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What Remodeling Contractors Worry About When Asked to Bid a Green Roof

on August 15, 2014 at 9:00 am under

 By Michael Chotiner


Traditional Scandinavian sod roof home.

The crowd I hang with these days — professionally speaking — consists mostly of small builders and remodelers.  To find inspiration for home improvement articles, I often prowl online professional forums like, forums.jlconline and to see what the pros are saying to each other about their work.  The topics most often involve structural problems and power tools rather than innovative gardening techniques.  But increasingly, there’s buzz about green roofs from remodeling contractors who may have never before considered installing one.  Small residential contractors are getting a chance to bid on them, and in the absence of much experience, they’re asking their peers for advice.

Here’s a typical thread-starter from the jlconline forums, by a West Virginia contractor:

Green Roofs in Cold Climates?
Looking at a set of preliminary plans that include a roof that is to be an “extensive green roof.”  Roof size to be 20′ x 44′.  There will be an access stair and door to roof.  The entire roof will be surrounded by solid 3′ tall sided walls with 4 scuppers to drain.  Plans call for:

“TYP. ROOF CONST: 14″ TJI @ 16” OC W/
JOISTS, & 1/2″ GWB CLG.”

The area in which it will be built averages 60+ inches of precip annually including 150″ – 250″ inches of snow.

My first thoughts: Hot or unvented roof assemblies are problem prone in this area.  Recessed lights cannot be spray foamed in and even if they were it would be impossible to get min.  R 40 above them.

I know it would look really cool, and I don’t want to say it to loud, but this does not seem like a good idea to me and likely to be nothing but problematic with the snow and freeze thaw cycles we get around here.  So am I being a spoil- sport here?  Any thoughts appreciated.

The post elicited many responses — most of them constructive.  One colleague from Toronto posted a sample of a green roof that had been completed in his area, noting that it, too, was subject to extreme cold and frequent freeze-thaw cycles.  But he also posted this cautionary note:

“Estimated membrane life of 35 years.  Anybody care to calculate the cost of removing 6 inches of soil etc. in case of a failure before then?”

There were also questions and comments about the spec for the I-joists (too small and far apart for the likely load), provisions for installation of the lights, ductwork and insulation.  The best technical advice came from a knowledgeable Portland, Oregon, architect who was able to suggest specifics for installing tapered insulation, protective sheathing, two waterproofing membranes and a drainage mat above the primary roof deck and beneath the planting medium.

WFYI Indianapolis-WinterSpring

WFYI Indianapolis (IN) offices in winter and spring using a single source modular system from Omni Ecosystems.

Professional Roofer’s Question About Green Roof Prep
I also saw this post on

“We’re doing a TPO roof that will have a green roof on top of it (by someone else).  What can we expect when we’re finished and out of there?  What precautions should be taken with regard to the warranty, etc.?”

A Chicago roofer replied with questions of his own:

“First off, was the roof designed to be a garden roof?  I mean is it 45-mil TPO?  Is it mechanically attached?  Was the insulation mechanically attached?  These would be no-nos.”

The response went on to advise that when the contractor finishes the TPO application, he should perform a “flood test” — block the drains and flood the roof — literally, try to make it leak.  This way, he can spot and fix any problems before the roof is covered.  He also advised applying a separation layer between the roof and planting medium once he was sure that the waterproofing layer was sound.  Finally, he suggested that the contractor contact the roofing manufacturer to find out what warranty limitations might apply to roofing covered by a garden.

Another respondent, a forensic engineer based in Colorado, advised the roofing contractor to have one of his own workers on site throughout the garden installation to make sure that the roofing wasn’t damaged in the process, and to make repairs immediately if it was.


Various stages of the custom built-in-place design at The Ohio State University Green Roof at Howlett Hall in Columbus by Higher Ground Green Roofs, Roofmeadow, rooflite®, Moerings USA / Sempergreen and more.

Reaching the Small Contractor Market
What I’m seeing in the forums tells me that there is growing demand in the small-contractor markets for green roofs, and a need for technical and how-to information that enables residential contractors to take on green roof construction with confidence.

Yes, a number of good courses are available, online and elsewhere, as well as books for small green roof construction.  Providers of green roof systems and components make a lot of good information available in less formal formats, including pamphlets, brochures and other collateral marketing materials.


Wind Dancer has a custom built-in-place greenroof on a single family home in Sonoma County, CA from the design/build firm of SYMBIOS ecotecture.

But for the small contractors in my crowd, each job has a life of its own. Most remodelers are familiar with structural and waterproofing standards for the sloped roofs that are typical on most homes around the U.S. Most residential contractors are comfortable re-roofing with asphalt shingles and metals.  They all have the circular saws they need to cut roof framing when it needs to be repaired or reinforced, drill/drivers and impact wrenches for sistering joist and rafters, and pneumatic nailers for getting shingles down in a hurry.


A built-in-place custom designed vegetated roof, Troy’s Green Roof is located in Tacoma, WA. Troy Wagner, the homeowner, is a roofing contractor.

But few have ever worked with built-in-place systems, lightweight modular planting panels or modules, and retaining systems (now available for green roofs in sloped applications), and most contractors don’t often work with the waterproofing membranes and root barriers that must be installed beneath a green roof.  And, if the system is built-in-place, it will also require drainage materials.  The engineered soil, or growing medium, should be site specific along with the appropriate plant selection.

While remodelers are often familiar with options for waterproofing flat and low-slope roofs — the kind that lend themselves to intensive green roof installations — they’re more likely to sub out the work to commercial roofing specialists, often because it doesn’t make sense for them to acquire the training and equipment necessary for jobs that they might do only occasionally.  Not many remodelers have heat welders required for sealing the plastic waterproofing membranes used beneath some green roofs.  I wonder how many commercial roofers bid green roofs without fear, unless they have received specialized training!


The Daniels Residence in Toronto uses a modular system from Vegetal i.D., a single source green roof provider.

To make good bids and reduce their risk, residential builders and contractors need the help of professional engineers to evaluate the load-bearing capacity of the roof frame and the loads that the finished project will be subject to.  They need warrantable materials and designs for insulation and waterproofing layers, bulletproof protection for the roofing and drainage systems that will lie beneath the greenery in a specific environment. They need training in maintaining and repairing green roofs over their service life.  What they don’t need are callbacks and liability for things they can’t control.

The green roof market is well established across North America, and professional organizations and companies support their members through initial and ongoing education, accreditation, and yearly conferences. Single source suppliers of green roof systems require installation by authorized applicators and installers. Their roofs may be warranted.  If you are going the independent, custom design route, you must become familiar with the standards of the industry.


Built-in-place custom Private Kiawah Island, SC Residence green roof by Living Roofs, Inc., a design/build company.

It takes a village to make a good green roof — at least an engineer, an architectural designer, landscape architect, designer or horticulturalist; product support and, of course, a competent roofing contractor. Manufacturers and proponents who wish to promote and support green roof options are urged to reach out to the growing market of small residential contractors through the communities that they establish online and in their local markets.


Home improvement expert Michael Chotiner writes about tools and home projects for Home Depot.  Using his experience as a general contractor, Michael provides tips and insights on the use of materials and power tools for roofing and other projects.  To view many of the power tools available at Home Depot, you can visit the website’s power tool section.

Comments or questions for Michael may be sent to Laura Nehse of Home Depot at:




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