FAQ

Green & Cool Roofs Frequently Asked Questions

What’s wrong with traditional black and silver-coated roofing?

Dark roofs absorb the heat of the sun, causing their temperatures to fluctuate drastically from up to 180° degrees Fahrenheit in direct sun, down to well below freezing in the winter.  This stress of temperature changes along with UV ray damage, cause roofs to wear out quickly – sometimes lasting as little as three to five years between major repairs.  Even silver-coated roofs only reflect about 50% of the sun’s heat, so they can heat up over 150° F.  Hot roofs contribute to the urban heat island effect raising temperatures in surrounding neighborhoods and increasing heat related health problems, such as asthma and heat strokes.  Hot roofs also greatly increase air conditioning costs, or make the upstairs of buildings unbearably hot.  Green and cool roofs both solve all of these problems.

What is a cool roof?

A cool roof is a highly reflective roof that absorbs less than 20% of the sun’s heat.  Cool roofs are generally bright white, but can come in other light shades of color.  Many large commercial buildings now use a white thermoplastic (TPO), EPDM, or PVC material to reduce air conditioning costs and increase the life of the roof.  Residential buildings typically go with a simpler white roof coating that rolls on like a thick paint.  Essentially any flat roof can be converted to a cool roof with the right preparation and coating.   A cool roof coating can cut summer utility bills in half on poorly insulated row homes, or even allow a homeowner to stop using air conditioning entirely.

What is a green roof?

A green roof is a rooftop with live plants protecting the waterproofing membrane from the heat and damage caused by the sun.  Some ‘intensive’ green roofs are built to carry heavy loads of soil or planters and have a wide variety of plants.  More typically, sedum or other succulent plants that don’t need much water are planted in a thin layer of aggregate and soil, called ‘growth medium’.  These lightweight green roofs are classified as ‘extensive.’

What are the benefits of a green roof?

• The waterproofing membrane of the roof lasts at least twice as long as a traditional roof, saving on maintenance costs.  Derry & Toms department store in Central London installed a green roof in 1938, and their roof membrane is still in good condition – never a leak  (Peck et al. 1999).
• A building with a green roof uses up to 50% less energy to cool and up to 20% less energy to heat saving substantially on long term utility expenses
• The soil absorbs rainwater saving the local municipality on storm water management costs
• The roof provides an aesthetically pleasing natural environment for birds and people to enjoy.
• Plants filter pollutants out of the air, cool the air by transpiration and produce oxygen.
• Green roofs can be low maintenance, and are generally cost effective investments.

How heavy are green roofs?  Could my roof support the weight?

The weight of a green roof depends on how deep and what type of soil is used.  Some lightweight systems can weigh as little as eight to ten pounds per square foot fully saturated with water, while more typical green roofs with four inches of soil weigh between 25 to 40 pounds per square foot.  Most row house roofs in Philadelphia can support an additional fifteen pounds of green roof material per square foot in addition to a heavy snow load, but having your roof double-checked by an engineer and at very least a roofing professional familiar with evaluating structural integrity is highly recommended.

What system of green roof does BioNeighbors use?

BioNeighbors’ Principal is certified by both LiveRoof and Elevated Landscape Technologies to install their assorted systems.  BioNeighbors has also had success pioneering their own low-cost planted in place variation that was developed partially through a partnership with Green Homeworks, Inc. and Philadelphia University, and has proven successful in Philadelphia.  This system has the essential components of a root barrier, drainage mat, soil retention system, growth medium, and a mixture of live sedum plants grown from plugs.  BioNeighbors tailors each roof to the preferences and budget of the roof buyer.

What maintenance is required?

Cool roofs that are coated need recoated every seven to ten years for maximum energy savings and roof life.  Green roofs can be as low maintenance as any other flat roof (check drains and exposed flashing yearly), but to keep them looking healthy, it is recommended that they are weeded three times a year or after any prolonged rainy season.  Also, if there is a long drought when the roof is still getting established, watering can help the plants stay lush.

Why do you use a mixture of sedum?  Could I plant grass or vegetables instead?

Sedum is a succulent plant that grows well in thin rocky soils and harsh climates, such as rooftops.  Grasses requires a minimum of four inches of soil with irrigation and six to eight inches of soil without irrigation.  Most vegetables also require more than the one to two inches of soil used on BioNeighbors special lightweight roofs.  To grow vegetables on roofs BioNeighbors recommends cool-coating your roof and then doing container gardening.  For optimal long term results most green roof experts recommending using at least five to eight varieties of sedum, so that if one variety doesn’t do well in a certain part of the roof or dies from disease another variety can take its place.

How much does all this cost?

Green roofs typically cost $9 to $30 per square foot to install, but BioNeighbors has been able to do a row house roof for as low as $4.00/ square foot or about $2500.  We’ve even done small porch roofs for as little as $500.00.  Many variables effect cost including what work needs to be done to the roof prior to installation, and the accessibility of the roof.  Cool roof coatings including trained labor by BioNeighbors and warranty usually run about $1.50 per square foot.

Where can I learn more?

You are welcome to visit more of our website or CONTACT BIONEIGHBORS for more information and/or a free roof consultation. You may also visit these informative sites:

Green Roofs for Healthy Cities has a comprehensive overview of the Benefits of Green Roofs.

Capitol Green Roofs has some good design sketches and questions for prospective green roof owners to consider.

Written and developed by Cory Suter, GRP for BioNeighbor Sustainable Homes LLC