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Attic Ventilation Formula

1 square foot (or 144 square inches) of net free area (NFA) ventilation for every 150 square feet of attic space.

The single most important variable in determining the longevity of a roof system is maintaining a properly ventilated attic. Having a properly ventilated attic requires 50% intake and 50% exhaust of air flow. Use the 1 in 150 Rule as shown below (hover over the tooltip to see calculation):

Attic Ventilation Using The 1 in 150 Rule

Soffit Net Free Area

Exhaust Net Free Area

500 square feet of attic space
240 square inches
240 square inches
600 square feet of attic space
288 square inches
288 square inches
700 square feet of attic space
336 square inches
336 square inches
800 square feet of attic space
384 square inches
384 square inches
900 square feet of attic space
432 square inches
432 square inches
1000 square feet of attic space
480 square inches
480 square inches
1100 square feet of attic space
528 square inches
528 square inches
1200 square feet of attic space
576 square inches
576 square inches
1300 square feet of attic space
624 square inches
624 square inches
1400 square feet of attic space
672 square inches
672 square inches
1500 square feet of attic space
720 square inches
720 square inches
1600 square feet of attic space
768 square inches
768 square inches
1700 square feet of attic space
816 square inches
816 square inches
1800 square feet of attic space
864 square inches
864 square inches
1900 square feet of attic space
912 square inches
912 square inches
2000 square feet of attic space
960 square inches
960 square inches

Example of how to calculate net free area:
1200 square foot attic divided by 150 square feet = 8 square feet (1200 / 150 = 8)
8 square feet times 144 square inches = 1152 square inches (8 x 144 = 1152)
1152 square inches divided by 2 = 576 square inches at soffit and at exhaust (1152 / 2 = 576)

The information below was originally compiled by www.hvac-buzz.com. I have since researched www.up.codes and discovered information that could be slightly different or information that pinpoints specific pipe venting termination lengths. Please do your own research on the building code relevant to each roof project before installing any products.

Building Code Requirements for Minimum Plumbing Vent Height Requirements Above Roof

There are 41 States (see below) that have a minimum plumbing vent height above the roof requirement of less than 9 inches (Roofing contractors are recommended to use Bullet Boots when installing roofs in these States.)

There are 7 states with 12 inch minimum height requirements for plumbing vents: Connecticut, Delaware, Indiana, Minnesota, Montana, Ohio, and Rhode Island (It's recommended for these States to use the Bullet California Boot because of its taller design meeting the 12 inch minimum. If the pipe is higher than 12 inches above the roof, the pipe may be reduced in length to stay within the minimum code requirements while allowing the Bullet California Boot to fit within the manufacturer specifications and installation instructions).

New Hampshire has a minimum requirement of 18 inches.

Vermont has a 6 inch minimum to a maximum of 12 inches for pipes plumbing vents to extend above the roof.

The details for each State is below in alphabetical order:

Alabama

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1 Residential Code 2015 of Alabama

Alaska

6” above roof OR 6” above firewall OR 7’ above roof (roof used as assembly or parking area)

Section 906.1 Section 906.3 Uniform Plumbing Code 2018 of Alaska

Arizona

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code for One- and Two-family Dwellings as amended by the city of Phoenix

Arkansas

6” above roof OR 7’ above roof (roof used as assembly or parking area)

Section 903.1 2018 Arkansas Plumbing Code

California

6” above rood OR 6” above firewall OR 7’ above roof (roof used as assembly or parking area)

Section 906.1 Section 906.3 California Plumbing Code 2019

Colorado

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code 2021 of Colorado

Connecticut

12” above roof OR 7’ above roof (roof used as living space)

Section P3103.1 2015 IRC portion of 2018 CT State Building Code

Delaware

12” above roof OR 7’ above roof (roof used as living space)

Section 903.1 2018 State of Delaware Plumbing Code

Florida

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 2020 FBC – Residential 7th edition

Georgia

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Georgia State Minimum Standard One and Two Family Dwelling Code

Hawaii

6” above roof OR 6” above firewall OR 7’ above roof (roof used as assembly or parking area)

Section 906.1 Section 906.3 Uniform Plumbing Code 2018 of Hawaii

Idaho

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 International Residential Code 2021

