Roof Decking & Sheathing: Useful Information Guide
By Jack Gray, Roof Online Editor • Last updated May 15, 2023
Introduction: Overview of Roof Decking & Sheathing
The deck is the foundation of all standard roofing systems. The roof’s performance is largely dependent on the foundation’s integrity, just like the performance of a building itself.
The type of roof decking used is usually determined by the materials used in the building, the design of the building, and the purpose of the building.
Choosing a deck type is not based solely on its physical properties, appearance, or price. Its effect on the installation and performance of the roof system should be considered as well.
Roof decking is considered to be the bottom (and foundational) layer of a roof assembly. It is not considered to be a part of a roof system, and that’s the main difference between a roof assembly and a roof system.
The roof deck is installed on top of the roof framing or roof support system (the rafters, trusses, or joists). Roof decks are typically made of steel, concrete, or wood.
The roof deck is usually the only part of a roof that is considered to have a structural function. The roof system itself has an extremely important function, but it isn’t structural.
The structural roof support system is not normally considered a part of the roof. On a house, for instance, the rafters or trusses are classified as a part of the framing.
The roof deck provides a stable and durable platform that forms the substrate for the attachment of the roof system. It serves as a structural base that supports the weight of the roof covering and any live loads, such as accumulated snow or materials loaded onto the roof during a roof installation.
The roof decking helps to tie the entire building structure together by connecting the roof support system components to each other and to the walls of the building. This helps the building resist lateral forces, such as those caused by wind or earthquakes. It distributes those forces to the roof framing system, mitigating stress or strain that could otherwise cause structural damage or failure.
An appropriately-designed roof deck can also help to improve the energy efficiency and performance of the building as a whole. It can act as a reflective barrier, reducing the amount of heat that is absorbed through the roof, and can serve as an air or vapor barrier, creating a more airtight seal that improves the insulation and ventilation performance of the building.
To summarize: the function of roof decking in a roofing system is to provide a sturdy and stable substrate for the roof covering, to support live loads and resist lateral forces, and to contribute to the energy efficiency and performance of the building.
Difference Between “Roof Decking” & “Roof Sheathing”
Both “roof decking” and “roof sheathing” are terms used to describe the structural component of a building that serves as the substrate for the attachment of a roof. They are often used interchangeably, and in many cases, properly so. There is a difference, though. I’ll try to explain it.
To start with, a roof deck is defined by the Roof Consultants Institute’s (now IIBEC) Glossary of Roofing Terms as “1) Any horizontal surface not defined as a wall. 2) A structural component of a building’s roof that provides the substrate to which the roofing or waterproofing system is applied”.
Sheathing is defined as “The covering placed over exterior studding or rafters of a building that provides a base for the application of wall or roof cladding.”
So there’s an overlap between the definitions, but clearly “roof decking” implies a horizontal application found on a flat roof, while “sheathing” implies the use of rafters and therefore a pitched roof. But strictly speaking, they’re still interchangeable as long as the building has rafters (or pitched trusses, which are used in place of rafters).
They are not interchangeable if it’s a low-slope or “flat” roof. Flat roofs don’t use rafters. On a low-slope roof, the correct term is “roof decking” or “roof deck”.
Due to its common use in residential pitched-roof construction, “roof sheathing” also connotes the use of wood products such as planks, OSB panels, or plywood. “Roof decking”, on the other hand, can be plywood, OSB, wood planks, steel panels, concrete, gypsum board, and so on.
So as far as common usage in the roofing industry goes:
“Roof decking” is typically used in commercial roof construction and covers any material used to construct a low-slope roof deck. It is also frequently used in residential pitched-roof construction to refer to what is otherwise known as “sheathing”.
“Roof sheathing” is typically used in residential roof construction to refer to wood products used to construct a pitched roof deck, or to refer to the pitched roof deck itself.
About the Author
Jack Gray is a principal roof consultant and vice president at the Moriarty Corporation, an award-winning building enclosure consultant firm founded in 1967. He is also the editor of the Roof Online website.
Mr. Gray has been involved in the roofing industry for over 25 years, with training and practical experience in roof safety, roof inspection, roof installation, roof condition assessment, estimating, roof design & specification, quality assurance, roof maintenance & repair, and roof asset management.
He was awarded the Registered Roof Observer (RRO) professional credential in 2009.
