Roofing
Attic Ventilation and Summer Heat: Protect Your Shingles
A well-vented attic pulls cooler air in at the soffits and lets hot, moist air out near the ridge. For a vented attic, Ohio's residential code generally calls for 1 square foot of net vent opening for every 150 square feet of attic. That can drop to 1 for every 300 when the vents are placed high and low as the code describes. Some attics are built unvented on purpose, and the code has separate rules for those. The Asphalt Roofing Manufacturers Association (ARMA) warns that improper ventilation "can lead to premature deterioration of your shingles and roof deck."
July is when you feel it. If the upstairs bedrooms stay hot no matter what the thermostat says, the attic is a good place to start looking.
Why does attic ventilation matter in summer?
Sun on a dark roof heats the attic. If that heat cannot get out, it sits under the roof deck all afternoon. ARMA's ventilation page says proper ventilation keeps the attic cooler, "reducing load on your air conditioning units."
GAF's technical bulletin on proper attic ventilation lists what too much attic heat can lead to:
- Moisture build-up in the attic
- Higher energy costs
- Ice dams in winter
- Damage to roofing products and the structure under them
Cincinnati summers are hot and humid, so heat and moisture show up together here.
Does ventilation matter in winter too?
Yes. In winter the problem is moisture. ARMA notes that a family of four puts 2 to 4 gallons of water vapor into the air each day. Some of it rises into the attic. If it cannot escape, it condenses on the cold underside of the roof deck. Over time that can rot the decking and grow mildew.
A warm, poorly vented attic also melts roof snow from below. The water refreezes at the cold eaves and forms ice dams. Our guide to ice dams in Cincinnati explains that cycle.
How much attic ventilation does Ohio code require?
The 2019 Residential Code of Ohio, Section 806.2, says: "The minimum net free ventilating area shall be 1/150 of the area of the vented space."
An exception allows 1/300 when "not less than 40 percent and not more than 50 percent" of the required vent area is in the upper part of the attic. Those upper vents must sit no more than 3 feet below the ridge. The rest goes in the bottom third of the attic, which usually means the soffits.
"Net free area" is the open area air can actually pass through, after screens and louvers. Vent makers print it on the product.
Here is what that works out to for common attic sizes:
| Attic floor area | 1/150 rule | 1/300 rule (balanced) | 1/300 split, high and low |
|---|---|---|---|
| 1,000 sq ft | 960 sq in | 480 sq in | About 240 high, 240 low |
| 1,500 sq ft | 1,440 sq in | 720 sq in | About 360 high, 360 low |
| 2,000 sq ft | 1,920 sq in | 960 sq in | About 480 high, 480 low |
Manufacturers mostly design to the 1/300 figure. GAF's bulletin uses it. ARMA says its preferred ratio is 1/150. Your local building department decides what applies to a permit.
What is balanced ventilation?
Balanced means about as much air comes in low as goes out high. GAF says a properly balanced system has "50% of the air flow through the soffit and 50% of the air flow through the ridge." It adds that exhaust "should not exceed" intake.
Two common mistakes make a system work against itself:
- Mixing exhaust types. GAF warns that soffit and ridge vents combined with a power fan "could result in reverse airflow that could result in water leakage into the attic." The fan can pull air in through the ridge vent instead of the soffits.
- Too little intake. ARMA says gable vents plus ridge vents with too little soffit venting can pull moisture into the house during heavy rain or snow.
Exhaust vents cannot do the job of intake. A ridge vent with blocked soffits is not moving much air.
Signs your attic is not venting well
When we inspect a roof, we look in the attic too. These are the things I check, and most you can check yourself:
- Hot upstairs rooms that stay hot into the evening.
- Blocked soffits. Look up at the soffit vents from the yard. Paint, dirt, or solid panels can close them. From the attic, insulation stuffed against the eaves can block them too.
- Dark stains or mildew on the underside of the roof deck.
- Rusty nail tips poking through the deck. Rust there often means moisture has been condensing.
- Frost on the deck on cold winter mornings.
- Ice dams along the eaves after snow.
- Several kinds of exhaust vents on one roof, such as a ridge vent plus box vents plus a power fan.
Be careful in the attic. Step only on joists or a solid floor, never on the drywall between them.
Does poor ventilation affect my shingle warranty?
It can. GAF's bulletin says its shingle warranty for manufacturing defects stays in effect "regardless of the ventilation used." But "any damage to the roofing system or the structure that is caused by inadequate ventilation is not covered by the Warranty." Other manufacturers have similar terms.
That is one more reason to get ventilation right when the roof is replaced. It is the cheapest time to add ridge vent or fix the soffits.
Can ventilation be fixed without a new roof?
Often, yes. Depending on the house, fixes can include:
- Clearing painted-over or clogged soffit vents
- Adding baffles so insulation does not block air at the eaves
- Adding soffit or other intake vents
- Removing or closing extra exhaust vents that fight each other
Adding a ridge vent is usually done during a roof replacement, but it can be done on its own when the shingles are in good shape. Some older houses with shallow attics or no soffits need a custom answer. That is a conversation to have with someone who has seen your attic.
If you are planning a replacement, ventilation should be a line item in every estimate you get. Our roof replacement guide covers what else to compare.
How Shamrock can help
Our roof inspections include the attic: ventilation, moisture, and how the intake and exhaust work together. We will tell you whether your attic needs work, and whether it can wait for your next roof. Request a free roof inspection or learn more about our roofing services.
FAQ
Can I have too much attic ventilation? Extra intake at the soffits does not hurt a passive system, according to ARMA. The trouble comes from too much exhaust compared with intake, or from mixing exhaust types.
Should I add a power attic fan? Not alongside a ridge vent and soffits. GAF warns that mixing them can reverse airflow and pull water in. Ask a roofer to look at the whole system first.
Do I need to close my gable vents if I add a ridge vent? GAF's guidance is that gable vents should be closed when soffit and ridge vents are used. Your roofer should confirm based on your attic.
Will better ventilation lower my cooling bill? It can help. ARMA says a cooler attic reduces the load on your air conditioner. Insulation and air sealing also matter, so ventilation alone may not fix a hot room.