Building codes have moved beyond recognition of “usual and customary” to their current state of “best practices.” Unfortunately, it appears as though our weather is changing as well, and best practices today may not be adequate for current (and future) stresses.
Wind, hail, and snow are the big three for roofing professionals. Looking back to the American Society of Civil Engineers (ASCE A58, Design Loads for Buildings and other Structures) and the revisions that followed, such as the subsequent versions under the scope of ASCE-7, we can see great improvements in the quantity and quality of input data.
Odds play a role in wind design. Designations such as 50- or 100-year design are thought of as the probability of an event occurring, such as odds of 1 in 50 or 1 in 100 that an event will exceed design velocity during a stated time period. Unfortunately, Mother Nature doesn’t seem to cooperate. In the past decade, we have seen several “100-year” events in a period of a few months. For example, Florida experienced four major hurricanes between Aug. 13 and Sept. 28, 2004. More recently, an EF-5 tornado devastated Joplin, MO.
Current return periods of ASCE 7-10 are as follows (though not all building codes have yet adopted the provisions of the 2010 version):
Category I Buildings 300-year return
Category II Buildings 700-year return
Category III and IV Buildings 1700-year return
ASCE 7-10 has also introduced the concept of windborne debris regions. The reality here is that an enclosed structure is less vulnerable to wind uplift failure, but if windows shatter during a storm event, the structure’s internal pressure and uplift forces increase considerably.
FMGlobal is a major resource in wind design, with designs based upon ASCE-7. The latest versions of FMGlobal’s data sheets are available free from www.fmglobaldatasheets.com, including Data Sheet 1-28 on design wind loads. FM’s data is highly reliable since its recommendations are based upon documented insurance losses. The organization recommends changes that will improve performance in future events. That differs from most building codes, which rely upon regional and customary data and may reflect bias from manufacturer or third-party data.
Data Sheet 1-49 is of particular significance, since membrane manufacturers generally do not include flashings and fascia/gravel stops in their specifications except in a general way. In many wind storms, it is the fascia metal that first blows off, sometimes taking the wood nailer with it. The unprotected membrane then lifts from the corner or edge of the roof first.
This situation may be changing, due in part to the Roofing Industry Committee on Weather Issues (RICOWI). This organization has established a number of blue ribbon task forces that visit storm-damaged structures (and structures that did not fail) and make observations on how they held up. Determining the weakest link can be very fruitful.
In the case of low slope roofing, failure could originate within the structure itself, the roof deck, fasteners, air or vapor barriers, thermal insulation, cover boards, flashing, edging, membrane, or rooftop units. It solves no problems at all if we focus on a detail that is not failing and overlook the ones that fail first.
The Institute for Research in Construction, part of the National Research Council of Canada, has greatly increased our understanding of roofing performance (especially for newer roofing systems such as single ply membranes). Of special note is NRCC-45693, Which Is the Weakest Link: Wind Performance of Mechanically-Attached Systems.
Hail damage and other forms of impact increase with the violence of thunderstorms and tornadoes. Underwriters Laboratories Inc. (UL) furnishes four levels of impact resistance: Class 1, 4.6 joules, to Class 4 at 31.2 J. FMGlobal designates Severe Hail (SH) at 19 joules and Moderate Hail (MH) at 10.8 J.
Missile diameter (mm)
ASTM E 3745
FM Class 1 SH
FM Class 1 MH
UL Class 1
UL Class 2
UL Class 3
UL Class 4
Recent testing indicates that higher compressive resistance coverboards, such as gypsum, improve impact resistance over softer underlays such as molded expanded polystyrene (MEPS).
Not only is impact resistance (dynamic load) of great importance to our roof systems, but resistance to static loads is important as well. In the past, treated wood “sleepers” were used to support above-membrane conduit lines, HVAC, and the like. Now, many rooftop photovoltaic (PV) systems are ballasted to reduce the number of penetrations through our roof systems. The pavers may weigh 20 psi or more, plus the weight of the panels themselves.
Here too, high compression resistance of selected coverboards can help. Membrane manufacturers now offer greater mil thickness membranes (i.e. 90 mil instead of 30 or 45), and strongly suggest that before installing PV systems, existing roof membranes should be replaced so that the expected life of the membranes more closely matches that of the PV panels (30 years or more). Once the PV panels are in place, fixing roof problems – and worse yet, finding roof problems – become major issues.
