IWS Material Width  (INCHES)
Soffit Overhang  (INCHES)
Exterior Wall Width (INCHES)
Pitch (RISE TO 12" RUN) 

Maximum Eave Width (inches) 

Before Second Length Of IWS Is Req'd


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<----If less than 0, you need
a second run of IWS


2009 and 2012 IRC Section R905.2.7.1, 2015 IRC Section R905.1.2 Ice Barriers.

In locations with a history of ice formation along the eaves that can cause a backup of water as described in

Table R301.2(1) install an ice barrier consisting of self-adhering polymer-modified bitumen sheet or two layers of underlayment cemented together.

The barrier should extend from the lowest edge of the roof to at least 24 inches above the inside surface of the plane of the exterior wall.

(IRC 2015 adds: If the roof slope is ≥ 8/12 the barrier should extend up at least 36 inches from the eave edge of the roof.)

RoofShots Waste Calculation

All major manufacturers describe a “preferred installation method” of a 6 row “stair step method”. This starts with a full shingle and each successive row has an average of 6” cut off to stagger the joint between each successive row. The 7th row starts the cycle again with a full shingle.
With an average 5 5/8” exposure, this creates 1.25 square feet of waste per linear feet of “vertical field edge”. The geometry of hips and valleys lengthens these lines and additional waste is created commensurately.
L = total linear feet of vertical field edge
The waste calculation is simple: L * 1.25
The percentage of waste can be expressed as such:
WASTE % = Area / L * 1.25

NOTE: Waste Considerations

  • 1) Manufacturers require a minimum of ¼ - ¾ inches of overhang on the first course.
  • 2) Material is wasted on the top row as a full 5 5/8” is not typically needed to reach the ridge.
  • 3) This calculation assumes typical W-valley installation (Closed, Woven or California Cut Valley installations will generate additional waste)
  • 4) While this calculation is mathematically based & superior to “rules of thumb”, it cannot account for manufacturers defects, poor handling, or inferior installation methods.
  • 5) Additional waste must be added for shingles used as starter or hip & ridge cap.


United States » Alabama



L - Adopted by Local Governments