In simple terms, it is a roof that fails to meet the design intent for the planned service life. Service life can be classified as the length of the warranty or the average life for the roofing system specified.
A failure is often the result of a leak, but it can manifest itself in many other ways.
Other forms of failure include membrane that becomes partially or fully unfastened from the roof assembly, blisters in the membrane or mold in the roofing system caused by moisture entrapment and accelerated membrane oxidation, among others. There are also catastrophic reasons for roof failures such as wind or hail events that exceed the roof design parameters.
However, this article is focused on common roof problems that can be minimized or avoided through roof design.
A frequent theme when talking to roofing experts is the use of "best practice" in design. According to Merriam Webster, a best practice is a procedure that has been shown by research and experience to produce optimal results. A design best practice is often not the same as the design requirements of the building code. Building code is a minimum standard. Best practices often exceed building code requirements.
Experienced architects and roof consultants know to stay out of trouble by exceeding the building code. But without long years of experience, how do you know those best practice design areas?
To find out, I spoke to two highly experienced roof consultants. Gary Mays is President May’s Consulting & Evaluation Services, Inc. Delaware, Ohio and Ray Wetherholt is Founder, Wetherholt and Associates, Inc., Redmond, Wash.
Design Issues Related to Roof System Failures
1.) Moisture Management
It may come as no surprise that moisture in the roofing system is the most common problem. But the source of the moisture might surprise you. The most prevalent source is not rain, but rather moisture drive from the interior of the building or a concrete deck.
Trouble develops in buildings with high relative humidity. Vapor drive in cooler climates sends the humid air from the warmer interior of the building to the cooler exterior of the building where it condenses on the underside of the roofing membrane.
That water can wreak havoc with the roofing system, leading to mold, reduced insulation R-value and delamination of the insulation paper facer. Insulation facer delamination in an adhered roofing system can lead to roofing system billowing and blow off in a high wind event.
The key is to prevent water vapor from entering the roofing system in the first place.
”A vapor retarder and/or an air barrier [climate zone dependent] can be critical to the life of the roof,” according to Mays.
“It is not uncommon to have dew point temperatures in the mid to high 40 degrees Fahrenheit range within the roof system. Vapor drive is always from hot to cold. Airborne moisture (water vapor) during cold weather destroys foam insulation cell structure, as well as paper facers.
“I have found multiple roof systems with delamination conditions immediately below the roof membrane. You must keep in mind that roof insulation and substrate boards are typically 4’x4’ or 4’x8’, and even with multiple layer systems, there are numerous openings between boards that allow moist air to travel to the top, or the underside of the roof membrane.”
While the installation of a vapor retarder and/or air barrier is best practice for many buildings, it is not required by building code in many locations. One situation where they are critical is over new concrete decks.
Concrete contains 15 to 20 percent water when it is placed. During the curing process water evaporates from the concrete, but the process can take many days and even months to complete depending on the initial concrete chemistry, relative humidity and weather conditions.
It is best to wait for the concrete to cure more thoroughly and the moisture content to decrease before roofing, but the pressure is always on to complete construction. The buildout of the building interior is dependent on having a weathertight exterior and that means that the roofing system must get completed.
“Dealing with moisture from concrete decks is a common problem that the roofing industry has yet to solve completely,” says Ray Wetherholt.
“Most adhesives, especially those that are water-based, don’t fully cure in wet conditions. As a result, many roofs that are installed as adhered systems on concrete decks are never fully adhered and are prone to wind uplift and blow off.
“A vapor barrier or air retarder can prevent moisture from entering the roofing system. The challenge is developing systems that don’t delaminate from wet concrete. There is new moisture-cured chemistry being developed and introduced that looks very promising.”
2.) Proper Roof Ventilation
Lack of adequate building ventilation can also lead to roofing system failure from moisture vapor drive, ice dams and, heat-related failures.
Anyone who lives in a cold-weather climate has seen icicles hanging from building roofs. Those icicles are the result of warm air reaching the underside of the roofing surface and melting the snow on the outside. The result can be destructive ice dams that cause the roof to leak to the interior of the building. These conditions can be especially prevalent in multi-unit residential buildings.
