July 2004 Guest Feature

The green roof industry has provided dictionary publishers with some new meanings to simple words. Don’t get me started on “extensive” and “intensive;” unless you can translate German, you may never really understand the true meaning of these terms. I am going to discuss three definitions that almost fall into the category of oxymoron: “Dirt,” “Value Engineering,” and the most magical of the three, “Green Roof.”

Green Roof

Those familiar with ASTM (American Society for Testing and Materials) may know what it takes to get a new standard approved. For those who don’t, the answer is ….. Years! So, during our second Green Roof Task Group Committee Meeting in 2002, some roofing manufacturer wandered into our meeting room and proceeded to take up almost an hour of our time arguing the point that the term “Green Roof” should not be limited to roofs that have a vegetated cover. This filibuster opened up a can of worms that included roofs of green color, roofs of white color that reflect heat, roofs made of recycled products, garden roofs, roof gardens, vegetated roofs, vegetative roofs, veggie roofs, grass roofs, planted roofs, sedum roofs, hippie roofs, thatch roofs, sod roofs, cool roofs vs. roofs that are not so cool, natural roofs, etc., etc., etc. After the hour was up we decided just to call it a green roof. We liked it because it sounded cool, or was it because green roofs are cool?

Back in my college days at Delaware Valley College of Science and Agriculture, my soils professor reprimanded the class for referring to soil as “dirt.” He said dirt is something that is tracked in onto the carpet. Politicians use it to gain an edge on their opponents. And in the green roof industry, “dirt” is something that the general contractor sneaks up onto the roof instead of the specified growing media. Most green roof professionals like the term growing media or medium, planting media, or root zone media. To avoid slip of the tongue and those confused stares, I like the term “engineered soil” although in actuality there may not be any soil in it whatsoever. However, one of the definitions of soil is the inclusion of particulate matter or substrate that anchors the plant roots to sustain plant growth. That is exactly what roof “soil” is supposed to do, but it does get complicated. For simplicity’s sake let’s call it just plain “media.”

It has to be understood that to maintain a green roof is very different than what is performed at ground level. Media designed for placement above structures may have a weight requirement, and lightweight mixtures are used for planting where it is desirable to reduce the media weight. On roof gardens or other large planting areas over structure, this can make a significant difference in the structural load. A planting media mixed with lightweight aggregate can reduce the saturated weight of ordinary soil by 25 to 40 percent.

Until recently, to provide a lightweight media suitable for green roofs and roof gardens meant bringing in a nursery mix consisting of amending sand with peat moss or pine bark. Over a short period of time, organics will decompose creating two specific problems for roof top gardens. First, the amount of mix decreases due to organic matter decomposition, requiring replacement - which means usually carrying product to the top of the structure in bags. Second, being of the most concern, as the organics break down, the fines filter out down to the separation fabric. Once on the bottom, the organic fines decompose further creating a slime, which may impede the drainage causing the water to build up in the media. This will also increase the weight and create plant health problems. Some structures have developed leaks because of this saturation problem. Many new to the green roof business will argue that organics in larger volume amounts is necessary. Well, as a proponent of compost for 20 years, I’m still a big fan, but not in excess on green roofs - no more than 10% to 20% in humid areas and not much more in the arid southwest.

The quality of compost is also very important. Another new word is “bio-solids,” just a pleasant sounding term for composted human waste (or sewage sludge for my more gentle readers). This type of compost has no business on a green roof, and in my opinion, anyone who argues against this point has probably been pressured by some government agency to help get rid of it. There is a lot of nasty stuff in this material including a heavy amount of a mysterious powdery substance that will end up on the filter fabric binding particles together causing havoc with drainage. Green roofs must be designed to drain and drain well, and concentrations of “bio-solid tea” into the local creeks don’t do much for the term “green.”

During the past several decades the performance of green roof media has been evaluated in Europe and it was determined that the most crucial physical property the planting media must have is good drainage. Recently this was made to be more complicated with the desire for the green roof to also retain a great quantity of water to not only reduce irrigation but also reduce runoff in urban areas. With this in mind there are six properties that a good media must possess to fulfill this requirement:

This leaves us with engineered soils as the only option, whether or not the project is extensive or intensive.

Components For Intensive

1. Lightweight Aggregate 35% to 75%

2. Sand 10% to 50%

3. Organics 5% to 20%

4. *Clay and Silt 0%-2%

Components For Extensive

1. Lightweight Aggregate 50%-100%

2. Sand 0%-30%

3. Organics 0%-40%

4. *Clay and Silt 0%

For intensive green roofs, the latest research has shown that a blend of coarse sand, porous lightweight aggregate, and only a 10 to l5% compost component is an acceptable planting media. The use of expanded slate, shale or clay as a lightweight component has provided a permanent, porous and structural benefit that also provides the cation exchange capacity that the reduction of the organic matter and clay in the mix left lacking. For extensive green roofs, the local climate and plant selection will dictate the media to be used but it is possible to consist of 80% lightweight aggregate and 20% compost.

