Hot Fluid Applied Green Roofs

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Author Message Randall A Chapple, AIA, SE, CCS, LEED AP Member Username: rachapple Post Number: 3 Registered: 12-2005 Posted on Saturday, May 12, 2007 - 09:53 pm:    Does anyone have any experience with wind loads on hot fluid applied green roofs? The roof manufacturer's will provide a 70 mph blow-off warranty but the code specified base wind speed for most of the US is 90 mph. I have not seen any other wind design requirements from the manufacturers. I am working on a high rise and I have wind uplift pressures from wind tunnel testing of 30-40 psf. Is it appropriate to specify the wind tunnel uplift loads? Does the 70 mph mean anything? Mark Gilligan SE, CSI Senior Member Username: markgilligan Post Number: 167 Registered: 05-2005 Posted on Sunday, May 13, 2007 - 04:38 am:    I find it interesting that Architects typically do not contact the structural engineer regarding roof wing pressures. Maybe somebody can explain why this is so. As a result I am admittedly on the learning curve trying to correlate how the roofing industry has historically addressed this issue and relating it to wind pressures. First saying a roof is rated for 70mph is not meaningful unless you describe wind exposure classifications, building geometry, building height etc. Each of these factors can have a real impact on the wind uplift pressure. Uplift resistance in pounds per square foot is what you really should be concerned about. You get theise pressures from either the structural engineer or from wind tunnel tests. In addition you need to be clear whether the wind speed is the 3 second gust or the fastest mile wind speed. 70 mph in the fastest mile speed corresponds roughly to 85 mph based on the 3 second gust. The current codes are based on the 3 second gust. Thus the 70 mph for a 3 second gust seems low. In your case you have it easy since you have wind tunnel results which give you the pressures directly. Thus you can ignore the wind speed. I would hope that the manufacturer’s rep could tell you whether their system is appropriate. J. Peter Jordan Senior Member Username: jpjordan Post Number: 238 Registered: 05-2004 Posted on Sunday, May 13, 2007 - 12:16 pm:    Mr. Gilligan's comments are helpful; however, manufacturer's warranties and design parameters are not always congruent. It is my understanding that most roofing manufacturers and installers do not routinely warrant membrane roofing systems for windspeeds greater than 73 or 74 mph, period. At that point, it is the insurance that kicks in, not the warranty. This is why insurers (like FMG) are interested in roof assembly design. The FMG ratings (I-60, I-75, I-90, etc.) indicate a maximum uplift pressure that the field of the roof assembly is designed to withstand with a safety of factor of 2. A roof assembly with an I-75 rating should be able to withstand uplift pressures in the field (not at edges or corners) of 37.5 lbs per sq. sf. From FMG's perspective, the roof assmbly includes the membrane, coverboard and insulation (if any), and structural roof deck (wood, concrete, or steel). For many single-story commercial buildings located in a surburban environment, a system designed to meet FMG's I-75 requirements will meet requirements for 110 mph (3-second gust) wind loading which is mandated by the City of Houston Building Code. Many of us have, for one reason or another, designed around FMG's I-90 ratings (owner requirements, experience, local custom, etc.). My recent inquiries indicate that this is probably an over designed system, especially in most of the US where the local code requires design to wind loads resulting from a wind speed of 90 mph. I have begun to pay a bit more attention to this expecially since FMG increased their fastener requirements last year. I usually try to consult with the structural engineer for wind load requirements and have begun referencing those as well as ASCE/SEI 7. I believe that local AHJs will accept wind tunnel criteria for wind loading criteria although some may want to review the testing procedures and results. It should also be noted that warranties have exceptions for damage to the roof membrane caused by impact from debris. So if you do have a wind storm event with maximum gusts of 60 mph and roof membrane failure is due to wind-blown debris tearing the membrane allowing the wind to peel it back, it will be your insurance that pays, not