Illinois

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of South Holland

Indiana

12” above roof OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 2020 Indiana Residential Code

Iowa

6” above roof OR 6” above firewall OR 7’ above roof (roof used as assembly or parking area)

Section 906.1 Section 906.3 Uniform Plumbing Code 2021 of Iowa

Kansas

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of Kansas

Kentucky

6 inches above roof OR 7' above roof (roof used as living space)

Section 903.1 Plumbing Code 2018 of Kentucky

Louisiana

6 inches above roof OR 6 inches above projected snow line OR 7' above roof (roof used as living space)

Section P3103.1 Residential Code 2015 of Louisiana

Maine

2 inches above roof OR 2 inches above firewall OR 7' above roof (roof used as living space) [it appears this statement might conflict with the interpretation of the following details: 906.0 Vent Termination 906.1 Roof Termination Each vent pipe or stack shall extend through its flashing and shall terminate vertically not less than two (2) feet (610 mm) above the roof nor less than one (1) foot (305 mm) from any vertical surface. ]

Section 906.1 Section 906.3 Uniform Plumbing Code 2021 of Maine

Maryland

6 inches above roof OR 7 inches above roof (roof used as living space)

Section 903.1 Plumbing Code 2018 of Maryland

Massachusetts

6 inches above roof OR 6 inches above projected snow line OR 7' above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 International Residential Code 2021

Michigan

1 inches above roof OR 7' above roof (roof used as living space)

Section P3103.1 2015 Michigan Residential Code

Minnesota

12” above roof OR 7’ above roof (roof used as anything but weather protection)

Section 906.1 Section 906.3 2020 Minnesota Plumbing Code

Mississippi

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of Mississippi

Missouri

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1 Residential Code 2015 of Missouri

Montana

12” above roof OR 12” above firewall

Section 906.1 Section 906.3 Uniform Plumbing Code 2021 of Montana

Nebraska

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code of 2018 of Nebraska

Nevada

6” above roof OR 6” projected snow line OR 7’ above roof (roof used as living area)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of Nevada

New Hampshire

18” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code for One- and Two-Family Dwellings of the State of New Hampshire

New Jersey

6” above roof OR 7’ above roof (roof used for anything but weather protection or maintaining equipment)

Section 12.4.1 New Jersey Plumbing Code 2018

New Mexico

6” above roof OR 6” above firewall OR 7’ above roof (roof used as assembly or parking area)

Section 906.1 Section 906.3 2021 New Mexico Plumbing Code

New York

6” above roof OR 6” above projected snow line OR 7′ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 2020 Residential Code of New York State

North Carolina

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1 2018 North Carolina Residential Code

North Dakota

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of North Dakota

Ohio

12” above roof OR 7’ above roof (roof used as living space)

Section 903.1 Ohio Plumbing Code 2017

Oklahoma

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of Oklahoma

Oregon

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 2021 Oregon Residential Specialty Code

Pennsylvania

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of Pennsylvania

Rhode Island

6” above roof OR 12” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 2021 Rhode Island State One and Two Family Dwellings

South Carolina

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 South Carolina Residential Code 2021

South Dakota

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 International Residential Code 2021

Tennessee

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living area)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of Tennessee

Texas

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living area)

Section P3103.1 Residential Code for one- and two-family dwellings of the Texas Industrialized Housing and Building Program

Utah

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1 Residential Code 2015 of Utah

Vermont

6” to a maximum of 12” above roof

Section 903.1 Plumbing Code 2018 of Vermont

Virginia

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 2018 Virginia Residential Code

Washington

6” above roof OR 6” above firewall OR 7’ above roof (roof used as assembly or parking area)

Section 906.1 Section 906.3 2018 Washington State Plumbing Code

West Virginia

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 Residential Code 2018 of West Virginia

Wisconsin

8” above roof OR 7’ above roof (roof used as living area)

Pg. 44 Vent Terminals Wisconsin Drain and Vent System Design by the Wisconsin Department of Commerce

Wyoming

6” above roof OR 6” above projected snow line OR 7’ above roof (roof used as living space)

Section P3103.1.1 Section P3103.1.2 International Residential Code 2021

Roof flashing refers to any roof component overlapping another component and protecting the roof area by allowing water to continue the path down the sloped roof so as to as to shed or guide the water to the eaves.

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