What are the most common types of deck substrates for metal roofing, and which one is best for your next project?
There are a variety of deck substrates to choose from for metal roofing projects, including plywood, B-deck, and OSB. The type of substrate you choose will dictate your installation process, so it’s important to understand how these materials differ.
At Sheffield Metals, we have an in-depth understanding of metal roofing materials. After spending more than two decades providing metal coils and sheets to our customers, we’ve learned what it takes to construct a metal roof that lasts. Our expert Technical Team assists us in putting together content that we can then distribute to contractors, installers, architects, and homeowners so they can make more educated decisions about their next project.
In this article, we’ll discuss the differences between metal roofing deck substrates and some tips for installation.
What Are the Most Common Metal Roof Deck Substrates?
Most roof decks will be made from either plywood or metal. These are two broad categories, however, and will include many details that further dictate the installation process, such as a metal deck with ISO or OSB instead of plywood.
Open framing is also an option — this is where there is no decking or substrate installed and the roof is installed directly over the framing.
Plywood and OSB
Plywood and OSB are two of the most common materials used for metal roof deck substrates.
So what makes these two materials different, and how do you know which one is right for your project?
The most significant difference is how they’re made. Plywood is made from thin sheets of wood that are glued together, whole OSB (oriented strand board) are wood chips that have been glued and hot pressed together to make up a solid board. Plywood has been the common choice for roofers in the past, although OSB has become more popular due to its cost-effectiveness.
In truth, plywood and OSB are very similar, but with OSB, you will lose some structural performance — something you’ll have to consider when you get to installation.
Fastener pull-out values for OSB are less than plywood and this can pose a problem for how the roof system should be attached. For example: The pull-out value for a #10 13×1” pancake-head fastener into ½ ” plywood is 375 lbs. That same fastener into 5/8” thick OSB is only 357 pounds. This can make a significant difference in performance if it is not accounted for when choosing the type and thickness of your substrate. 7/16” OSB is not considered structural and should never be installed on a roof.
For a more in-depth look at OSB vs. plywood, check out our video on the topic:
OSB vs. Plywood: Which Should You Choose for Your Roof Deck? (youtube.com)
B-Deck
B-deck is corrugated metal decking. It’s a structural decking typically used on commercial projects, with 22-gauge metal or thicker. For B-deck, you’ll use a different type of fastener instead of a gimlet/sharp point like you’d use for plywood. You’ll also use a self-driller to get through the metal.
Installing a roof over a metal deck will require unique processes that are different from plywood and OSB. This is especially true regarding the perimeter edges of your roof. Since you’re dealing with a corrugated substrate, you’ll need to make sure you have a solid attachment for your fastener spacings all along your rake edges, valleys, sidewalls, ridges, and eaves. When you’re doing a 4” or 6” fastener spacing, you need to make sure you’re not missing the deck.
For Sheffield Metals’ standard details, we show a 22-gauge continuous support on metal decking, which acts as a support angle. The support angle is an additional flashing installed around all edges of a roof, including valleys and sidewalls. This same support applies when installing insulation over B-deck — you’ll be adding it all around your perimeters for fastener attachment. This option can save time and money by eliminating the need for longer fasteners in those areas.
What Are the Requirements for B-Deck with ISO During Roof Installation?
Since B-deck with ISO is a thicker system, you’ll need longer fasteners to match to get through all the parts of the assembly. ISO is a common type of commercial roof insulation that offers various benefits, including sustainability, improved durability, and fire resistance.
#10 fasteners are essentially the starting point for fasteners on plywood. With a metal deck, you’ll likely want to use a #12 fastener, and with a metal deck with ISO, you will probably need to use #14s due to the length of the fastener required to reach the decking through the assembly. These fastener types are examples and an installer should always refer to the technical and engineering information provided for specifics.
Open Purlins
Installing a metal roof over open purlins will require different testing since the roof you put down will also act as the decking.
With solid decking, you’ll only need a UL 580/1897 test for wind uplifts to meet testing standards. With open framing, you need a panel that can perform structurally because the panel is technically acting as the barrier from the outside of the building to the interior. For this, you’ll need ASTME 1592 testing.
With open framing, your clip attachments are dictated by your purlin spacing. You need to have continuous support all around the perimeter of your roof to span your purlin spacings and to ensure all of your perimeter flashings, including eaves, rakes, valleys, sidewalls, headwalls, ridges, have a continuous component to attach to.
This is also required for penetrations — when you have curbs or pipes in the center of the roof, you’ll need to account for a structural component under your panel system to get around those penetrations.
Not all panel systems are recommended for use on open frame projects. SMI offers the 2” mechanical seam panels for use on open framing conditions as this panel offer the best structural performance as well as has the necessary ASTME 1592 uplift testing.
How Does the Deck Substrate Affect Engineering for Metal Roofing Projects?
The type of material you’re using for your substrate will determine which accessories you’ll use to meet the engineering requirements of that system.
If you want to meet Sheffield Metals’ engineering requirements, the deck substrate has to match the specific type used in the engineering tests. Each project is unique, and engineering on different substrates will require different fasteners, clip types, and clip spacings.
Matching the engineering to your unique project is essential to maintain the uplift integrity of your roof. This is especially true for commercial projects with weathertight warranties, as the deck assembly will need to match the specific engineering requirements provided to fulfill that warranty.
Where Can You Find More Information on Deck Substrates?
When it comes to deck substrates, you have a few different options to choose from. And while this article gives a good overview of the different types, you may be looking for information and advice that relates to your specific project. You could be looking for how to replace your project’s roof decking, or looking for more in-depth videos on how to install certain details.
At Sheffield Metals International (SMI), we have a Technical Team that can assist contractors with many aspects of their installation projects, including testing, panel selection, and engineering requirements. Feel free to reach out today to learn more about which deck substrate would be right for you.