the warranty. This is something that you may always want to keep in the back of your mind. As windspeeds increase so does the likelihood of damage from wind-blown debris. Anonymous   Posted on Monday, May 14, 2007 - 11:47 am:    What precisely do you mean by "hot fluid applied green roofs?" Are you referring to vegetated roofs? or roofs made of low albedo or reclaimed materials? (I assume you do not mean green color!) Re "vegetated roofs," wind (as well as fire-performance) has been virtually ignored by both the green roofing industry and authorities having jurisdiction. In a conversation I had with a Miami code official, this non-attentiveness may be because the blow-off of vegetated roof components is not considered significantly hazardous or because there is insufficient data to formulate requirements. Presently, the industry addresses only the roof membrane as if there were no vegetation above it. It would be wise to verify for each project what the code official wants. Wayne Yancey Senior Member Username: wyancey Post Number: 358 Registered: 05-2005 Posted on Wednesday, May 16, 2007 - 10:22 am:    "Hot fluid applied green roofs" may be referring to American Hydrotech's Garden Roof Assemblies in which the roofing membrane is their hot fluid-applied rubberized asphalt (MM 6125). Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 550 Registered: 07-2002 Posted on Wednesday, May 16, 2007 - 12:44 pm:    you need a roof that is rated for I-90 and the roofing manufacturer can tell you that. it generally has as much to do with the installation method as anything else, and you will probably need to have a fully adhered system. I would suggest that in the future, you define your terms "green roof" more carefully and also pay attention to what the code actually requires. You can't submit something that "seems" equivalent -- if the code requires an I-90 uplift, you will need certification that it has been tested to that number. if the roofing company can't provide that information, I would find another roof. Code authorities are not in the testing business. they are in the verification business so they need information in a form that will make it understandable to them and to the lay public if it goes to court. Ronald L. Geren, RA, CSI, CCS, CCCA, SCIP Senior Member Username: specman Post Number: 458 Registered: 03-2003 Posted on Wednesday, May 16, 2007 - 02:14 pm:    FM 1-90 designed roofs may far exceed what is actually required. The application of FM Global's Loss Prevention Data Sheet 1-29 is entirely voluntary, unless the owner is insured by FM Global. The International Building Code only requires ASCE 7 for determining wind loads on a structure. And, if you haven't seen the changes to FM 1-29 lately, you should do so. For a FM 1-90 designed roof, the perimeter needs to be designed to FM 1-150 and corners to FM 1-225--extreme overkill in my opinion (and we complain about structural engineers and their safety factors--sorry, Mark G., I couldn't resist.) Mark Gilligan SE, CSI Senior Member Username: markgilligan Post Number: 170 Registered: 05-2005 Posted on Thursday, May 17, 2007 - 01:44 am:    As I have previously said our structural designs are excessive if there are no problems and unconservative if there is a failure. The FM recommendations are probably about right. Per ASCE 7 the pressures at the corners can be about 2.5 times that experienced in the field. Thus 45psf*2.5*2.0=225 psf. The good part is that this is only a small region. The bad part is that if you do not get the corners right this is where the failure will likely occur. An interesting perspective on the 2.0 factor of safety. The theoretical factor of safety for a steel roof is about 1.7. Thus if the stars were aligned right you could theoretically have the structure failing before the roofing. In reality the structural FS is typically higher and the field installed resistance of the roofing to uplift is likely less than the test values. Randall A Chapple, AIA, SE, CCS, LEED AP Intermediate Member Username: rachapple Post Number: 4 Registered: 12-2005 Posted on Thursday, May 17, 2007 - 09:23 pm:    OK for those who are stickler the roof system is vegetate protected roof, with perimeter pavers, over a hot fluid-applied membrane (similar to Hydrotech) over a concrete deck. The applicable code is the Chicago Building Code and FM insurance requirement do not apply. I am not sure that FM applies at all because you cannot test a hot fluid-applied membrane on a concrete deck even though the manufactures can get an FM rated roof when required for insurance. I believe the point about warranty and insurance requirements is a valid one based upon some research that I have done and it may be valid to specify the 70 mph blow off requirement to provide a line in the sand where the warranty stops and insurance begins. The wind tunnel loads are in excess of ASCE 7 used by the Chicago Code and therefore should be the design requirement. 