FM Issue Bonus: Your Building Will Tell You What Kind Of Roof You Need
By Phil LaDuke
Published in the August 2007 issue of Today’s Facility Manager
Theprocess of determining the right roof for a building has changedsignificantly over the last two centuries. As society has evolved froma pre-industrial revolution era to the modern day business climate, sohave people evolved the way they construct buildings to meet theirneeds.
In keeping with the times, current roofing options areextremely varied; the number of choices may be overwhelming to somefacility managers. But by listening to their buildings, theseprofessionals can begin to narrow down the appropriate possibilities.
Backin the early 1800s, there were basically two options for decking:concrete or wood. The science of energy efficiency did not exist,therefore, the building required waterproofing only. Typically,waterproofing consisted of multiple layers of cotton rags embedded intohot tar—either coal tar pitch or asphalt.
With the turn of the century, the industrial revolution was in full gear. Consequently, buildings began to speak a new language.
Therewas a demand for larger span buildings, and steel answered that demand.Now, the building was not only saying that it needed waterproofing, butit also needed an additional substrate (because the steel deck couldnot have a roof applied directly to it).
In modern times,the building had even more demands. With the energy crisis in the1970s, facility managers listened to a new need, and insulation becameintegral to the building for heating and cooling efficiencies.
Today,given the countless options of roofing materials and the multitude ofkey components to consider, facility managers rely on the help ofspecifiers, consultants, and contractors to determine a building’s roofneeds. These components are critical to the life of the roof and varybased on the following criteria: location, insurance coverage, buildinguse and occupancy, building codes, environmental requirements, andaesthetics. It is very important for the facility manager to listen towhat the building is saying and consider all of these factors whendetermining which roofing system will work best with a building.
Wind And Ground Roughness
Building location is an extremely important factor in determining windload and resistance—issues key to the system’s success. If the buildingis in a location with high winds, such as Miami, FL, not only shouldthe roof be specified to withstand a certain wind speed, but the entiresystem, including the structural steel and the deck, needs to bedesigned to meet wind uplift requirements.
Facilitymanagers and specifiers should turn to roof detailing and edgeconfiguration as a first line of defense against the wind. By overspecifying the attachment of the roofing system, without substructureenhancements, damage may occur to the metal deck and possibly thestructural steel below. Both can be very costly to replace when damaged.
Thebuilding’s ground roughness factor also plays a key role. Buildingcodes reference ASCE-7 as the basis for determining the groundroughness factor.
Ground roughness is categorized as follows:
- Exposure A: Large city centers with at least 50% of the buildings having a height in excess of 70′.
- Exposure B: Suburban or wooded areas, or other terrain with numerousclosely spaced obstructions having the size of a single family dwelling.
- Exposure C: Open, level terrain with scattered obstructions of less than 30′.
- Exposure D: Flat, unobstructed areas exposed to wind flowing over open water for a distance of at least one mile.
(Itshould be noted that versions of ASCE-7 dated 2002 and later havedeleted Category A due to the close proximity of buildings with variousheights.)
As previously mentioned, there are some exceptionsto the exposure ratings in certain dense urban areas. The City ofChicago is a good example of this exception.
It would seemlikely that buildings located in the city center would be an Exposure Awhile buildings located directly along Lake Michigan would be anExposure D. However, if there is a three-story building in the citycenter that is considered an Exposure A and two very tall towers arebuilt on either side, a wind tunnel could occur. Thus, the building’sexposure rating could change.
Safety And Insurance
The importance of a building’s use relative to life safety is anothercritical factor to consider in a roof’s design. Insurance coverage fromunderwriters, such as Factory Mutual (FM), and the building’s use workhand in hand. Building ratings fall into the following categories basedon those who will occupy them:
•Category I: Buildings and other structures that represent a low hazardto human life in the event of failure including, but not limited to:agricultural facilities, certain temporary facilities, and minorstorage facilities.
• Category II: All buildings and other structures except those listed in categories I, III, and IV.
•Category III: Buildings and other structures that represent asubstantial hazard to human life in the event of failure, including,but not limited to: more than 300 person congregation in one area;schools/day cares with more than 250 people; colleges or adulteducational facilities with more than 500 people; healthcare with 50residents; jails; power generating stations; and buildings containingsufficient quantities of toxic or explosive substances considereddangerous to the public if released.
• Category IV: Buildingsand other structures designated as essential facilities including, butnot limited to: hospitals and other healthcare facilities havingsurgery or emergency treatment facilities; fire, rescue and policestations and emergency vehicle facilities; designated earthquake,hurricane, or other emergency shelters; power generating stationsrequired for emergency; and buildings having critical national defensefunctions.
