Membrane-Over-Metal Roof Recover Explained

October 13, 2022

Your metal roof is failing, and you have to do something. You’ve got three options:

These approaches range in cost, with the total tear-off being the most costly and a coating being the least; however, this economical approach may not be feasible if the roof has been coated in the past and the coatings have deteriorated, are flaking, or loss of adhesion has occurred. Additionally, if dynamic forces (e.g. expansion and contraction or two adjoined buildings moving independently of each other) have caused the metal roof to fail; it is difficult to seal those junctions with conventional coating products. Another pitfall of coating is its shorter life. “Why take a short-term approach to a long-term building asset? In other words, why do something that’ll last 5-10 years when you can get 20-30?” asks Ben Fashimpaur, Vice President National Account Manager, D. C. Taylor Co., Cedar Rapids, IA, about a tear-off or membrane-over-metal recover project.

If your building can handle the additional weight capacity of a new roof over the existing one, a recover project can be beneficial for several reasons. As explained in the National Roofing Contractor Association’s Metal to Metal whitepaper by W. Lee Shoemaker, Vincent E. Sagan, and Dale Nelson, “The advantage of re-covering a roof is the original roofing materials can remain in place to protect the building interior during installation of new roofing materials, allowing for building operations to continue. Re-covering takes full advantage of existing insulation with the option of easily adding more insulation over an existing roof, increasing the energy efficiency of a building.”

How Membrane is Installed Over a Metal Roof

The challenge of installing single-ply membrane over a standing seam or conventional rib metal roof is that the surface is not flat. This is rectified with the addition of flute fill. Long planks of this insulation are typically loose laid between the ribs or standing seams of the metal roof. Flute fill is available in square or beveled edges to match the profile of various metal panels. Your roofing contractor will either order precut insulation specific to the width and profile of the depressions in your metal roof or cut it onsite if necessary.

You realistically have two options for flute fill insulation: polyisocyanurate or expanded polystyrene (EPS). R-value (polyiso has a higher thermal resistance), cost (EPS is less expensive), and fire rating may influence which type of insulation is specified for your project. Insurance requirements must be considered when choosing flute fill.

The addition of the flute fill makes it possible to apply either a coverboard or rigid polyisocyanurate insulation over the metal roof, spanning the ribs without it cracking. A coverboard provides more durability if the roof is heavily trafficked, but it can also be heavier putting more strain on the roof’s structural members. If this is a concern, conduct an engineering assessment to determine the structural load and weight capacity before proceeding. “It’s common for metal roofs to be built with little tolerance remaining for additional loads,” warns Fashimpaur.

The overlay insulation (commonly polyisocyanurate) is mechanically fastened through the flute fill and into the metal panels. Single-ply membrane is installed on top and mechanically attached into the purlins that run under the metal roof. Seams are heat welded. “These purlins are heavy gauge and can support the fasteners with high pull-out value. It’ll have high wind uplift resistance,” notes Fashimpaur. Most reputable manufacturers will not provide a warranty if the roof system is only attached to light-gauge metal panels; that doesn’t provide enough wind uplift resistance. Your insurance carrier and manufacturer warranty requirements will dictate the attachment methodology.

Wondering if a flute fill membrane-over-metal project is the best solution for your roof? Contact D. C. Taylor Co. at 319.731.4118 today.

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