Choosing Proper Substrates for Metal Roofing Systems

Posted on February 16, 2017 by Randy Tweedt

How to choose an appropriate substrate to support a metal roof is a frequently asked question and one which is equally important for quality installation and long-term durability of the product. Some of the considerations include ensuring a structurally sound metal roofing base, knowing the type of underlayment required, and how that required support can vary depending upon the type of metal roofing product being utilized. In its most basic terms, metal standing seam roof (SSR) and through-fastened roof panel systems can be supported on either open framing or solid substrates. The choice will often depend on the type of construction and the purpose of the building, including residential, commercial, industrial and agricultural applications.

Importance of Substrates

As a key part of the metal roofing system, the substrate (or substructure), which rests underneath the metal roof panels, serves two main functions.

  1. It acts as a base to which the metal roof material is attached-securing the fasteners and clips used to hold the panel. (Note: fasteners and clips would be attached into the substructure.)
  2. It serves as a structural member, transferring the weight of both live (ex: wind, snow, rain) and dead (ex: HVAC units, roof materials, etc.) loads to the supporting joists or purlins.

Types of Substrates

The two main types of substrates used with metal roofing systems are open framing and solid substrates (composite or plywood).

Open Framing Substrates

The open frame substrate uses wood or metal framing members (such as bar joists or min. 16-gauge purlins) to secure the metal roof panels. These members may be individual pieces of wood, formed steel or truss-like members built from multiple individual pieces. Open frame substrates should be used with structural-style panels in which the design can support the weight and can also span the dimensions of open areas.

BattenLok® HS Substrates
Examples of MBCI structural-style panels include Double-Lok®, Ultra-Dek®, SuperLok® and BattenLok® HS

Solid Decking Substrates

A solid deck substrate may consist of panels, solid sheets, or closely spaced deck members (made of wood, metal or composite materials) to which the roof membrane is applied.

  • Solid decking should be used with non-structural style panels, such as MBCI’s Craftsman™ Series – Small Batten panels. (Note: While structural panels do not require a solid deck, they may be installed over one, depending on preference.)
  • Composite deck roof designs may or may not include rigid board insulated as part of the installation.
  • Plywood, oriented stranded board (OSB), and individual wood planks can all be used for the construction of a solid roof deck.

Other Considerations

It goes without saying that installers must follow OSHA practices and make sure to follow installation details of the manufacturer. Beyond this common-sense advice, there are other considerations to look at when matching substrate to a metal roof system, such as moisture control and acoustics. For instance, if you’re going over a plywood deck, you would always want to be sure you have a moisture barrier between the wood and the panel; metal decks should have a vapor barrier, and sound transfer through certain substrates could be an issue. It’s always wise to do your research–confirm with the manufacturer, check installation manuals, etc.

The main takeaway here is an awareness that depending on your project’s panel profile, you need to take steps to ensure you have the appropriate substrate. For detailed information on MBCI roof panels and substrates, view MBCI’s standing seam panels.

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