Decking/Sheathing Case Study

By: Stanton Smith, P.E.

Code Compliant, Warranty Void

In a recent assignment from an insurer, the instructions started with: “[We] have determined the roof is a total loss due to hail damage and we have written an estimate to replace the roof.”

However, the contractor countered, as described here by the insurance company:

The roof decking is 1″ x 8″ (1×8) planks. The insured’s contractor contends the decking does not meet code and must be removed and replaced if the roof is replaced. Please inspect and provide a determination of whether the 1″x8″ decking complies with building code for solid decking.

In a meeting on site with the contractor, he indicated the roof decking was “not up to code,” and that the decking would have to be replaced when the roof was replaced.decksheeth1

His reasoning is similar to the rationale used during the replacement of a water heater.  A city permit process will often force a contractor to replace a water heater’s flue at the same time the water heater is replaced.  The flue replacement is an upgrade, but the goal of the upgrade is to have the flue conform to code.

decksheeth2Therefore, it’s important to determine whether the decking complies with the building code. As seen in the photos, the attic framing consists of wood trusses and wood decking.  The decking size is 1×8.  Our locale uses the 2009 version of the International Residential Code (IRC 2009). Note: the sheathing requirements in IRC 2009 have not changed from those of the previous code.

From IRC 2009 section R803, “Roof Sheathing,” dimensional lumber is allowed for use as sheathing in this house (on a 2 foot span) as long as it has a minimum net thickness of 5/8 inch.  The 1×8 lumber in this house conforms to that standard.  Therefore, the decking complies with the code.


Since the decking complies with code, how can the roofer insist that there is a problem?  What is the problem?

The problem is that the shingle manufacturers have begun to insist on a new standard.

The contractor provided documentation to support his assertion.  The documentation is illuminating.  Let’s look at how roofing is addressed by some of the manufacturers.

GAF Shingles
The contractor provided two GAF Technical Advisory Bulletins.  The first, dated 03/11/2011, number TAB-R 2011-101, is titled “Acceptable Substrate for GAF Asphalt Shingle Applications.”  The second, dated 04/11/2011, number TAB-R 2011-139, is titled “Deck Requirements for Existing Deck Boards.”  Both documents state that wood planking should be nominal 1” thick (minimum) and 6” wide (maximum).  They also state that “Roof decks must meet local codes.”  As stated earlier, the deck complies with the codes.

The “Deck Requirements” document does not address board widths greater than 6”, which is likely an intentional oversight, since the document purports to address existing decks.  According to a strict interpretation of these documents, GAF shingles cannot be used over the unmodified 1×8 decking on this house without the likelihood of voiding the warranty.

CertainTeed Shingles
The contractor provided a copy of the CertainTeed Shingle Applicator’s Manual, Chapter 4, “The Roof Deck and Tearing Off of Roofing Over,” (no provenance).  The document states, “If you apply a shingle roof over a deck surface that is unacceptable to the shingle manufacturer and damage results, the warranty might not be honored” (emphasis added).  Further, the document states, “Boards wider than 6” may need too large of a gap between them and may move more than the shingles can permit.  The result can be unsightly buckling . . . .”

Atop one page of the document is this tip:

Be sure to determine what type of decking or sheathing is in place when you’re inspecting older homes prior to making a cost estimate.  If you discover wide wood board decking that needs to be cut or covered over to avoid potential buckling problems, be sure to take this cost into account.

CertainTeed is clearly implying that its warranty will be voided by any unmodified, dimensional lumber decking wider than 1×6.

Owens Corning Shingles
The contractor provided an ICC Evaluation Service document ESR-1372, dated January 1, 2012.  The document is specific to Owens Corning asphalt shingles.  From section 4.1, “Installation of Owens Corning Shingles must comply with this report and the manufacturer’s published installation instructions.”  It also says that, “The roof deck must be in conformance with the applicable code, except as modified in this report,” (emphasis added).

This is the key point: the shingle manufacturers are insisting on a new standard which goes beyond code requirements.  Their documents “modify” code.

The ICC document requires a maximum width of 6” for solid sheathing.  Thus, Owens Corning shingles cannot be used over the unmodified decking on this house without the likelihood of voiding the warranty.

How can the problem be resolved?
The CertainTeed document suggests several methods of addressing deck boards wider than 6 inches:

To reduce the risk of shingle buckling, you can cover the boards with 4’x8’ nailable decking which could be plywood, OSB, or COM-PLY, or cut through the center of the entire length of each wide board with a hand-held power saw, being careful not to cut into the rafter more than 1/8”.  This will create a new joint in the center of each board, resulting in wood decking that is narrower than 6”.

According to their documentation, CertainTeed shingles can be installed on 1×8 decking if it is either overlaid or cut.  The decking does not have to be replaced.

The CertainTeed suggestion of cutting the decking boards longitudinally would solve the problem of compliance with the manufacturers’ requirements as well as eliminate the expense of additional decking.  Cutting would meet the requirements for the use of GAF and Owens Corning shingles as well.

Other Requirements
Shingle manufacturers are also requesting additional requirements.  For example, one of the GAF bulletins states that when roofing over an existing deck, “The deck boards must have been in place for a minimum of 5 years.”  In context, the statement follows a discussion of both sheet material and dimensional lumber.  No differentiation is made between the two, so the statement applies to both.  If the statement is enforced as written, any shingles installed over a deck which is less than 5 years old will require decking replacement, regardless of the condition of the decking.