Reroofing: When to Retrofit and When to Replace

Eventually, even the best roof system will fail. So it is inevitable that, at some point in time, every building will require reroofing. This article will help you explore two metal roof retrofit alternatives, providing you with the information you need to determine when to retrofit over an existing roof, or, when to tear-off and replace it.

Metal roof retrofits and replacements are not limited to metal-roofed buildings. Many flat roofs (membranes over insulation) can be upgraded to accept a metal retrofit or replacement, to take advantage of the added slope that metal roofing provides. But regardless of the type of existing roof, the quality of its assembly must be thoroughly evaluated prior to determining whether retrofit, or tear-off and replacement, is your best option.

WHEN THE EXISTING ROOF IS METAL

When the existing roof is a metal one, your first step is to visually evaluate the condition of the panels. Specifically, what you are looking for is obvious signs of rusting and deterioration and/or a profusion of popped-out fasteners. It is rare for metal roofs to require total replacement. But when the damage is severe, it may be advisable to entirely remove the existing roof prior to reroofing.

More often than not, metal roofs can be successfully retrofitted by installing a framing system between the existing roof and the new one. Such framing systems attach the new roof to the framing of the building, thereby transferring the load from the new roof to the building's structure. Design loads should always be checked when adding weight to any building structure, especially pre-engineered metal buildings which have limited weight capacity.

To summarize, when replacing metal with metal, there are two major concerns: • How serious is the existing damage to panels and the fastening system? • Does a second roof already exist?

WHEN THE EXISTING ROOF IS FLAT

The ability to add slope to flat roofs is one of the most exciting capabilities of metal retrofitting. But before entertaining this option, there are several things to consider.

First, you'll want to consider your building's load restrictions. Typically, the framing and metals used in a metal retrofit will add three pounds of weight per square foot to the deck's structural load. It is imperative that a qualified engineer evaluate your building's ability to accept that additional

BY JOHN L. PIERSON, PE

weight.

Equally important is the issue of wet insulation. Trade organizations and design professionals uniformly agree that wet insulation should always be removed and replaced prior

to reroofing, regardless of the type of retrofit. The good news is that only the wet insulation requires replacement, so a total tear-off is not required. The further good news is that retrofit provides an ideal opportunity not only for adding slope, but also for increasing the R-value of your roof by installing additional insulation.

The framing system required to install a metal roof over a flat one is more elaborate than that which is used for metalto-metal retrofits, since it has the additional purpose of adding slope to the finished installation. It should be noted that, depending on its dimensions, such framing may constitute an enclosed space, according to building code requirements, thereby necessitating the installation of sprinkler systems. Ventilation requirements, too, may be affected by the installation of a framing system and retrofit metal roof. The engineering services of a roofing professional, acquainted with the nuances of your local building code, can help you determine what additional requirements might apply should you opt to retrofit a flat roof with a metal one.

Finally, since performance requirements for flat roofs and metal roofs may differ, you'll want to review specific building code requirements with an engineer, such as fire-resistance and wind-uplift requirements, in order to ensure compliance when upgrading from a flat to a metal roofing solution.

CONCLUSION

Retrofitting a new metal roof system over an existing roof assembly, whether metal or flat, offers cost-saving and timesaving benefits, in comparison with total tear-off, related disposal, and replacement. Retrofits also protect the building’s interior from inclement weather conditions and construction debris, while minimizing disruption to occupants.

However, a retrofit isn’t the panacea for all situations. Attractive as this option is, the decision to retrofit should only be made after a thorough engineering analysis that takes into consideration all relevant factors, including the condition of the existing roof and specific building code requirements.

About the Author: John L. Pierson, PE, is the engineering services manager and product manager for metal roofing systems for The Garland Company, Inc., a Cleveland-based manufacturer of high-performance roofing materials. He is a member of Garland’s Speaker’s Bureau and delivers seminars and AIAaccredited classes on installation techniques and roofing technology. Prior to his work with Garland, he was employed in the construction industry as a field engineer and consultant. ❏