We must keep an eye on what goes up on the roof. Contractors not familiar with the importance of the growing media may be talked into a substitution that will only satisfy the lawyers. This brings us to the last oxymoron… “Value Engineering” or what I like to call it, “De-value Engineering.”

As landscape architects we are expected to be competent in the design of landscape plans that the landscape contractor is expected to implement as specified. To design and specify without following proper horticultural practices may be considered by some to be incompetent, especially to a competent landscape contractor. On the other side of the fence, I have been discouraged many times by the compulsion by some landscape contractors to not follow specifications. This action may bite them back both professionally and legally.

In my opinion, the single most annoying and difficult part of protecting the design intent of a project occurs after the bidding process. It starts when the general contractor mutters the oxymoron “Value Engineering.” This by far is the dirtiest word in the business. It doesn’t matter who utters the phrase; it always lands in the lap of the landscape architect. Why is this, you ask? It’s very simple; the budget is blown at the end of the project, not in the beginning. The growing media and planting is almost always the last thing on the critical path to completion. A talented landscape architect can work with the project owner and work out design issues to reduce costs without totally jeopardizing the intent of the design. However, in the real world, we as landscape architects are usually the last ones to find out that our designs have been bastardized in the back office of the construction trailer by the sub-contractor (one of those bottom feeder types) and the general contractor. When the landscape architect finds out what has happened, he or she had better be ready to whip out a very convincing rejection letter to squash the attempted sabotage.

Let me let you in on the secret of this clandestine band of subversives. It usually starts early in the game, during the bidding process, and with no doubt the lowest bidder has already devised a plot to undermine the specifications and pad the low bid. These guys are easy to spot; they are usually the ones that were calling vendors for material prices the final day before the bid letting or just after when they are told they left something out. As the overruns pile up during construction, the budget starts to get squeezed. Our low bidder finally starts to breathe again; he’s ready to pounce. Soon the big day comes, the weekly job meeting begins, the construction superintendent asks for some “value engineering” suggestions for the remaining items on the punch list. Of course, our lowest bidder has some wonderful substitutions to suggest (excluding the ones that have already been secretly buried in the ground). What the general contractor doesn’t know, or is hoping not to find out, is that our bottom feeder has already prepared his co-conspirator vendor buddy to whip up a sample of his wonderful green roof soil product. He then pulls it out and shows everyone at the meeting how it can replace the more “expensive” specified product with another at a much lower cost. Now here is the secret that the shafted specified vendor will eventually find out: Already knowing it’s very late in the game, the low bidding sub-contractor has gotten the best deal on the substitution product early on and can now mark it up twice as much more than when he bid it under competitive conditions, thus making more money by “de-valuing” the job. This is not a conspiracy theory, it has happened on more than a few projects with which I have been involved. One situation like the above scenario was finally settled in a Georgia court between two subcontractors for an $800,000 judgment.

Now that we are savvy of the Bottom Feeder’s plot, how do we control it? It is up to us as professionals to challenge every modification made to our designs without our seal of approval. Most of the time, the offended vendor who just unfairly lost out to an unworthy competitor alerts us. He is more than willing to blow the whistle on the offending party and help you write a juicy rejection letter. The folks who get hurt the most by this practice are the good contractors who bid the projects to specifications, as they never had the opportunity to win the bid fairly. It seems no matter what the project management relationship is between the landscape architect and the general contractor, the budget rules. When this happens late in the project, proper communications between the parties do tend to drift. Sometimes the “approved equal” specification is ignored completely. The solution is for the landscape architect (LA) to get paid to manage that portion of the project and take hold of the contract and keep the green portion of the roof separate from the general contract. Final green roof work can be a separate bid item away from the general contractor’s budget. Granted, this sometimes can open up a new can of worms with scheduling issues, but someone other than the owner should never assume the LA’s control. Budget issues will always include the landscape plan; however, the LA and the Licensed Landscape Contractor can go over the line items together and work out the best way to take the “de” out of value engineering.

Everyone with professional integrity and the proper knowledge to stay current will benefit, and the long-term results will benefit everyone.

Chuck Friedrich is a registered landscape architect, member of the American Society of Landscape Architects, and member of the ASTM Green Roof Task Subcommittee E06.71. Chuck is also a representative of the Carolina Stalite Company, manufacturers of horticultural aggregates Stalite and PermaTill®, which are rotary kiln expanded lightweight aggregates.

Don’t Call It Dirt! By Chuck Friedrich, RLA, ASLA

Green Roof

Don’t Call It Dirt!
By Chuck Friedrich, RLA, ASLA