Ice and water shield (IWS) is one of the most critical components of a modern roofing system, designed to prevent water intrusion at the most vulnerable points of the roof: the eaves and valleys.
Because it is a premium material, insurance carriers frequently try to minimize its use on their estimates. They will often approve it only for valleys, or approve an insufficient quantity for the eaves.
If you aren't supplementing for the correct amount of ice and water shield, you are either eating the cost yourself or installing a non-code-compliant roof.
Understanding the Code Requirements
The foundation of any successful supplement is the building code. For ice and water shield, the International Residential Code (IRC) is very clear.
IRC Section R905.2.7.1 (Ice Barrier): "In areas where there has been a history of ice forming along the eaves causing a backup of water... an ice barrier shall be installed for asphalt shingles... The ice barrier shall consist of not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet [Ice and Water Shield]... The ice barrier shall extend from the lowest edges of all roof surfaces to a point not less than 24 inches (610 mm) inside the exterior wall line of the building."
The "24 Inches Inside the Exterior Wall" Rule
This is where carriers make their "mistake." They often calculate the amount of IWS based on a single 3-foot roll applied at the eave.
However, the code requires the barrier to extend 24 inches inside the exterior wall line.
If a home has a 2-foot overhang (soffit), a single 3-foot roll of IWS will only extend 1 foot inside the exterior wall line. This fails code. To comply with the IRC, you must install two courses (two 3-foot rolls) of IWS to reach the required 24 inches inside the warm wall.
The Supplement Strategy: When the carrier only approves one course of IWS (Xactimate code RFG 191), you must measure the overhangs. If the overhangs are 12 inches or greater, one course is mathematically insufficient. You must supplement for the second course and cite IRC R905.2.7.1.
Valleys and Penetrations
The IRC also dictates how valleys should be handled.
IRC Section R905.2.8.2 (Valleys): Valley linings must be installed before shingles. While metal is an option, a self-adhering polymer-modified bitumen sheet (IWS) is the standard in most modern installations.
Carriers often "forget" to include IWS for the valleys (RFG 190). Always check the linear footage of the valleys on the roof measurement report and ensure the corresponding quantity of IWS is on the estimate.
Furthermore, best practices and manufacturer specifications often require IWS around critical penetrations like chimneys and skylights.
How EstimateDelta Catches IWS Shortages
Calculating overhangs, valley lengths, and cross-referencing them against the carrier's approved quantities takes time and precise math.
EstimateDelta does this instantly.
When you upload the carrier's estimate, our engine looks at the quantity of RFG 191 (IWS at eaves) and compares it to the total perimeter of the roof. If the ratio suggests only a single course was approved, EstimateDelta flags it as a high-probability missed item.
It then generates a supplement letter that explicitly cites IRC R905.2.7.1, explaining the math of the overhang and demanding the second course to achieve code compliance.
Stop doing the math by hand and fighting adjusters over building codes. [Let EstimateDelta build your IWS supplement today](/pricing).