Home > Issue by Date > August 2007

Your Building Will Tell You What Kind Of Roof You Need

This bonus article explains how sustainable design concepts have been incorporated into the latest roofing product options.

By Phil LaDuke

Miller Motorsports Park Club House The Miller Motorsports Park Club House Building is covered with metal, TPO, and EPDM products illustrating some of the many choices roofing customers now have. (Credit: Firestone Building Products.)

The process of determining the right roof for a building has changed significantly over the last two centuries. As society has evolved from a pre-industrial revolution era to the modern day business climate, so have people evolved the way they construct buildings to meet their needs.

In keeping with the times, current roofing options are extremely varied; the number of choices may be overwhelming to some facility managers. But by listening to their buildings, these professionals can begin to narrow down the appropriate possibilities.

Back in 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 into hot 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.

There was a demand for larger span buildings, and steel answered that demand. Now, the building was not only saying that it needed waterproofing, but it also needed an additional substrate (because the steel deck could not have a roof applied directly to it).

In modern times, the building had even more demands. With the energy crisis in the 1970s, facility managers listened to a new need, and insulation became integral to the building for heating and cooling efficiencies.

Today, given the countless options of roofing materials and the multitude of key components to consider, facility managers rely on the help of specifiers, consultants, and contractors to determine a building’s roof needs. These components are critical to the life of the roof and vary based on the following criteria: location, insurance coverage, building use and occupancy, building codes, environmental requirements, and aesthetics. It is very important for the facility manager to listen to what the building is saying and consider all of these factors when determining which roofing system will work best with a building.

Wind And Ground Roughness
Building location is an extremely important factor in determining wind load and resistance—issues key to the system’s success. If the building is in a location with high winds, such as Miami, FL, not only should the roof be specified to withstand a certain wind speed, but the entire system, including the structural steel and the deck, needs to be designed to meet wind uplift requirements.

Facility managers and specifiers should turn to roof detailing and edge configuration as a first line of defense against the wind. By over specifying the attachment of the roofing system, without substructure enhancements, damage may occur to the metal deck and possibly the structural steel below. Both can be very costly to replace when damaged.

The building’s ground roughness factor also plays a key role. Building codes reference ASCE-7 as the basis for determining the ground roughness 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 numerous closely 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.

(It should be noted that versions of ASCE-7 dated 2002 and later have deleted Category A due to the close proximity of buildings with various heights.)

As previously mentioned, there are some exceptions to the exposure ratings in certain dense urban areas. The City of Chicago is a good example of this exception.

It would seem likely that buildings located in the city center would be an Exposure A while buildings located directly along Lake Michigan would be an Exposure D. However, if there is a three-story building in the city center that is considered an Exposure A and two very tall towers are built on either side, a wind tunnel could occur. Thus, the building’s exposure rating could change.

Safety And Insurance
The importance of a building’s use relative to life safety is another critical factor to consider in a roof’s design. Insurance coverage from underwriters, such as Factory Mutual (FM), and the building’s use work hand in hand. Building ratings fall into the following categories based on those who will occupy them:

• Category I: Buildings and other structures that represent a low hazard to human life in the event of failure including, but not limited to: agricultural facilities, certain temporary facilities, and minor storage 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 a substantial 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 adult educational facilities with more than 500 people; healthcare with 50 residents; jails; power generating stations; and buildings containing sufficient quantities of toxic or explosive substances considered dangerous to the public if released.

• Category IV: Buildings and other structures designated as essential facilities including, but not limited to: hospitals and other healthcare facilities having surgery or emergency treatment facilities; fire, rescue and police stations and emergency vehicle facilities; designated earthquake, hurricane, or other emergency shelters; power generating stations required for emergency; and buildings having critical national defense functions.

These category ratings play into design because they assign an importance factor. For example, the importance factor of an agricultural building is 13% lower, because the probability for injury during a catastrophic event is not as high.

If the building falls under a Category III or IV rating, the designer must add 15%, making the rating 1.15. This is key to determining wind uplift performance criteria, because the uplift pressures will be multiplied by the importance factor.

