water table) there is a possibility of the plant roots standing in water.  Additionally, standing or ponding water has a tendency to go anaerobic.  Anaerobic water will kill most plants (think of your house plants sitting in water – they turn yellow, die and stink).

Plants grown in Perlite need good drainage and do not need a perched water table. This aspect of the design may be important to building owners who have fears about leaks. If no water is perched on the roof – there is less opportunity for leaks to occur from standing water.  Most waterproofing membranes incorporate some type of biocide aimed directly at anaerobic growth on top of a membrane which can affect the growth of the plants and the quality of the perched water table under specific conditions. T he perched water table detail used for structural soil blends should not be used for a Perlite green roof.  Perching water under a Perlite green roof will cause the plants to stunt their own growth to survive and may cause anaerobic conditions which may be detrimental to the entire project.

Cost
Perlite is far more cost effective than most of the structural soil blends.  Approximately 450 pounds of Perlite (enough to cover 160 square feet at a 4” depth) in Houston costs about $150.00 plus installation time.  (Costs for Perlite in your area may vary with transportation costs . A great source of Perlite is Ron Maurer at Schundler Perlite (www.schundler.com).  Perlite can be blown on to a roof with an insulation or mulch blowing truck – thus reducing the time to install a green roof as well as reduce the installation cost.

Climates
I do not have enough personal experience with Perlite in cold climates to ascertain the viability of using Perlite in locations that are subject to prolonged periods of freezing.  It has snowed in Houston since the first Perlite green roof was installed.  We did not lose any plants or trees and we did not cover anything.  I know Perlite is used as the only planting media at the Schundler Perlite Company in New Jersey – and it freezes there – and they haven’t lost any plants either – but insulation of the parapets and location of plant materials with respect to wind shear is a critical design issue that must be considered.

Maintenance
Perlite roofs should not be maintained with synthetic chemicals.  Primarily because plants used in green roof environments can only use about 8 pounds of Nitrogen PER YEAR!!!  In a structural soil, the excess Nitrogen (20-20-20) ends up dissolving within the media or runs off the roof polluting ground or surface water sources.  High Nitrogen fertilizers tend to burn the roots of plants planted in Perlite, because there is no mechanism for the synthetic chemicals to break down prior to being deposited on the plant root, drainage layer or roofing membrane.

We solved the maintenance issues for Perlite green roofs by incorporating our Compost Tea through our patented Vitamin Tank on the irrigation system at a rate of one gallon per 1,000 square feet once per week.  The Compost Tea should be applied immediately to the green roof.  Most 3,000-5,000 square foot green roofs will use a 3 gallon Vitamin Tank.  Compost Tea is Liquid Compost (when it is made correctly).  A gallon of Compost Tea may have the same positive effect as adding a yard of compost but weighs only 10 pounds per gallon.  Compost, which weighs approximately 1200 to 2200 pounds per cubic yard, is what the forest uses to feed itself (and it seems to do pretty well without additional help).  Properly made Aerobic Compost Tea does not burn plant roots and has the proper biology to grow healthy low water requirement flowering plants.  Information about making your own Compost Tea with Granular Vitamins can be found at www.landscapevitamins.com.  For those readers who do not have a microscope or a solid understanding of soil microbiology, I would only recommend compost tea that has been tested by www.soilfoodweb.com or a Soil Food Web Advisor or you might be buying very expensive dirty water.

For those green roofs that do not include an irrigation system, add 1 pound of Granular Vitamins per 100 square feet each month to increase plant establishment and carbon sequestration or longer bloom periods.  The Granular Vitamins will help the plants to establish faster and use less water than a conventional fertilizer.

Retrofit sloped residential green roofs?
I know I have argued against a 4” green roof in the Houston area for 6 years, but we did it and without any compost or irrigation.  The outer edge restraint is Geofoam – and it had no trouble staying in place through Hurricane Rita last year and has taken a considerable beating when we remove structural components, caps or plant materials and replace with new concepts for evaluation.

This test has only been active for two weeks. The plants have doubled in size.  It is pretty obvious that it was planted and left unattended except for watering it twice and feeding it with Vitamins to get it started.  It rained here three times and one had wind gusts in excess of 40 mph with no loss of media.  The Grasspave2 has had a positive effect on media stabilization.

Although we were trying to design a media that weighed about 20 pounds per square foot, this project weighed in at about 5 pounds per square foot with a 4” depth of Perlite.  The mock up on the Tuff Shed below helped us to understand how little a green roof needs attention.  We learned that native plants could survive without additional water when genetically modified plants did not.  It was watered 6 times between March '06 and May '06 and fed monthly with Granular Vitamins.  This test was designed as a mock up for the Lady Bird Johnson Wildflower Center Test Green Roof Plots (Austin, Texas). 

Publisher's Note:  See the Lady Bird Johnson Wildflower Center project profile in The Greenroof Projects Database.

The following pictures include: waterproofing, parapet walls, bio barrier, drain fabric, Perlite and Grass Pave2 before planting, at planting and three weeks after planting.

Waterproofing, bio barrier, drain fabric & Grass Pave2 before planting;

Waterproofing, bio barrier, drain fabric & Grass Pave2 at planting time;

Waterproofing, bio barrier, drain fabric & Grass Pave2 at three weeks after planting. Photo taken on April 3, 2006 by Jennifer Appel.

Design Assistance
When designing a green roof the designer must have a working knowledge of structural systems, wind and cultural factors, plant growth characteristics and watering requirements.  Knowledge of these requirements combined with Perlite, foam, compost and packing peanuts have allowed us to assist Architects, Landscape Architects, Engineers and home owners to create lightweight landforms, beautiful green roofs on steep pitched roofs and use a wider variety of plant materials to create lightweight outdoor rooms than previously thought.  My archived June 2004 Greenroofs.com article contains some early design information about Perlite use in green roofs.  It seems that we are always learning something new about Perlite and green roofs… more to come with each new project!


Jennifer Appel is a registered landscape architect and licensed irrigator in Texas, with a BLA from Texas A&M in 1993. She is the owner of landscapehouston.com, a design build firm specializing in innovative and low maintenance landscape architectural solutions to high end residential and commercial drainage clients in Houston, Texas.

Jennifer is also the developer and proprietor of Jennifer’s Landscape Vitamins, maintenance reducing feed products for green roofs. www.landscapevitamins.com.  Ms. Appel can be reached at 713.263.1682 or at design@landscapehouston.com.