40 psf negative pressures equate to a wind speed in excess of 100 mph. I would just like to know that the insulation paver, and vegetative roof components will remain in place for the design loads! Anonymous   Posted on Friday, May 18, 2007 - 07:52 am:    If the Chicago code official wants assurance that the vegetative roof components will stay put, I think you have a difficult choice: Either invest in costly calculations of wind loading and how the assembly of assembled vegetative roof components will behave, or find a manufacturer who will warrant that the vegetative roof components will withstand the wind loading and obtain the testing or calculations from that manufacturer. I have found wind warranties only for the roofing membrane. If you find a manufacturer who grants wind-condition warranties for vegetative roof assemblies, please post it to this group. Thanks. Susan McClendon Senior Member Username: susan_mcclendon Post Number: 59 Registered: 01-2005 Posted on Monday, May 21, 2007 - 07:49 pm:    The wind speed at which the manufacturer ceases to warrant against blow-off varies from mfr to mfr and from system to system and is not usually negotiable (meaning if they don't offer it, specifying it will do no good whatsoever -- you get either their standard warranty or no warranty at all). You need to know what the mfr will offer, so the warranty details become a product/manufacturer selection issue. The maximum wind speed is also often a lot lower than you would expect, mostly because they don't want to raise the cost of their roofs -- they expect that owners won't want to pay extra up front for something their insurance would cover if it happens. It's a good idea to get the owner's concurrence on the risk -- some industrial and public owners are self-insured, which may change the equation. It's hard to explain to a self-insured owner that the roof covering is not really expected to withstand winds that the building must withstand -- when his roof has just torn or blown off. I guess that would be hard to explain to any owner, but it's true. I don't know whether ASCE 7 covers the vegetated cover on a green roof, but it wouldn't surprise me if the behavior in wind of vegation was not comparable to any other component of the roof, nor calculable. Remember what happened with loose-laid ballasted roofs before they figured out that the corners needed more ballast? The gravel rolled and piled up leaving corners bare and ripe for blow off. The current ballast recommendations were derived using wind tunnel testing because the ballast didn't behave the way it was expected to based on its weight. Vegetation probably has some different behavior, since it usually sticks up into the wind -- weight is probably not a sufficient measure. I'd be astonished if a roofing manufacturer would warrant the vegetation and related components against blow-off -- unless he had complete control of them. It seems to me that a vegetated roof covering is more like a ballasted loose-laid roof than anything else, since you can't screw the plants down. ICC IBC requires ballasted roofs to comply with SPRI RP-4, which is all about blow-off, which is different from the wind uplift resistance of the structure or cladding. John Bunzick, CCS, CCCA Senior Member Username: bunzick Post Number: 718 Registered: 03-2002 Posted on Tuesday, May 22, 2007 - 08:41 am:    I've pondered this issue as well. Depending on design, the soil with roots and stabilization fabric could act as a membrane, making it more resistant to blow off. Europe has been doing these systems for decades, so it would be interesting to see what their experience has been. As to the warranty issue. Just because the warranty only covers up to a given wind speed does not mean that the roof can't survive significantly higher speeds. This is attested to by the fact that many FM tested systems have a warranty well below what they've tested for. It's useful to remember that