Roofing Underlayment and Its Attachment Requirements

The International Residential Code (IRC) is commonly considered to be a prescriptive code, which means there are many requirements included that provide specific directions. Prescriptive-based code language provides a simpler method of enforcement for inspectors. And shouldn’t that be the case for one- and two-family dwellings, where well built and affordable is the goal?

Underlayment Requirements

In Chapter 9 of the 2015 IRC, the underlayment requirements for steep-slope roof coverings are included in three tables—material types, application and attachment requirements. Each table includes specific information for metal panels and separates out high-wind areas (defined as greater than 140 mph, and is only in the southernmost portion of Florida).

Roofing underlayment by our sister company ABC.
Roofing underlayment by our sister company ABC.

Material Type Requirements

Underlayment types for metal panels needs to only comply with manufacturer instructions. D226 and D4869 underlayments are viable options, as long as metal panel manufacturers allow them. And very importantly for metal panels, synthetic- / polymer-based underlayments are a viable option, again, as long as the panel manufacturer allows them to be used.

For metal panels in high wind areas, only D226 Type II and D4869 Type IV are allowed. In other words, only the heaviest materials are allowed in the highest wind zones.

Application Requirements

Simply put, underlayment should be applied according to the manufacturer’s installation instructions. For high-wind areas, specific application requirements are provided:

“For roof slopes from two units vertical in 12 units horizontal (2:12), up to four units vertical in 12 units horizontal (4:12), underlayment shall be two layers applied in the following manner: apply a 19-inch strip of underlayment felt parallel to and starting at the eaves. Starting at the eave, apply 36-inch-wide sheets of underlayment, overlapping successive sheets 19 inches, and fastened sufficiently to hold in place. For roof slopes of four units vertical in 12 units horizontal (4:12) or greater, underlayment shall be one layer applied in the following manner: underlayment shall be applied shingle fashion, parallel to and starting from the eave and lapped 4 inches. End laps shall be 4 inches and shall be offset by 6 feet.”

Underlayment Attachment Requirements

Underlayment should be attached according to the manufacturer’s installation instructions. For high-wind areas, specific attachment requirements are provided:

“The underlayment shall be attached with corrosion-resistant fasteners in a grid pattern of 12 inches between side laps with a 6-inch spacing at the side laps. Underlayment shall be attached using metal or plastic cap nails or cap staples with a nominal cap diameter of not less than 1 inch. Metal caps shall have a thickness of at least 32-gage sheet metal. Power-driven metal caps shall have a minimum thickness of 0.010 inch. Minimum thickness of the outside edge of plastic caps shall be 0.035 inch. The cap nail shank shall be not less than 0.083 inch for ring shank cap nails and 0.091 inch for smooth shank cap nails. Staples shall be not less than 21 gage. Cap nail shank and cap staple legs shall have a length sufficient to penetrate through the roof sheathing or not less than 3/4 inch into the roof sheathing.”

Self-Adhesive Underlayment Options

Of course, there are exceptions to these requirements. The first is to use a self-adhesive underlayment (i.e., ice dam protection) over the entire roof. The material needs to comply with ASTM D1970, “Standard Specification for Self-Adhering Polymer Modified Bituminous Sheet Materials Used as Steep Roofing Underlayment for Ice Dam Protection” and be installed per the metal panel manufacturer’s requirements. The code also points out that roof ventilation must be considered because a self-adhesive sheet is most often an air barrier and a vapor retarder. Concerns with moisture are quite relevant when these types of materials are installed over the entire roof deck. The second exception is to tape the seams of the roof deck with 4-inch wide strips of D1970 material, and then cover the deck with underlayment. The second exception is not widely used, except when trying to reduce, or eliminate, air flow through the deck while allowing moisture to escape.

IRC Requirements for Attaching Metal Panels

The IRC also includes some, but not many, requirements for the attachment of metal panels. The IRC requires metal panels be attached per manufacturer’s installation instruction and “be secured to the supports.” This implies fasteners should be attached to purlins or rafters, but one could easily argue the roof deck is the support for the metal panels. However, the IRC does provide specifics for fasteners used to attach metal panels, but the following is only applicable if manufacturer’s instructions don’t include fastener requirements. The IRC states:

“In the absence of manufacturer’s installation instructions, the following fasteners shall be used:

  1. Galvanized fasteners shall be used for steel roofs.

  2. Copper, brass, bronze, copper alloy and 300-series stainless steel fasteners shall be used for copper roofs.

  3. Stainless steel fasteners are acceptable for metal roofs.”

The Importance of Following IRC and Manufacturer Instructions

The IRC is a prescriptive code and there are many specific requirements for underlayment and metal panels. But because of the wide variety of styles, the IRC appropriately requires installation according to manufacturer’s instructions. It’s important to specify a new roof using both manufacturers’ instructions and IRC’s specific requirements. And, remember, a metal roof will have a long service life, so the underlayment’s service life should equal that of the metal roof. Don’t be shortsighted when designing for longevity.

Codes: More than the IBC and IRC

IBC IRC CodeWe all know to look to IBC Chapter 15 and IRC Chapter 9 for information about roof systems.  These two “Roof Assemblies and Rooftop Structures” chapters include the requirements for fire, wind, impact, materials, and reroofing.  But did you know the scope of the building code (IBC Section 101.4) references additional model codes that are considered to be part of the requirements of the IBC?  From a roofing perspective, this scoping reference brings into play the International Energy Conservation Code (IECC) and the International Existing Building Code (IEBC).

The creators of the model codes are attempting to ensure that buildings (and roofs, in our case) are designed and built according to the most recent model codes even if they haven’t been specifically adopted by a state or local jurisdiction.  If a jurisdiction adopts and enforces the 2015 IBC, by reference the 2015 IECC and 2015 IEBC are in effect.

How do 2015 IECC and 2015 IEBC affect roofs?
The IECC Commercial Provisions include energy efficiency requirements for the same buildings for which IBC Chapter 15 roofing requirements are required.  The IECC includes minimum insulation, air barrier, and reflectivity requirements for building envelopes.  Prescriptive R-values and U-values are provided for roofs, and they are based on climate zone, metal buildings, and attics.  Minimum levels of solar reflectance and thermal emittance are required for low-slope roofs on buildings with air-conditioning in climate zones 1, 2 and 3.

Air barriers—used to reduce or eliminate air leakage—are required for new construction.  These are based on materials, systems, or the whole building.  Sheet steel and aluminum are listed as materials that meet the air barrier requirements.  Of course, the joints and seams are critical to the effectiveness of metal roofing panels when considered to be air barriers.  When reroofing, air barrier requirements are not triggered, which is significant.  But the insulation requirements are triggered.

Roofing and structural considerations
The 2015 IEBC includes sections about reroofing (Section 706, which is new in the 2015 IEBC) and structural considerations (Section 707).  The IEBC divides “Alterations” of buildings into three types: Levels I, II and III.  A level I alteration includes the removal and replacement of existing materials.  Reroofing is a level I alteration, which triggers the requirements of Chapter 7.  The Structural section includes a requirement to upgrade a wind-resisting roof diaphragm when more than 50 percent of the roof is removed where the design wind speed is greater than 115 mph, and in special wind zones.  While these are small portions of the United States, it’s important to understand this requirement.

Build roofs with the full scope in mind
Look beyond the roofing chapters to ensure that you design and build buildings according to the most recent building codes.

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