These category ratings play into design becausethey assign an importance factor. For example, the importance factor ofan agricultural building is 13% lower, because the probability forinjury during a catastrophic event is not as high.
If thebuilding falls under a Category III or IV rating, the designer must add15%, making the rating 1.15. This is key to determining wind upliftperformance criteria, because the uplift pressures will be multipliedby the importance factor.
Fire resistance of roof assemblies is considered from both interior andexterior aspects. Common testing bodies include FM and UnderwriterLaboratories (UL).
Whenmeasuring for fire resistance from the interior, a number of factorscome into play. The primary concern is a roof system’s contribution offuel to an internal fire. Can it burn and/or contribute fuel to a fire?
Buildingsmay need a thermal barrier to reduce this potential if an insulationthat is flammable is being used. If the area under the deck is set upwith a sprinkler system, facility managers will have additionalinsulation choices.
Roof systems are also fire rated from theexterior, while roof assembly ratings are designated as Class A, B, andC. These ratings are based on a specific amount of time that the roofsurface is exposed to a flame and the amount of damage to the roofassembly.
The slope of the roof will affect the burningcharacteristics due to the flame spread up the slope. Class A willprovide the longest fire resistance; however, Class B or C may be theonly option based on the slope.
Other important areas toremember include deck type—combustible and non-combustible—as theycontribute to fire prevention efforts. It should be noted that thecurrent building code requires a Class B, and it is the facilitymanager’s decision to increase this relative to specific performancerequirements.
Property loss prevention is another keycomponent for FM and Industrial Risk Insurance (IRI), two of thenational insurance companies. Facility professionals should check withtheir insurance companies to see if the roof design will affectpremiums. Insurance carriers may have strict requirements for the roofassembly.
The Importance Of Codes
Building codes are set at local and national levels, and it is criticalfor facility managers to be cognizant of the fact that they are inplace—after all, they are the law. These codes will dictate a number ofthings including: wind uplift performance, fire rating, metal edgeaffixment, roof slope, the number of times a roof can be re-covered,and other factors.
Facilitymanagers should also be aware of the designed weight allowance for theroof, since all buildings carry a certain amount of load—live or dead.Live loads can include snow or heavy rain, while dead loads includeHVAC systems and other permanent equipment located on the roof.
Forthose areas that carry a large live load from snow or rain, drainage isextremely important. Proper roof slope is often one of the areasoverlooked, and a roof that does not have positive slope may notperform properly.
Today’s codes require an overflow drain, soif the main drain becomes plugged, there is alternate relief for thewater to get off the roof. This minimizes the potential for roofcollapse.
If water is escaping from the overflow, there is apossible problem. Facility professionals should make sure the drain isvisible and have someone address the issue.
Aesthetics can be a very important factor of the building’s appearancefrom above. It is critical to avoid putting too many mandates on thedesigner as the overall performance of the roof will be affected.
Forexample, if a facility manager wants the roof to be bright red or wouldlike to incorporate a logo, the designer may be limited to the type ofroofing system that can be installed. Many thermoplastic membranes canbe pigmented to a desired color, but there are very few guarantees thatthe color will last without fading over a long period of time.
Coatingsare also an option for many roofing membranes including thermoplastics,EPDM, and modified bitumen membranes. Given the plastic nature of mostthermoplastics systems, many coatings may not adhere well to them.Adding these kinds of constraints makes it very difficult to find asystem that will work and perform well.
To Top It Off
Environmental issues have also entered the roofing world. If a buildingis located in a heat island, it may be necessary to consider a greenroof. [For more on the environmental aspects of this subject, see theaccompanying article, “The Right Overhead.”]
Some options for green roofs include white coatings, reflective thermoplastic roofing systems, and vegetative roofs.
Vegetativeroofs today add a number of additional choices to the facility managerwhen it comes to helping the environment. Intensive, extensive, andballasted systems are available with a wide variety of plants that candictate what the roof system needs to be. [To read more on these typesof vegetative systems, see the TFM article, “Green Extremes” by BillScalia, August 2006.]
Asthe industry has continued to grow and develop, facility managers nowhave even more options and responsibilities when it comes todetermining the right roof for a building. Working closely with thespecifier and contractor is important to ensure the right design forthe building is created and proper codes are being added to the design.Facility managers who are aware of what the building is telling themwill have peace of mind that their roofs will perform for years to come.
LaDukeis director of national account services for Firestone BuildingProducts, based in Indianapolis, IN.