He also served as an infantry paratrooper in the 82nd Airborne Division and attended Cornell University. Read full bio.
- Air Barriers, Vapor Barriers, and Weather Barriers
- Building Codes and Roofs
- Roof Decking Manufacturers List
- Wall Sheathing
- Weight of Plywood and OSB
- Weight of Roofing Materials
- Weight of Sheet Steel
External Sources & References for Roof Decking & Sheathing
Note (5/15/2023): A couple of the links to IIBEC Interface articles are not currently working, as IIBEC has recently changed their website structure. We’ve removed those active links for now, but we’re leaving the references in place in case we can link to them again in the future.
1. Book Recommendation: Manual of Low-Slope Roof Systems: Fourth Edition by C.W. Griffin & Richard Fricklas
2. General: For a good overview article describing various types of roof deck, see “Roof Decks: Don’t Underestimate the Backbone of the Roof System” by Thomas W. Hutchinson. The article is from 2014 and is archived on the Roofing Magazine website.
3. General: The National Roof Deck Contractors Association website has some useful information about various roof deck types.
4. Building Codes: Commercial: Lumber sheathing: see 2304.8.2 Structural Roof Sheathing in the 2018 International Building Code.
5. Building Codes: Commercial: Steel roof deck: see 222.214.171.124 Steel Roof Deck in the 2018 International Building Code.
6. Building Codes: Residential: Wood sheathing (wood plank, plywood and OSB): see Section R803 Roof Sheathing in the 2018 International Residential Code.
7. Clay Tile (Structural): See “Roof Decks: A to Z – Part 3: Structural Clay Tile and Plywood” by Lyle D. Hogan, Donald Kilpatrick, and Richard Koziol in the November 2011 issue of IIBEC Interface, the technical journal of IIBEC (formerly RCI, Inc.).
8. Concrete: Cast-in-Place Concrete: See “Roof Decks: A to Z – Part VII: Cast-in-Place Structural Concrete” by L. D. Hogan and Rob Kennerly in the April/May 2015 issue of IIBEC Interface.
9. Concrete: Post-Tensioned Concrete: See “Roof Decks: A to Z – Part XI: Post-Tensioned Concrete” by L. D. Hogan in the July 2018 issue of IIBEC Interface.
10. Concrete: Precast Concrete Planks: See “Roof Decks: A to Z – Part 2: Wood Planks and Precast Concrete Planks” by Lyle D. Hogan in the April 2011 issue of IIBEC Interface, the technical journal of IIBEC (formerly RCI, Inc.).
11. Fiberglass-Reinforced Plastic: “FRP Roof Deck: A Solution for Challenging Conditions” is a decent starting point for information about this type of roof deck. From the November 2011 issue of IIBEC Interface and available on the IIBEC website. (Note that the author was employed by an FRP manufacturer at the time the article was published.)
12. Gypsum: Poured Gypsum: See “Roof Decks: A to Z – Part 4: Poured Gypsum” by Lyle D. Hogan in the October 2012 issue of IIBEC Interface.
13. Lightweight Insulating Concrete: See “Roof Decks: A to Z – Part 1: LWIC and AWC” by Lyle D. Hogan in the September 2010 issue of IIBEC Interface.
14. Lightweight Insulating Concrete: “Lightweight Insulating Cellular Concrete: A Seamless Roof Deck Installation” by Leo A. Legatski is a good basic introduction to this material. Available on the IIBEC website in the May 2000 issue of IIBEC Interface.
15. OSB/Plywood: Great article for history and comparison: “Choosing Between Oriented Strandboard and Plywood” by Paul Fisette is available on the website of the Building and Construction Technology Program at UMASS Amherst.
16. Plywood: See “Roof Decks: A to Z – Part 3: Structural Clay Tile and Plywood” by Lyle D. Hogan, Donald Kilpatrick, and Richard Koziol in the November 2011 issue of IIBEC Interface.
18. Steel: “Fundamentals of Steel Construction” by Lyle D. Hogan is a must-read article for anyone involved in penetrating or replacing the roof where there’s a steel roof deck. From the May 2001 issue of IIBEC Interface.
21. Wood Planks: See “Roof Decks: A to Z – Part 2: Wood Planks and Precast Concrete Planks” by Lyle D. Hogan in the April 2011 issue of IIBEC Interface.