warranties are primarily a marketing issue, not a technical one, and the owner is paying for it. She may be better off financially without the warranty if she has insurance to cover the failure. Why pay twice? Bob Woodburn Senior Member Username: bwoodburn Post Number: 188 Registered: 01-2005 Posted on Tuesday, May 22, 2007 - 10:03 am:    Is there a point at which a membrane covered by dirt and vegetation could be considered underground waterproofing instead of roofing? William C. Pegues, FCSI, CCS Senior Member Username: wpegues Post Number: 664 Registered: 10-2002 Posted on Tuesday, May 22, 2007 - 10:41 am:    Bob, Yes, except for very few exceptions, when it is at grade level, then its waterproofing. Otherwise, it should be considered a roofing membrane Randall A Chapple, AIA, SE, CCS, LEED AP Advanced Member Username: rachapple Post Number: 5 Registered: 12-2005 Posted on Tuesday, May 22, 2007 - 10:51 am:    We have been using an erosion control mat over the soil prior to planting to stabilize the growth medium. The mats are staked down to prevent uplift. The mat can be permanent or temporary until the planting gets established. I would caution against temporary mat as they can clog cooling towers when they erode. Bob Woodburn Senior Member Username: bwoodburn Post Number: 189 Registered: 01-2005 Posted on Tuesday, May 22, 2007 - 11:14 am:    Thanks, William. So, after all, the solution is simple: Just build underground! (I said simple, not easy...) John Bunzick, CCS, CCCA Senior Member Username: bunzick Post Number: 719 Registered: 03-2002 Posted on Tuesday, May 22, 2007 - 12:53 pm:    Going back to the 60s with earth sheltered housing! Mark Gilligan SE, CSI Senior Member Username: markgilligan Post Number: 172 Registered: 05-2005 Posted on Wednesday, May 23, 2007 - 12:50 am:    ASCE 7 does not explicitly address anything above the structural roof. Thus it done not address any specific roofing systems, nor does it cover blow-off or movement of the ballast. If you design a vegetated roof cover on a green roof you will need to determine the wet and dry weight of the vegetated cover. My understanding is that there is an ASTM standard to determine the weight. If you are even contemplating a vegetated roof notify your structural engineer early since this added weight could cause him to have to re-do a number of early calculations. This is definately not something you can add on at the last moment without consequences. Lynn Javoroski CSI CCS LEED™ AP SCIP Affiliate Senior Member Username: lynn_javoroski Post Number: 585 Registered: 07-2002 Posted on Wednesday, May 23, 2007 - 08:59 am:    I received a brochure from LiveRoof LLC yesterday. While they don't address wind loads/uplift, they do offer a 20 year warranty against material defects and photo degredation. It's basically a pre-packaged, pre-planted, engineered set of flats that are installed on the roof, eliminating the brown roof period of most green roofs until the plantings get established. They state the saturated weight with mature vegetation is 27-29 lb. per sq. ft. 800-875-1392; www.LiveRoof.com or www.LiveRoof.net. John Bunzick, CCS, CCCA Senior Member Username: bunzick Post Number: 723 Registered: 03-2002 Posted on Wednesday, May 23, 2007 - 09:17 am:    Mark is correct about the additional loads, but they often do not require additional structure because many green roofs are, in fact, relatively light (even when saturated.) ASCE 7 may not address the roof membrane specifically, but it does tell you what the design uplift pressures are. Todd C. Skopic, CSI, CDT Junior Member Username: tremskope Post Number: 2 Registered: 02-2007 Posted on Friday, May 25, 2007 - 09:45 am:    One item that may be of interest is that FMG has a relatively new (1/07) data sheet that pertains to this discussion: FM 1-35 Green Roof Systems, see: http://roofnav.fmglobal.com/RoofNav/pdf/1-35.PDF Lynn, beware of the limited liabilty warranty. How long has this product been in use? What if the waterproofing membrane leaks? Anne Whitacre, FCSI CCS Senior Member Username: awhitacre Post Number: 556 Registered: 07-2002 Posted on Friday, May 25, 2007 - 03:15 pm:    Carlisle does a "green grid" roof system and will warrant from the deck up; as does Hydrotech. these two manufacturers understand how their membranes work, and how the green part influences the performance of their membranes. they do not provide the actual plants, but they do know how it all works together.