Solano County Government Building, Fairfield, Calif., features tapered ENRGY 3 Insulation and GlasKap cap sheet. (Photo courtesy of Johns Manville.)

Put in the simplest terms, a roof is required to provide shelter from the outside elements. It is a barrier that is constructed to prevent the intrusion of the environment into the space beneath it. Since primitive times, roofs were designed solely to protect man from the elements. Today, roofs are not only expected to provide shelter from the elements - they must also satisfy numerous other functions. A roof must control the flow of heat and the passage of air and water vapor, and it must resist noise, dust, fire and airborne pollutants as well. A roof must also have adequate drainage, high thermal insulation, compatible materials and efficient flashings. It should also be costeffective and provide a long waterproofing service life.

Satisfying these functions is difficult enough bearing in mind all the negative elements that affect a roof on a constant basis. A roof is consistently exposed to sunlight, rain and the forces of wind. All of these elements can degrade a roof system. In addition to weather elements, a roof system is subject to deformations in the structural frame, seismic forces, and wear and tear from traffic over the surface. Furthermore, unbalanced loads that are caused by the accumulation of snow and ice could also be detrimental.

While roof systems have changed considerably since primitive times, the mind-set regarding their purpose has remained the same. As long as the roof remains watertight, little attention is paid to it. The roof is normally the least maintained subsystem of the building, when in reality it should be continually maintained. In some respects, the roofing system is like an automobile; it needs constant maintenance to provide a long service life. Just as an automobile degrades when it is not serviced properly, a roof will slowly deteriorate to the point of failure if it is not maintained.

A roof is actually a complex system that relies on several components that must interact and be fully compatible. (Photo courtesy of CertainTeed.)

Avoiding Common Roof Problems

Proper roof design and annual maintenance are two of the most important aspects of a successful roof. These two attributes, coupled with good workmanship, will result in a roof system that provides a long waterproofing service life. That is why it’s important to briefly describe each of these areas and their relationship to roof deterioration before the roof repair analysis is discussed. It is the experience of most building owners and property managers that roofs of commercial facilities, particularly in temperate climates and those having a low slope, are a chronic problem. Nationwide statistics and historical reviews of building systems’ performance offer plentiful testimony to this condition. For the purpose of more thorough understanding of the situation, we should point out that the chronic problems with roofing systems generally stem from two conditions:

1. Of the major generic building systems - structures, mechanical systems and protective systems - it is the protective systems that are subject to the most intensive distress.

2. Roofs, while the most critical of the protective systems, are difficult to effectively maintain because of:

• Their massive size.
• Their system complexity.
• The intensity of their exposure to weather effects.
• The complexity of problem diagnosis and the uncertainty of the effectiveness of the treatments.
• The typical administrative process employed to deal with them.

These factors do not preclude the obvious problems. The American Institute of Architects states that over 50 percent of all building construction litigation involves roofing. This is the single leading building subsystem involved in this area. This is an astonishing rate when it is taken into consideration that the roof system only accounts for approximately 10 percent of the entire building cost. Most of the litigation involves moisture intrusion and premature failures. It is estimated that 4 percent to 5 percent of all roofs completed annually experience premature failure. A premature failure could be defined as a roof system that fails to provide waterproofing service for a 10- to 15-year period. Most failures occur within five years after their application. The expenses of litigation, and the repair and replacement of the roof system, cost building owners millions of dollars each year. In most of these cases, the costs could have been avoided had the roof system been properly installed.

A roof is actually a system that relies on several components that must interact and be fully compatible. The integration of this system is comprised of three equally important stages: design, application and maintenance. All three of these stages serve an exact function that contributes to the success of the roof as a system. A lapse in any one of these areas often will result in a premature failure.

Roof design decisions can be instrumental in helping to lower energy usage. This roof features Flintglas mineral surfaced cap sheet with CoolStar reflectant surface. (Photo courtesy of CertainTeed.)

The Roof Design Decision

The roof design decision is the most important decision in the roofing process. It encompasses a number of important issues that can only be determined at this stage of a roof’s progression. The assurance of proper roof performance is the main issue with roof design. Consideration must be taken in determining proper design, material and application techniques.

