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How to Calculate Roof Ventilation

When it comes to understanding what the roof ventilation requirements are for residential structures, we must look to the International Residential Code (IRC). Section R806 of the 2018 International Residential Code (IRC) outlines when roof ventilation is required and how to calculate it.

When is Roof Ventilation Required?

Section R806.1 of the International Residential Code states the following:

Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow.

Based on this code section there are two scenarios that will require attic ventilation: Enclosed Attics and Enclosed Rafter Spaces.

Both scenarios will require cross ventilation to be provided. If an attic space is divided into separate spaces, each space is required to be vented, just like each enclosed rafter space is required to be vented.

Attic spaces are formed when ceiling joists are provided and enclosed rafter spaces are typical when a vaulted ceiling is provided. Both enclosed conditioned will require ventilation.

These ventilation openings must also be protected from rain, snow, birds, rodents, and similar creatures while still providing enough opening to properly vent. We will discuss how these openings are required to be protected a little later in the post.

Now before we talked about how to calculate roof ventilation, let us briefly go over why roof ventilation is required.

Why is Roof Ventilation Required?

Common activities around the house such as cooking, bathing, laundering, and even human breathing and perspiration can produce a large amount of water vapor that can make its way through the building envelope and into the attic.

Due to the temperature difference between the attic and the conditioned space, the water vapors can reach a dew point where condensation can be present on the wood members in the attic. Overtime this can cause the roof members to rot and decay.

To prevent this, the space within the attic must be vented to not allow the water to collect on the building components. Ventilation openings allow air to flow through the attic to remove any moisture and possibly minimize condensation problems within the attic and enclosed rafter spaces.

How to Calculate Roof Ventilation

To properly calculate the required roof ventilation, Section R806.2 specifies that the minimum net free ventilation area shall be 1/150 of the area being vented.

For example, an attic being 1,500 square feet in area requires 10 square feet of ventilation. If you are trying to determine the area in square inches, you can convert it by multiplying 144. The 10 square feet X 144 square inches per square foot = 1,440 square inches required.

So let’s say the size of 1 opening you have provides 300 square inches of ventilation. Take 1,440 square inches and divide by 300 square inches = 4.8

Round up and the required number of ventilation openings to be provided shall be 5.

Now lets look at another example using different types of vents to achieve compliance:

How to Calculate Roof Ventilation

Net Opening vs. Gross Opening

An important note to consider when you are calculating the required ventilation opening area is that since the openings are being covered, the area of the ventilation openings should account for the net free area of the covering material used instead of the gross area of the opening. When determining the minimum ventilation openings, you are calculating the net area. Therefore if the covering reduces the gross opening area by 50% then twice as many openings would be required.

If you purchase a roof vent, the manufacturers specifications/catalog will have information on it as to what the “net free area” is of the vent. So for example a gable vent might be 24 inches square meaning it is 576 square inches but the manufactures specs might say that the net free area that the vent provides is 300 square inches to account for the covers that go over the opening to protect it from the elements as we discussed above.

Exceptions to Calculating Roof Ventilation

Section R806.2 does provide an exception to reducing the required venting area to a minimum of 1/300 of the vented space in comparison to what is typically required which is 1/150 of the vented area.

In order to allow for the required venting to be reduced, two conditions must be met. They are the following:

  1. In Climate Zones 6, 7 and 8, a Class I or II vapor retarder is installed on the warm-in-winter side of the ceiling.
  2. Not less than 40 percent and not more than 50 percent of the required ventilating area is provided by ventilators located in the upper portion of the attic or rafter space. Upper ventilators shall be located not more than 3 feet (914 mm) below the ridge or highest point of the space, measured vertically. The balance of the required ventilation provided shall be located in the bottom one-third of the attic space. Where the location of wall or roof framing members conflicts with the installation of upper ventilators, installation more than 3 feet below the ridge or highest point of the space shall be permitted.

The first condition recognizes, in colder climates, that a vapor retarder is required and allows for a reduction based on the vapor retarder being installed on the warm-in-winter side of the ceiling.

The second condition recognizes that attic vents providing cross ventilation are located higher up into the attic.

Attic Vent and Insulation Clearances

With the installation of attic insulation being very common practice now to achieve energy compliance, it is important to make sure that the roof vents providing attic ventilation is not blocked my the insulation. To ensure that vent openings are not blocked by insulation, the code requires the insulation to be held back from the vent openings a minimum of 1 inch.

The same is applied to enclosed rafter spaces. When ventilation is provided to rafter spaces, not less than a 1 inch space shall be provided between the insulation and the roof sheathing. This clearance must be maintained throughout the rafter space as well as throughout the attic for all types of openings providing attic ventilation.

Ventilation Openings for Attic Spaces

We discussed earlier about how ventilation openings must also be protected from rain, snow, birds, rodents, and similar creatures while still providing enough opening to properly vent. So let us discuss how these openings are required to be protected.

The ventilation openings must be at least 1/16 inch to no more than 1/4 inch. If the openings are larger than 1/4 inch, then material such as corrosion-resistant wire cloth screening, hardware cloth, perforated vinyl or similar material with openings not less than 1/16 inch and no more than 1/4 inch shall be provided.

Whichever one of these is used it is important to exercise care to ensure that the vent openings remain unobstructed while providing protection to prevent entry of unwanted visitors.

Also it is important to note that all required attic ventilation openings shall open directly to the outside air.

Unvented Attics

What about unvented attics and enclosed rafter spaces, are these allowed?

The code does allow for unvented attics and rafter spaces given that all 5 conditions that it lists are met. To read about what these 5 conditions are, be sure to check out Section R806.5 by clicking HERE.


To Learn more about roof ventilation requirements for residential structures, be sure to check out Section R806 of the 2018 International Residential Code (IRC).

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* Reference Source – 2018 International Residential Code[Buy on Amazon]