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  • Writer's pictureTimothy Hill

Free Your Air & the Rest Will Follow

Updated: Sep 23, 2022

Roof Venting is important for many reasons. In summer (in Ohio), proper venting allows hot air to escape the attic, which keeps cooling load down. In the winter, attic air will stay closer to the outside temperature to prevent dangerous sheets of ice from forming & sliding off the roof. Also, with ventilation air passing over the surfaces inside the attic, moisture can escape which helps keep roof sheathing dry & safe from mold & rot.

How to size roof vents: The Ohio Building Code (OBC), Section 806 states that Net Free-vent Area (NFA) ratio must be 1:150. That means for every 150 ft² of attic, 1 ft² of vent opening is required. This includes total open area of ridge & soffit vents combined. For example, a 2000 sf house calculation would look like this:

2000 ft² ÷ 150 = 13.3 ft²

13.3 x 144 (in. per s.f.) = 1915 in² TTL NFA REQ’D

If a ridge vent is used & let’s say our example house has 70 lineal feet of usable ridge at 18” NFA (ridge vent products vary), then the ridge vent calc would be:

70 x 18 = 1260 in² TTL RIDGE NFA

In this example, the required NFA for soffits would be:

1915 – 1260 = 655 in² REQ’D SOFFIT NFA

Since, a typical 8 x 16 soffit vent provides 56 in² NFA, the total number of vents required is:

655 ÷ 56 = 11.7, or in other words, 12 SOFFIT VENTS MINIMUM

There is also an exception that allows the NFA ratio to but cut in half (1:300). To qualify for this exception in the code, a specific balance of ventilation must be achieved between the ridge & soffit, 40% - 50% at the ridge to be exact. This code exception is based on the building science theory that if an attic has positive pressure, then air inside the house will be less likely to leak into the attic through cracks in the drywall, lights, hvac, etc.

Conclusion: A minimum amount of soffit, ridge & gable venting is required for each specific project & the balance between ridge & eave can also be important.

If this information has been useful to you, please like & subscribe to my blog. Also, if this information has sparked an interest or raised questions, I hope you will feel free to give me a call, text or email so we can walk through it together so you can get the peace of mind you need knowing your home will be constructed to last.

Thank you for reading & happy building!!

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