Proper roof design is the basis from which the roof system is assembled. It is at this point where all aspects of the roof system are devised. The roof assembly generally falls into two component groups: substrate and roofing. The substrate consists of all components that are protected from the environment. This group would include the roof deck, any vapor retarders and the insulation. Roofing is considered to be all of the components that provide protection for the substrate. This group includes the membrane, flashings, exposed trim, drains and all other items required for a complete roof installation. The type of roof that is designed is normally limited by several factors. The pitch or slope of the roof will largely designate the type of system that can be applied. Roof slopes that are less than three inches in twelve (3:12) are considered to be low-slope roofs. Low-slope roofs will accommodate the majority of nonresidential systems available and will be the focus of all further discussion. The roof’s slope is a key consideration when determining the type of bituminous material that is applied on a built-up roof. Roofs with a slope of ½ :12 or less, which are referred to as dead level, should use coal tar pitch. Coal tar pitch is highly resistant to water penetration, but it has a low sloping point. If it is applied on roofs that are sloped over ½ :12, there is a possibility that it may release in high temperatures, causing membrane slippage.

Asphalt is applied on sloped surfaces in accordance with their softening points. Type I asphalt, which has a low softening point, is recommended for a roof slope of 1:12 or less. This type of asphalt has a better resistance to water absorption and protection, but because of the low softening point, membrane slippage can occur at high temperatures. The Type III asphalt has a higher softening point and is recommended for roof slopes above 1:12. The shape of the roof also limits the roof designers’ choices when it comes to the roof system. Certain types of systems can be altered to more complex or warped roof designs; others are more restrictive. The number of roof penetrations, such as those required to accommodate HVAC units and skylights, will also limit the type of systems that can be applied. This is important in today, as it seems more building areas are designed with rooftop HVAC equipment to utilize more space inside of the building.

The selection of the roof system is not complete until the roof designer has carefully analyzed the past performance of the system chosen. The performance characteristics or requirements to be considered when analyzing a roof system should include durability, cost, aesthetics and the system’s compatibility to other roof and building components.

The number of penetrations and the type of HVAC units installed could limit the number of options when selecting a roof system.

Roof Design for Maintenance

To effectively control roof maintenance, there must be an understanding of the design aspects of repair and reroofing specifications that may ultimately impact maintenance requirements. The degree of maintenance required for any roof is often determined at the time of its construction, whether it be the original system installation or reroofing application. Maintenance can be unwittingly “built in” to the original roof system by certain faulty elements of the design. Some of these faulty design elements can be corrected to some extent during repair or reroofing; others will continue to be constant maintenance nuisances. It is mandatory for those charged with the management of maintenance to become familiar with the effects of design elements on roofing repair and maintenance.

New Construction

In new construction, a designer has more control over a building’s configuration, including items such as wall heights, perimeter treatments, roof slope and drainage facilities - all of which can be designed in an optimum fashion. In addition, the designer will undoubtedly have a larger available base of membrane systems from which to select the optimal choice.

However, designers often build maintenance requirements into the documents rather than designing them out. Following are examples of common design practices in new construction that can directly relate to increased maintenance requirements:

• The use of pitch pans to flash a penetration through the roof. Pitch pans lose their fill material with time; in addition, the material will often harden and shrink away from the penetration, allowing the potential for water entry.
• Improperly designed flashings. It is a well-documented statistic that 90 percent of all roof leaks relate to flashings. Flashing heights of less than 8 inches may place the flashing in the water line, inviting problems. Flashings should be provided with a minimum height of 8 inches, if possible, or otherwise detailed in a manner that keeps the top edge of the flashing out of the water line. Flashings that are inadequately attached at their top edge (due to lack of nailing) may slump below their counter flashing, allowing for direct access of water into the building.
• Improper flashings used with perimeter sheet metal. Depending on the fastening frequency and pattern, construction may allow for movement of the sheet metal, which often fractures the bituminous materials which strip-in the sheet metal, providing potential for leakage and necessitating ongoing maintenance. Perimeter sheet metal should be fastened in a pattern and frequency that will restrain this movement.
• Improperly designed drainage. Treatments that do not allow for rapid runoff of water will result in ponded conditions, which accelerate deterioration of the membrane in the ponded areas.

Existing Construction

With existing construction requiring remedial treatment, a designer is faced with modifying pre-existing conditions to maintain technically sound treatment approaches, a design task often more difficult than new construction design.

After its original construction, a roof, in addition to aging and weathering, may undergo other changes. A new drain may be installed to improve drainage, a roofmounted heating or cooling unit may need to be replaced, or entire areas may need repair or reroofing. The manner in which such activities are conducted may have a drastic impact on roof maintenance. Such modifications are often attempted without design guidelines and by individuals not familiar with, or not concerned with, the technical efficiency of their activity. When designing a roof system, the emphasis begins at the roof deck and continues up through the membrane surfacing. Each component of the roof must be carefully orchestrated to ensure complete compatibility throughout the system. It is the roof designer’s job to develop a system of many separate components that once assembled will serve as a single system. The roof system that is designed in this manner will provide a long waterproofing service life.