Roof Building Codes: Chapter 9 Of The IRC
Explained For Seattle Homeowners

August 22, 2022

Everything You Need To Know About The Requirements
For Asphalt Roof Coverings

Are you getting a roof replacement on your Seattle home? With all the jargon thrown around during exterior remodeling, you might be wondering: What kind of rules and regulations protect you as a homeowner?

A set of guidelines called Building Codes, specifically the International Residential Code (IRC), breaks down the requirements for what contractors have to do at a bare minimum. Chapter 9 details what is required for proper roof coverings, such as asphalt shingles. The IRC is full of hard-to-read language, so here’s a breakdown of exactly what this all means for you.

Asphalt Roofing Terms Explained

If you choose asphalt shingles as your roof replacement material, there are specific guidelines your roofing company must follow.


Asphalt shingles can only be installed on a roof with a slope of at least 2:12. What this means is that for every 12 horizontal inches, your roof must increase by two vertical inches. Anything less and asphalt shingles cannot be used. Furthermore, for roofs with low slopes between 2:12 and 4:12, a double underlayment is required or rubberized ice & water shield on the entire surface.



The underlayment is required to conform to specific classes: ASTM D226 Type I and ASTM D4869 Type I. This means your roofing company must use an organic felt underlayment saturated with asphalt. It is a breathable, weather-resistant barrier applied over the decking before the installation of your new shingles. Alternatively, some synthetic underlayments made using woven polypropylene are now accepted. These synthetics are less likely to wrinkle when damp, are more tear resistant, and generally safer to walk on for the roofer. We use these products at RoofSmart.

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Roofs with slopes 4:12 or greater will have a single application underlayment. However, in areas where the average January temperature is 25℉ (-4℃), an ice barrier must be created by merging two layers of underlayment for added protection at the eaves up past where the attic starts. This helps prevent ice damming, but usually is not required in the Seattle market.

Asphalt Shingles

The asphalt shingles themselves are required to have self-sealing strips or an interlocking design. This will give the shingles exceptional wind resistance by locking down their leading edges. The self-sealing strips are typically made from an asphalt-based adhesive specifically designed to adhere shingles together.


To secure your shingles in place, your roofing contractor is required to use roofing nails crafted from galvanized steel, stainless steel, copper, or aluminum. All nails must also be 12 gauge with a minimum ⅜-inch head and can penetrate a minimum of ¾-inch into the roof sheathing, or go all the way through. Sometimes this is an issue at overhangs without a soffit. If you have ½” plywood and nail that has to go ¾” deep, you’re going to get some poking through.

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Shingle Attachment

Shingles will be attached using the minimum number of fasteners the manufacturer requires, and no less than four per shingle strip. However, special fastening methods will be required if the roof slope exceeds 20:12 or the home is in a location where the basic wind speed exceeds 110 mph. Sometimes that means more nails, sometimes it means hand-sealing each shingle in addition to the factory sealant.


Flashing is a thin metal sheet that roofers use to direct water away from critical areas on a roof. The IRC states that base flashing, the continuous flashing that may be used around the chimney, and the cap flashing, or an L-shaped structure laying flat against the roof, are both made of corrosion-resistant metal.

Sidewall flashing should be installed via the step-flashing method that utilizes a regular piece of flashing bent at 90 degrees in the middle. Any flashing for a vertical front wall, chimney, or vent pile should be installed according to the asphalt shingle manufacturer’s specific directions.

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Valley linings need to be installed according to manufacturer directions before the installation of the asphalt shingles. That’s going to be 24” wide “W” metal valleys, or ice and water shield. We use both on most applications at RoofSmart, because we absolutely do not want a leak. Ever..


Are You Ready For A New Roof Replacement By A
Company That Goes Above And Beyond the Minimum?

When it comes to getting a roof replacement in the Greater Seattle area, choose a roofing company that will go above and beyond the minimum requirements of the IRC. Our roofs are engineered to our exacting Egghead Installation Standards, and guaranteed for life.

Here at RoofSmart, we strive for excellence in everything we do to deliver the best quality results for our customers. Our ceaseless attention to detail allows us to perfect our craft and deliver the perfect roof every time

Call us today at (206) 487-4877 to schedule your free estimate

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