The Metal-Era Learning Center

This article discusses how to calculate the proper information to select the right products for use on your building projects.

• For a discussion on the importance of ventilation and its critical impact in different regions, see The Importance of Ventilation.
• For a discussion on the different types of ventilation for sloped roofs, their environmental impacts, and their ventilation requirements, see Types of Ventilation & Guidelines.

A Balanced Ventilation System

A balanced ventilation system is vital to the longevity of your roof and building. Every inch of exhaust air requires an equal or greater amount of air intake. Intake and exhaust vents should be installed with equal amounts of net free area (NFA). NFA  is the amount of total unobstructed area that will allow air to enter or exhaust a vent system.

Building Code Requirements (Commercial vs. Residential)

Commercial and residential building code requirements are similar; however, commercial buildings are usually much larger and require a greater amount of ventilation. 



The calculation needed for an attic ventilation system determines the amount of air space.

(af/300) x 144/LF= Inches² of NFA/LF

(This calculation needs to be run once for the LF of eave and again for the LF of ridge)

• af = Attic Floor Square Footage

• LF = Lineal Feet of Eave or Ridge (run calc separately for eave & ridge)

• NFA = Net Free Area

NOTE: Based on the 1/300 rule for attic ventilation. For more conservative results based on the 1/150 rule, multiple the results by 2.

Fortunately, we offer resources to help you with your calculations for your attic ventilation requirements…

Attic Ventilation Calculator

The Attic Ventilation Calculator will help you calculate the NFA for your project and the necessary vent width for your ridge vents or vented fascia products. Enter either the square feet of attic space or the NFA that your project requires and the vented eave and ridge length. 

Vented Nailbase Calculator

When there is no attic space to ventilate, there are some unique challenges to establish a balanced ventilation system. The 1/300 rule does not apply here. For nailbase ventilation systems, you need an air gap between 1-2 inches. The overall idea of the vented nailbase system is to exhaust the same amount of air you are taking in. 

The Vented Nailbase Calculator can help you determine the proper air gap, which provides benefits no matter the region of your building: 

• In cold regions, the right air gap and insulation R-value will prevent ice damming. In other words, on the graph of the calculator results, you do not want a blue limit of effectiveness line, as this could create damming.

• In warm regions, the right air gap and insulation R-value will keep your roof cooler to prevent your shingles from curling and prolong the life of your roof. 

The Vented Nailbase Calculator accounts for the R-value of insulation and slope or the roof that affects how fast the air exhausts. Your air gap and R-value of insulation can be adjusted to create the proper ventilation to get a balanced roof system/cool roof system (preventing ice dams).

Examples:

• If you have a 10 ft. length and you take in 12 NFA per foot (1” Ridge Opening)

• 120 square inches of air/NFA exhaust through ridge (1:1 eave to ridge ratio – this would be a single pitched roof)

• 5 ft. ridge length you still need to exhaust 120 NFA (2” Ridge Opening) (2:1 eave to ridge ratio – this would be a roof that has pitch on both sides, typically a gable roof)

Ready to build? Explore our sloped roof ventilation product solutions for a wide variety of room types, backed industry-leading wind and ventilation warranties.

​Questions on proper ventilation? Reach out to our team of experts today!