Reroofing: When to retrofit and when to replace BY JOHN L. PIERSON, PE
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
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Ops Talk Fall 2009
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