“Building code requires five air changes per hour for office buildings but only one air change per hour, more or less, in residential buildings,” says Wetherholt. “But residential activities such as cooking and bathing generate a lot of humidity. The result can be excess moisture in the occupied living space that needs to be vented outside, and not leave the building through the roofing system.”
Lack of air circulation can be especially bothersome in steep slope roofing systems where the attic space between the heated building and the roof is meant to stay properly ventilated.
"Steep-slope roof systems fail as a result of lack of ventilation,” says Mays. “Adequate air circulation is required to cool the roof by convection currents to prevent heat-related failures and to remove unwanted moisture from the roof system that can lead to biological growth in organic materials, component failures, and moisture-saturated insulation issues."
There are many studies that indicate methods for calculating the amount of Net Free Air per Lineal Foot (NFA/LF) needed to provide the required ventilation to cool the roof and remove unwanted moisture.
“Based on my research for a typical roof system, a minimum two-inch opening at the eave and a four-inch opening at the ridge for a conventional gable roof is needed on most commercial construction projects,” says Mays. “This is the minimum that I normally find needed but more NFA/LF may be required on large roofs. I like to use 150 square feet area for each square foot of ventilation needed.
“For standard gable roofs, I typically design 50 percent of ventilation to come in at the eave and 50 percent to flow out the ridge. This is often 50 percent more than required by building code. I have never seen a failure with this system design over the last 50 years.
“Many residential roof system manufacturers do not manufacture ridge vents with adequate NFA/LF needed for larger commercial building roofs. There are alternative metal ridge vent systems available that can provide adequate NFA/LF needed for a balanced system.”
3. Roof Traffic and Hail
Low-slope roofing systems are increasingly being used for more than just keeping water out of the building. Roofs have become a favored platform for renewable energy, communications, and outdoor living space. With this use comes increased foot traffic and wear and tear on the roofing system.
“Roof membrane systems require enough compressive strength to prevent deformation of the substrate materials during roof traffic and expected roof-mounted equipment maintenance,” says Mays.
Planning for roof traffic in the design phase can avoid damage. It is best practice to install higher compressive strength underlayment in areas such as roof hatches, penthouses, and walkways to rooftop equipment.
“When cost is not the object, high-density cover boards can be specified,” says Mays. “But when the cost has greater importance, 25 psi coated glass-faced polyisocyanurate insulation is available. I find this is cost-effective, as well as sufficient compressive strength for most applications.”
Hail storms are another condition where roof underlayment compressive strength can save a roof or at least minimize the damage.
“If your building is in a hail prone area of the country then you may be required by building code and/or your insurance company to design a hail resistant roofing system,” says Wetherholt. “This means the use of high-density cover boards and thicker membranes. While this adds cost to the system, it can save the building owner in the long run from roof damage.”
In general, the construction industry is slow to change and this is especially true in roofing. Tried and true beats new and improved. There are countless stories of new products and systems that were touted to provide better performance only to fail prematurely.
While this is not always the case, it has happened frequently enough that experienced roof consultants know to require a track record of success in other installations before they are willing to take a chance on a new product or system.
“There is no substitute for experience and proven performance,” says Wetherholt. "That is not to say there is no room for innovation in the industry. You just have to be careful with where and how new products are used.
“Make sure that all of the proper tests have been completed and be conservative in the beginning with where and how the products are used. Over time you gain confidence through your experience and that of others and the product becomes assimilated into common roofing practice.
“Roof system failures occur when the roof system designer does not specify a roof assembly that has been tested to resist uplift pressures and other conditions for the project location,” says Mays. “We most often require roof systems with an FM Global RoofNAV number. FM provides independent testing criteria for roof assemblies and a RoofNav number indicates the roofing system has successfully met certain performance criteria.”
At a minimum, Mays typically states and specifies the following:
Wind Up-Lift and Hail Characteristics: Provide a membrane roofing system that is identical to systems that have been successfully tested in accordance with FM 4474 by a qualified testing and inspecting agency to resist uplift pressures and hail as listed below. The perimeter and corner areas shall be prescriptively enhanced in accordance with the current edition of FM Global Loss Prevention Data Sheet 1-29.
