According to the Institute for Business and Home Safety (IBHS),roof cover damage continues to be the largest, most frequentsource of non-surge related failures related to hurricanes¹.In response, roof covering manufacturers have provided a multitudeof products that have demonstrated through independent testing tobe able to withstand design event forces. However, it is apparentin the wake of recent windstorm events that much is lost in translation between theidealized laboratory setting and an actual constructed roof. Thisarticle will provide a brief review of the current codes, design,and selection process. It will also present a number of commonconstruction-related defects that result in the loss of millions ofdollars annually.

The selection of the roof covering in a hurricane-prone regionwould likely start with the locally adopted building code. For mostof the U.S., the building code is likely a recent version of the InternationalBuilding Code² with local- and regionallyspecific, amendments. Some jurisdictions—Florida, for example—havespecific codes that apply. The purpose of the code is to set aminimum standard of construction and design for the purpose of lifesafety. It typically does not purport to address other aspects ofthe functionality of the roof that might include resistance towater infiltration, service life expectations, hail resistance, andother functions that the facility owner may also require. Anotherdistinction to make is that codes, especially those related tocommercial buildings, have become less prescriptive and moreperformance-based in the last few decades. The code dictates thedesign criteria but not the number of fasteners per squarefoot.

Performance-Based Guidelines
Once the basic life-safety design criteria is established, localmunicipalities and insurance requirements take the design processfurther, by requiring the selection of assembly that has shownthrough independent laboratory testing to have a particularresistance to applied wind loads. It is the manufacturer of thespecific assembly that undertakes to have it rated by third-partytesting. The tested and rated assembly is specific with respect tothe materials, attachment, and configuration. Any variation in,say, the type of fastener used, would constitute a differentassembly that would require its own rating. Therefore, uponselection of an assembly that is meant to meet building code, localmunicipality, and, in some cases, insurability requirements, itbecomes imperative to closely follow the manufacturer’srecommendations for the installation of the assembly.

Before examining some common construction-related defects that affect the wind-related performance of a roof, here is a quick recap ofthe way it is supposed to work. For thebuilding-code-derived design wind speeds are used to calculatewind-related pressures imposed on the roof. An assembly is eitherselected based on its laboratory-obtained rating or designed to beadjoining attached to the structure to resist these pressures. Theactual attachment of the roof is done in one of two basic ways, theassembly is either mechanically attached to the structure withfasteners or adhered with adhesive. The base sheet of an assemblymay also be mechanically attached and the cap sheet adhered to thebase sheet. In either case, the assembly must be attached to thestructure or component with a resistance that is greater than thedesign pressures. Like a chain, the assembly is only as strong asits weakest link. The following examples are used to illustratesome common disconnects between the ideal laboratory conditionsused to determine ratings that are then select an assembly toresist the design pressures.

Modes of Attachment
Fasteners are used to mechanically attach components to thestructure. These come in many shapes, sizes, and types depending onthe application. The fastener will have an anticipated resistanceto failure (as determined by the manufacturer of the fastener).Assuming the fastener is properly installed, the resistance of theindividual unit divided by the spacing gives an allowableresistance to the design pressure. The number of fasteners, perunit area, then, is critical to the wind related performance.

In the case of a low-slope modified bitumen or single plymembrane over a metal deck, the base sheet and/or the membranewould likely be mechanically fastened to the deck. These fastenersshould penetrate the deck and should be observable if access can beobtained to the underside of the deck by lifting ceiling tiles orby accessing a mechanical room. Consider Figures 1 and2; both are taken in hurricane-prone regions but atdifferent locations. Without performing arduous calculations,suffice it to say the required number of fasteners in a hurricaneprone region for a mechanically fastened assembly would probably becounted by the dozens per a 10-foot by 10-foot area (as seen inFigure 1). All things being equal, one can easilydiscern in Figure 2 that this roof may be lackingthe appropriate number of fasteners.

Another common mechanically fastened roof system is the asphaltshingle. Asphalt shingles typically are specified to have either a‘four-nail’ pattern or a ‘six-nail’ pattern, the latter being meantfor regions susceptible to high-velocity winds. The nails aresupposed to be installed just below the adhesion strip for threetab and laminate shingles according to ARMA³. Thelocation is critical because with the shingles properly exposed(overlapped), the placement of the nail in this location wouldpenetrate the top shingle and one underneath. Putting the nail toohigh (Figure 3) on the shingle misses the lowerone, in effect, cutting the number of fasteners per shingle inhalf. Thus, the ‘six-nail’ pattern actually produces 12 nails pershingle holding the shingle to the roof.

Adhesion Resistance Standards
Fully adhered systems are required to meet the same resistancestandards as the mechanically fastened systems, given that thelocation and application are the same. Also, like the mechanicallyattached systems, the adhered applications are only as strong astheir weakest component.

With a ‘hot-moped’ application the temperature of the asphaltwhen applying the insulation or overlying plys of felt becomescritical to its adhesion. Asphalt has an Equiviscous Temperature(EVT) range that is type specific and represents the point at whichthe asphalt has the ideal viscosity of workability and adhesionbetween the layers⁴. Application of the asphalt belowthe EVT can prevent the asphalt from penetrating the felt andcreating the appropriate bond. Working with asphalt above its EVTcan breakdown and cause degradation of the asphalt. Figure4 depicts an example of the blow-off of a membrane whereasphalt was applied below its EVP. This is evident by the smoothsurface of the asphalt which is indicative of lack of transferbetween the asphalt and the adjoining layer. Also noted inFigure 4, proper inter-ply adhesion would beexpected to cause some delamination of the insulation.

With fully adhered single ply membranes the attachment of themembrane to the roof is adhesive. The condition of the substrateplays an important role with respect to how well the membrane isattached. The substrate should be clean and free of dust, anddebris, or materials not compatible with the adhesive. The‘Achilles Heel’ of a fully adhered single-ply membrane, however, isthe edge attachment. Once the edge is compromised by wind, thesystem has little resistance to the wind that is now under themembrane and the peeling action of the combined pressure.

Figure 5 shows a roof located in the Gulf of Mexico that failed during a storm that produced windvelocities well below the applicable code required velocities.Inspection of the perimeter condition (Figure 6)revealed that an inappropriate termination bar was used to fastenthe edge of the membrane to the parapet. In this case, the flat barhad little clamping ability to adequately hold the edge of themembrane. In effect, the edge of the membrane was only attached atthe fasteners and not continuously as is the intent of thetermination bar.

Limits of Technology
The requirements of a roof system are many, and are dependent onregion and function. Locally adopted building codes and othercriteria narrow the search for an appropriate system. Manufacturersthen go to great time and expense to have their products rated asappropriate for use in a particular application. This long,involved process requires input from designers, code officials,contractors, and manufacturers. This process is all for naught,however, if the intent of the design is not fulfilled in the fieldduring installation of the roof. How does one ensure that the roofis properly installed in the first place? The answer is throughproper procurement and quality control, the discussion of which isentirely outside the scope of this article. The technology doesexist to attach a roof to a building to withstand a design eventbut when determining causation after a loss, it is important to bemindful that the technology does not always make it to theroof.

¹ Hurricane Ike: Nature’s Force vs. StructuralStrength, Institute for Home and Business Safety, Sept.2009

² International Building Code, International CodeCounsel

³ Asphalt Roof Manufacturer’s Association, ARMAResidential Roofing Manual, 2010

⁴ The Roofing and Waterproofing Manual,5^th Edition, National Roofing Contractors Association,2003