One might think that problems from snow accumulation would be most serious in climates that have a lot of snow. Ironically, that is not always the case. Regions of the country that routinely have a lot of snow are designed for these loads, with the entire structure beefed up. The science of compensating for drifted snow, moisture content of the snow, and other factors are well established.
However, with nature playing her trick cards, fringe areas are now seeing unpredicted accumulations that result in eaves damming, sliding snow or ice, hail blocking the strainers of roof drains, and roof membrane damage from inexperienced crews trying to shovel snow off the roofs before they collapse.
Recently, a couple of snow-damaged sports domes made major headlines – 6 people were injured in Dallas when snow and ice fell off the roof of Cowboys Stadium just before the Super Bowl, and a similar blizzard shredded 5 fiberglass panels at the Metrodome in Minneapolis, leaving gaping holes in its wake. However, there are plenty of instances of roof collapse that don’t get such publicity.
Consequences of adding thermal insulation
Newton's Third Law of Motion says it all: “For every action there is an equal and opposite reaction.” In our case, ever since the oil embargo of the early 1970s, we have been trying to make our buildings more energy-efficient. Some of you may remember the term “superinsulation.” A new standard, ASHRAE #189, has increased the minimum thermal insulating value for all roofs on commercial and high-rise residential buildings by 33% from R-15 to R-20 (or more) and many designers are insulating way beyond that.
However, the reactions to more insulation (and cool roofs) are several:
- Thicker insulation increases the need for thicker nailers at edges and curbs.
- Deep fascia metal will need to be of heavier gauges and with more stiffening ribs and cleats to resist thermal buckling as well as wind effects.
- Cool roofs are cooler! Single ply roofs are slippery when wet, and very slippery when there is a film of ice present. Dark-colored roofs look wet when wet, but light colored (cool) roofs give us no such warning. The texture of a gravel-surfaced roof provides good friction, granulated roofs (cap sheets) give us some help, but a smooth polymeric membrane is, by definition, smooth. Textured roof walkways are probably needed more on single-ply roofs than any other types.
- We are maxed out on thermal insulation and return on investment. The best return came when we added the first inch of insulation to our buildings, providing R=2.78 per inch for wood fiber and perlite and slightly more for glass fiber insulation. Unfortunately, the cost of insulation goes up in a straight line with thickness, plus we need increased thickness of fascia metal and nailers as mentioned above. But the energy conserved per inch flattens out. Since designers use the U value, expressed in Btus per hour that flow into or out of our structures, the U is then the inverse of the R.
Energy saved per unit of thermal insulation
1 inch wood fiber
R = 2.78
U= 1÷2.78 = 0.36
Savings/unit thickness over compared to non-insulated construction = 0.36 btu/h•sq ft•degree F
R = 2•2.78 = 5.56
U= 1÷5.56 = 0.18
R = 4•2.78 = 11.12
U = 1÷11.12 = 0.09
4 inches Isoboard
R = 4•5.56 = 22.24
U = 1÷ 22.24 = 0.045
6 inches Isoboard
R = 6• 5.56 = 33.36
U = 1÷ 33.36 = 0.030
It should be clear that the first couple of inches of insulation have by far the best financial return, but there is virtually no gain between 4 inches of Isoboard and 6 or more inches of the same material. The difference may be small in terms of Btus and the saving could very well be lost in open doors, air leakage, etc. Perhaps we ought to spend at least some of our money on full-time quality assurance instead of overdesigning or over-insulating our roofing systems,. As RICOWI’s field teams found out, it does little good to make our building codes more and more stringent if the constructions are not even meeting existing codes.
Richard (Dick) L. Fricklas was technical director emeritus of the Roofing Industry Educational Institute prior to his retirement. He is co-author of The Manual of Low Slope Roofing Systems and continues to participate in seminars for the University of Wisconsin and RCI Inc. - The Institute of Roofing, Waterproofing, and Building Envelope Professionals. His honors include the William C. Cullen Award and Walter C. Voss Award from ASTM, the J. A. Piper Award from NRCA, and the James Q. McCawley Award from the MRCA. Dick holds honorary memberships in both ASTM and RCI Inc.
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