Commercial roof ventilation: Calculations & requirements

Roof ventilation is critical, not just to adhere to International Building code but also for safety and energy savings. If your roof isn't well ventilated and causes humidity to build up, you risk mold growth and roof deterioration.

Additonally, a poorly ventilated roof can trap heat, leading to a "hot attic." This means your building gets too warm, substantially increasing the costs and energy required to cool a commercial building.

This article will explore how to calculate roof ventialtion, essential considerations, and tools you can use to ensure proper airflow in your commercial building.

How is roof ventilation calculated?

Typically, commercial buildings require one square foot of ventilation for every 150 square feet of attic space. This includes both inlet and exhaust ventilation, which must be balanced. If your attic is 1,500 square feet, you’ll need ten square feet of ventilation, including five square feet each of intake and exhaust.

However, there are exceptions to the above-mentioned roof ventilation calculation guidelines. In buildings located in certain climates, roofs with vapor barriers installed, or steep roofs with more than 40% of the ventilation located at the peak, ventilation can be reduced to one square foot for every 300 square feet of attic space. These climates include zones 6, 7, and 8, which are temperate climates with low to moderate humidity (30-60%).

How is roof ventilation tested?

To determine the quality of roof ventilation, smoke tests can be conducted by the building owner. During this test, smoke is pushed into the attic through the inlet vent or inside the attic. Then, someone observes how it flows and leaves the attic.

Additionally, HVAC technicians can conduct airflow tests for a more detailed ventilation analysis. Meanwhile, a commercial building inspector may simply visually confirm the presence of roof ventilation.

Roof ventilation is typically conducted on new construction roofs or as requested by the building manager based on noted issues, including:

Rising temperatures in the summer, especially on upper floors
Roof damage, including rot and shingle deterioration
Ice dams
High humidity within the attic

Even if none of these issues are noted, it’s a good idea to conduct a ventilation check as part of your bi-annual roof maintenance plan. Consider checking your attic every six months to ensure the temperature is normal and there is no mold built up.

What roof ventilation options exist?

Ventilation has two critical components: fresh air intake and exhaust. Both must be present and balanced to ventilate a roof properly.

Hot or stale air must be removed from the building via exhaust to prevent mold and mildew build-up. However, cool air must be brought into the building via intake to push out the old air.

- Exhaust solutions

A common type of roof ventilation is a ridge vent installed at the highest point of the roof. Since warm air naturally rises, this placement allows the hottest air to flow out your roof. Ridge vents may leak during heavy rain, so it’s best to avoid this type of vent if your building is located in an area that frequently experiences heavy precipitation and high winds.

Another popular option is the box vent, which works similarly to ridge vents by allowing warm air to escape the attic. Box vents are smaller and best suited for smaller areas that cannot accommodate a ridge vent.

Powered attic vents, which can be hard-wired or powered by solar panels, use a fan to draw out old air. While these can help to lower attic temperatures significantly during warmer months, they can reduce energy efficiency by removing too much air. Heavily air-conditioned buildings won’t benefit from powered vents, as they can remove too much cold air.

Fresh air intake solutions

Soffit vents are among the most popular intake solutions and pair perfectly with the ridge vent. Soffit vents are installed directly below the roof line and feature small holes to allow air intake only while mitigating the risk of rodents entering the roof.

Fascia vents, or over-fascia vents, are alternatives for roofs without sufficient eaves for installing soffit vents. Drip edge vents are another standard air intake option. Drip edge vents are placed directly under the first row of shingles.

How does roofing material impact ventilation?

Vapor barriers can be used to improve the moisture resistance of the roof and can lower the ventilation requirements. Some roofing materials may impact the type of vents installed or how they are installed. For example, a tile or slate roof will require more care when installing ventilation to avoid damage.

Do flat roofs require different ventilation solutions?

Flat roof ventilation requires equal air exhaust and intake. Balancing the two will ensure your roof can properly “breathe” with ventilation.

Since flat roofs are particularly susceptible to leaks, it’s critical to look for vents with sufficient height to minimize leakage.

Take the guesswork out of roof ventilation solutions

While there are many factors to consider for proper ventilation, identifying the best product for your commercial roof doesn’t need to be challenging. Our VistaVent® finder makes it easy to find the right ventilation solution for your needs. Just enter what you know about your panel, and we’ll provide the right ridge ventilation product.

SFS has designed versatile commercial roof ventilation and closure products to be easy to install while protecting your building from moisture, debris, and rodents.

Learn more about constructing and maintaining your commercial roof with these SFS resources:

Solar Panel Installation Methods — Discover best practices for installing solar panels on different types of commercial roofs.
Different Types of Commercial Metal Roofing & Material Options — Explore the different types of metal roofing, including why you would choose a metal roof and the best varieties for your project.
Replacing Metal Roof Screws — Learn about best practices for replacing metal roof screws on commercial structures.