1. Field Wind UP-Lift Rating: 60 psf (Minimum)
2. Hail Resistance: SH
5.) Roof System Drainage
A common misnomer in the roofing industry is to call low slope roofing systems “flat”. By building code all roofs must have a slope of at least a quarter-inch per foot (1/4:12 or 2 percent) to drain water from the roof.
Ponding water on the roof can lead to problems as catastrophic as roof collapse from excess weight to chronic problems such as biological growth that can shorten the roof membrane life span or cause odor issues near fresh air intakes. In reality, however, many low slope roofs designed to building code do pond water and problems can occur.
“Specifying and detailing positive roof drainage can greatly improve the lifespan of a roofing system,” says Mays. “I see many new roofs designed with tapered insulation having 15 degrees of slope to drain. This is not enough to compensate for structural roof deck deflection. I always recommend that the roof system designers consider deck deflection when laying out tapered roof insulation systems.
“Designing with 30 degrees of slope to drain is a best practice that will overcome this issue. Keep in mind that on a code-required ¼:12 roof slope, a 30-degree slope-to-drain cricket reduces the drainage to an eighth-inch per foot. Structural deck deflection can reduce the drainage even further.”
Getting the water moving to the drains is one thing, having a working drain is another. Avoiding clogged drains is often a maintenance issue. A minimum of two roof inspections per year, as well as after storms, is recommended. This can go a long way towards eliminating drainage problems.
“The increase in severe storms with more and heavy rainfall means we need to design overflow drains to compensate,” says Wetherholt. “I often see overflow drains that are improperly designed or located. Drain size is also a consideration. A four-inch drain at the opening should be four inches minimum to the outlet. This ensures that anything that can get sucked into the opening doesn't get stuck somewhere in the drainage pipe. This seems logical, but I have seen this problem in many buildings."
Understanding and avoiding common roof design problems, and when and where to exceed building code, is extremely valuable knowledge. However, many roof problems are the result of poor workmanship rather than design shortcomings.
Workmanship issues are largely out of the roof designer’s control, but there are steps you can take to minimize the occurrence.
- Provide adequate and clear detail drawings for the roofing system specified. Don’t leave it up to the contractor to design roof details in the field.
- Mandatory pre-bid conferences allow contractors to ask clarifying questions and helps to ensure all bidders understand the design and are on the same page.
- Require that only qualified and trained contractors are chosen for the roofing system selected. Roofing contractors are often rated by the roof system manufacturers based on the quality of workmanship, the volume of projects (experience) and other financial factors. The rating is publicly available and should be a consideration in the final contractor selection.
- On-site monitoring by a Registered Roof Observer is an excellent option to ensure quality control. This does add cost to the roofing installation but is a good return on investment if installation mistakes are caught and future problems avoided.
Achieving an Optimal Roof Design
Roofing systems are subject to severe conditions day-in and day-out and as a result are the most problem prone areas of a building. Understanding the common problem areas and the best design practices to preempt problems can avoid roof failures and significant damage to the roofing system and possibly the building.
While knowing the common roofing problem areas is important, hiring a roof consultant to help with system design can keep you out of a lot of trouble. Best practices are based on years of experience and lessons learned the hard way. Hiring a Registered Roof Consultant is a cost-effective way to gain that experience on your team and avoid a lot of costly headaches down the road.
All photos courtesy of May’s Consulting & Evaluation Services, Inc. Delaware, OH with the exception the vapor barrier photo. This photo is courtesy of Sika Sarnafil, Canton, MA.
Where to find a Roof Consultant
Roof Consultants are roofing system experts hired by building owners and architectural firms to assist with new roof design and construction quality assurance and existing roofing system trouble shooting. They are often called upon to solve difficult roofing problems or to provide an expert and unbiased roof specification for use in the procurement process.
The profession is represented by the International Institute of Building Enclosure Consultants (IIBEC). There are approximately 1,900 professional members of the organization. A membership directory is available on the IIBEC website at https://rci-online.org.