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Been trying to find a reputable and concrete answer as to what type of ventilation we should use on our ranch house that has a 3/12 pitch shingled roof. The house is located in the Cincinnati Ohio area.

The house was built in 1958 and according to our neighbor who has lived here since 63 the house originally had a roof turbine for ventilation that has long since been removed.

I know of two types of ventilation we can use; either gable vents or soffit vents with a ridge vent. I just cannot find a specification from something with authority that says that on a roof with 3/12 pitch that soffits with a ridge vent will be effective or correct for the application.

We are being pushed strongly in the direction of gable vents by a busy body family friend who is advising and helping us, but that will require some significant modifications to the sides of the house not least of all the relocation of electric service line coming into the house.

Meanwhile most others recommend soffits with a ridge vent.

Unfortunately my situation has us stuck between a rock and a hard place with regards to our "advisor" that is out of my control mostly, which is why I'm hoping for an authoritative specification on what we should be using.

I've tried many Google searches as well as looking into the Ohio code with no definitive answers

Thanks for any assistance.

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  • Have you considered an unvented (cathedralized) roof assembly?
    – ThreePhaseEel
    Commented Oct 20, 2021 at 3:58
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    On our roof we had soffit and gable vents. Had icicles forming off the roof. Re-did the roof and went with soffit and ridge vent(along the whole peak), no more icicles forming.
    – crip659
    Commented Oct 20, 2021 at 11:02
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    Slope is somewhat irrelevant until you get very flat. Soffit and ridge venting is clearly superior due to the convection (rising warm air) action they create. Gable vents rely much more on weather and breeze action and are hampered by home location and orientation. Almost no one installs gable vents in new homes. That should say something.
    – isherwood
    Commented Oct 20, 2021 at 12:50
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    If you were not family friends with this person who you say is "out of control advisor", would you be inclined to trust their opinion on the matter more than "most others"? Be careful of falling prey to the "appeal to trust" fallacy... just because you trust someone doesn't mean they are knowledgeable about this at all.
    – TylerH
    Commented Oct 20, 2021 at 14:17
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    One of my properties is a ranch built in 1963. It has gable venting. Anecdotally, it does a terrible job of venting and is an ingress point for pests. If you want to keep the character of a ranch of that era, sure build a gable vent. If you want actual venting, use modern tech: ridge + soffit.
    – bishop
    Commented Oct 20, 2021 at 14:28

3 Answers 3

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Code only determines how much ventilation, not what kind. There are different specifications of how much ventilation for the 2 most popular kinds which are considering.

With soffit and ridge vents, code requires a 1/300 ratio. 1 sq. ft. of ventilation for every 300 sq. ft. of attic area. Usually continuous ridge and soffit vent satisfy this.

With gable vents the ratio is 1/150, a much larger area is needed for venting. The vents, just because there are a certain size, doesn't mean you will get what you expect in "free area" which is what the code refers to. For example, a 1 ft by 2 ft gable vent does not give 2 sq ft of venting. The louvers and insect screening give resistance to the free flow of air, therefore reducing the amount of free area. Each vent whether rectangular, octagonal or adjustable triangle has stamped on it what the free area is.

The downside, possibly of the soffit vents is that the insulation cannot be tight to the roof sheathing, otherwise it blocks the flow of air. Insulation baffles are installed between the rafters where they meet the top plate, so when the insulation is placed, it still allows air movement into the attic space.

There is a 3rd way that I have seen for this situation, multiple rectangular or square roof mounted vents

enter image description here

Picture courtesy myroofhub.com

These are cut in the roof, through the sheathing, between the rafters. You need to calculate the free area of each vent and figure how many you will need.

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You want soffit / ridge vents.

One of the main purposes of the attic ventilation on asphalt shingle roofs is to allow the roof sheathing to cool such that the shingles are not subjected to temperatures that cause them to degrade.

The best cooling airflow is achieved by having the air intake at the soffit and run up the underside of the roof sheathing and exist at the ridge such that the air has passed over the full underside surface of the sheathing. If you have gable vents the cooling effect of the air moving over the underside of the sheathing is going to be fairly minimal. I had soffit and gable vents but when I had the roof redone I blocked the gables, installed baffles to the soffit and installed a continuous ridge vent. First roof lasted 15 years, current roof is at 10 years. Hard to really know for sure as the quality of asphalt shingles varies but I am hoping the change results in the shingles lasting longer.

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Attic Ventilation shall be “cross ventilation “ and shall be 1/150th the area that is to be vented. (See ICC R806 Vents.)

However, there is one exception: The attic ventilation can be reduced to 1/300th the area to be vented provided one of the following items are met:

  1. a Class I or II vapor retarder is installed on the “warm-in-winter” side of the ceiling, or

  2. not less than 40% and not more than 50% of the ventilation is located in the upper portion of the attic. The upper portion is defined as: “Upper ventilation shall not be lower than 3’ below the ridge or highest point of the roof. “ (See ICC R806.1.2.)

So, you can see there is clearly a benefit to having ridge vents, soffit vents, gable vents, etc. , but in a proportion that allows COMPLETE CROSS-VENTILATION to the attic. (The code does not mention 3:12 or 4:12 requirements, just dimensions. )

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  • Looks like the code has changed slightly since my remembrance.
    – Jack
    Commented Oct 20, 2021 at 4:42

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