Fire Ratings
Fire resistance of roof assemblies is considered from both interior and exterior aspects. Common testing bodies include FM and Underwriter Laboratories (UL).

When measuring for fire resistance from the interior, a number of factors come into play. The primary concern is a roof system’s contribution of fuel to an internal fire. Can it burn and/or contribute fuel to a fire?

Buildings may need a thermal barrier to reduce this potential if an insulation that is flammable is being used. If the area under the deck is set up with a sprinkler system, facility managers will have additional insulation choices.

Roof systems are also fire rated from the exterior, while roof assembly ratings are designated as Class A, B, and C. These ratings are based on a specific amount of time that the roof surface is exposed to a flame and the amount of damage to the roof assembly.

The slope of the roof will affect the burning characteristics due to the flame spread up the slope. Class A will provide the longest fire resistance; however, Class B or C may be the only option based on the slope.

Other important areas to remember include deck type—combustible and non-combustible—as they contribute to fire prevention efforts. It should be noted that the current building code requires a Class B, and it is the facility manager’s decision to increase this relative to specific performance requirements.

Property loss prevention is another key component for FM and Industrial Risk Insurance (IRI), two of the national insurance companies. Facility professionals should check with their insurance companies to see if the roof design will affect premiums. Insurance carriers may have strict requirements for the roof assembly.

 

The Importance Of Codes
Building codes are set at local and national levels, and it is critical for facility managers to be cognizant of the fact that they are in place—after all, they are the law. These codes will dictate a number of things including: wind uplift performance, fire rating, metal edge affixment, roof slope, the number of times a roof can be re-covered, and other factors.

Facility managers should also be aware of the designed weight allowance for the roof, since all buildings carry a certain amount of load—live or dead. Live loads can include snow or heavy rain, while dead loads include HVAC systems and other permanent equipment located on the roof.

For those areas that carry a large live load from snow or rain, drainage is extremely important. Proper roof slope is often one of the areas overlooked, and a roof that does not have positive slope may not perform properly.

Today’s codes require an overflow drain, so if the main drain becomes plugged, there is alternate relief for the water to get off the roof. This minimizes the potential for roof collapse.

If water is escaping from the overflow, there is a possible problem. Facility professionals should make sure the drain is visible and have someone address the issue.

Appearance Matters
Aesthetics can be a very important factor of the building’s appearance from above. It is critical to avoid putting too many mandates on the designer as the overall performance of the roof will be affected.

For example, if a facility manager wants the roof to be bright red or would like to incorporate a logo, the designer may be limited to the type of roofing system that can be installed. Many thermoplastic membranes can be pigmented to a desired color, but there are very few guarantees that the color will last without fading over a long period of time.

Coatings are also an option for many roofing membranes including thermoplastics, EPDM, and modified bitumen membranes. Given the plastic nature of most thermoplastics systems, many coatings may not adhere well to them. Adding these kinds of constraints makes it very difficult to find a system that will work and perform well.

To Top It Off
Environmental issues have also entered the roofing world. If a building is located in a heat island, it may be necessary to consider a green roof. [For more on the environmental aspects of this subject, see the accompanying article, “The Right Overhead.”]

Some options for green roofs include white coatings, reflective thermoplastic roofing systems, and vegetative roofs.

Vegetative roofs today add a number of additional choices to the facility manager when it comes to helping the environment. Intensive, extensive, and ballasted systems are available with a wide variety of plants that can dictate what the roof system needs to be. [To read more on these types of vegetative systems, see the TFM article, “Green Extremes” by Bill Scalia, August 2006, page 30; online at www.todaysfacilitymanager.com/tfm_06_08_news4.php.]

As the industry has continued to grow and develop, facility managers now have even more options and responsibilities when it comes to determining the right roof for a building. Working closely with the specifier and contractor is important to ensure the right design for the building is created and proper codes are being added to the design. Facility managers who are aware of what the building is telling them will have peace of mind that their roofs will perform for years to come.

LaDuke is director of national account services for Firestone Building Products, based in Indianapolis, IN. For more on his company, visit www.firestonebpco.com.

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