Asphalt Shingle Underlayment Where Wind Design is Not Required

[jar546]

This applies to most of the USA which is why I am using it as an example. Asphalt shingles are the most popular roofing system across the US for single-family homes in most areas. Codes can be confusing because you always have to reference something else. Let's break down the underlayment requirements for an asphalt roof in an areas not subject to wind design specifications.

Table 905.1.1(1) gives multiple underlayment options to choose from, but what are they? The IRC does not tell you, so we are.

1. ASTM D226 Type I or II: This standard is for asphalt-saturated organic felt used in roofing and waterproofing. Type I is commonly known as No. 15 felt, and Type II is No. 30 felt. The difference primarily lies in their weight and thickness, with Type II being heavier and more durable.
2. ASTM D4696 is a standard that addresses polyethylene film used as an underlayment in roofing. Unlike the asphalt-saturated felts covered by ASTM D226 and D4869, ASTM D4696 is about plastic underlayments. These types of underlayments are often used in specific applications where water vapor transmission and resistance are critical factors. Here's a breakdown of the types:
   1. Type I: This would be the basic grade of polyethylene film underlayment.
   2. Type II: A step up from Type I, offering improved characteristics, potentially in terms of thickness, durability, or other performance metrics.
   3. Type III and IV: These would represent further enhancements in quality or specific performance features over Types I and II.
3. ASTM D6757: This standard specifies polymer-modified bitumen underlayment used for steep slope roofing. This type of underlayment is known for its superior waterproofing qualities, resistance to tearing, and stability under extreme temperatures. It's more advanced than the traditional felt underlayment and is often used in high-end residential and commercial roofing applications.

Before I get into the requirements for installing the underlayment, lets look at the two exceptions first:

Exceptions:

1. 1.As an alternative, self-adhering polymer-modified bitumen underlayment bearing a label indicating compliance with ASTM D1970 and installed in accordance with both the underlayment manufacturer’s and roof covering manufacturer’s instructions for the deck material, roof ventilation configuration and climate exposure for the roof covering to be installed, shall be permitted.
2. 2.As an alternative, a minimum 4-inch-wide (102 mm) strip of self-adhering polymer-modified bitumen membrane bearing a label indicating compliance with ASTM D1970, installed in accordance with the manufacturer’s installation instructions for the deck material, shall be applied over all joints in the roof decking. An approved underlayment complying with Table R905.1.1(1) for the applicable roof covering for areas where wind design is not required in accordance with Figure R301.2.1.1 shall be applied over the entire roof over the 4-inchwide (102 mm) membrane strips. Underlayment shall be applied in accordance with Table R905.1.1(2) using the application requirements for areas where wind design is not required in accordance with Figure R301.2.1.1. Underlayment shall be attached in accordance with Table R905.1.1(3).

Now we can look at the underlayment requirements for asphalt shingles in an area not subject to wind design:

For roof slopes from 2 units vertical in 12 units horizontal (2:12), up to 4 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. Distortions in the underlayment shall not interfere with the ability of the shingles to seal. End laps shall be 4 inches and shall be offset by 6 feet.
For roof slopes of 4 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 2 inches, Distortions in the underlayment shall not interfere with the ability of the shingles to seal. End laps shall be 4 inches and shall be offset by 6 feet.

 

[Mr. Inspector]

PA does not require a permit for reroofing, but if any sheeting is replaced it does require a permit. A couple of our townships require a permit for re-roofing. We don't inspect the underlayment for new houses, just for the Ice and water barrier when reroofing unless they are going over an existing one layer of shingles.
Timing for an inspection of the Ice and water barrier is hard since the contractors don't want to stop working and sometimes we need to rely on photos.

The most fails we have is the missing kick out flashing:
2018 IRC Flashing shall be installed to divert the water away from where the eve of the sloped roof intersects a vertical sidewall.

We are undecided on if reroofing requires a cricket at a chimney where one is missing (built before we had codes) especially if just installing a second layer of shingles.

How do other inspectors do this?