Penetrating a Metal Roof: Specifying leak-free assemblies

All images courtesy Butler Manufacturing

by Rodger Russ
Once a building owner invests in the longevity promised by a standing-seam metal roof, there will be little patience for leaks. A standing-seam roof—one of the longest-lasting options available—is often specified because of its durability and minimal maintenance requirements. Some metal assemblies include a 360-degree double-lock seam to create a truly monolithic watertight surface. Alas,
a hole is a hole.

Roof penetrations are the number-one source of leaks in these roofs, and even a single poorly installed fastener can lead to millions of dollars in damage to the inventory or equipment below. A detailed specification encompassing several best practices for penetrating a standing-seam metal roof helps protect a building owner from the havoc of leaks.

Considerations for common roof penetrations
While common sense dictates cutting holes in any roof should be avoided whenever possible, there are five typical reasons to penetrate a standing-seam metal assembly. These include:

  • ventilation piping;
  • air circulation exhaust fans;
  • HVAC equipment and associated ductwork;
  • solar applications; and
  • daylighting.

Ventilation piping
Leaks related to ventilation piping penetrations are relatively easy for an experienced professional to remedy, but it is important to thoughtfully specify them, given the implications they have on the roof’s weathertight warranty.

One tactic is to specify the penetrations be performed by the roof installer, rather than the mechanical contractor. This is a deviation from common practice, but can be an important step in ensuring they are done properly. Not all mechanical contractors are educated in how to produce a weathertight seal around the pipe they put through a metal roof. Many do not carry fasteners or sealants required to ensure a proper installation. Further, the responsibility (and thus liability) for the flashing falls typically on the roofing contractor.

It is also critical to require the penetrations be performed according to the manufacturer’s guidelines to protect warranty compliance. The roofing contractor is usually well-versed in installation details required by manufacturers; this includes ensuring the metal roof is permitted to expand and contract as needed per the system requirements while not compromising the weathertight integrity of the penetration flashing. Specifications should require the roof installer be responsible for fitting all penetration flashings to ensure trained personnel are weatherproofing them. If this is left to others, improper methods might be employed, resulting in leaks and damage not covered by the manufacturer’s warranty.

Using manufacturer-approved clips is the preferred approach to minimizing roof leaks resulting from solar panels.

Air circulation exhaust fans
There are two primary focal points when specifying how roofs should be penetrated in order to accommodate exhaust fans while avoiding leaks—the curb and adequate secondary structural supports.

Ensuring the former can integrate with and be supported by the metal assembly is the challenge, because, ultimately, the curb—not the fan—is key to keeping a roof weathertight. It is imperative the specifier research the metal roof basis of design in any specification and approved substitutions to ensure the manufacturer has curbs that integrate with the specified assembly to provide a weathertight installation. This often eliminates several manufacturers, as not all have approved curbs and details for this purpose. If left up to others to determine, the least expensive conventional curbs are often installed, jeopardizing the warranty, or worse yet, the weathertight integrity of the roof system.

Typically, the curbs need to be supported by systems not initially designed as part of the building structure. These secondary structural members tie into the structure and support the weight of the curb and exhaust fan, as well as allow for an attachment point for the new curb. Without them, the roof panels may start to deflect, creating a potential for leaks. Lack of proper secondary structural support is one of the most common defects in curb installation.

Repairing leaks caused by air circulation and exhaust fans can be much more challenging than fixing penetrations for ventilation piping. These types of repairs may require multiple trades, including electricians. In some cases, it must not only include replacing the damaged curb, but also the structural members.

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