Roof Ventilation in St. Louis

What attic ventilation is for

A vented attic is meant to stay close to the outdoor temperature, with outside air coming in low at the eaves and leaving high at the ridge. ARMA, the asphalt shingle industry's association, gives the reasons in plain terms:

• Winter moisture. A family of four puts roughly two to four gallons of water vapor into the air each day from breathing, showers, cooking and dishwashing. Warm, damp air rises into the attic, and in a cold attic it condenses on the rafters, the nails and the underside of the deck. Nails corrode, wood rots, and wet insulation can bring on mold.
• Summer heat. Sun on the roof heats the attic, and without ventilation that heat radiates into the rooms below and keeps the air conditioner running.
• The roof itself. ARMA says improper ventilation can lead to premature deterioration of shingles and the roof deck, and FORTIFIED, the building program of the research institute IBHS, lists curled or cupped shingles as a possible sign of poor attic ventilation.
• Ice dams. Warm spots on a snowy roof start the melt that feeds them; more on that below.

Ventilation is one part of a system that also includes air sealing and insulation; the ice dam section below explains the order the Department of Energy puts them in.

What the code requires: the vent ratios

Section R806 of the International Residential Code requires enclosed attics and rafter spaces to be cross-ventilated. St. Louis County's re-roofing checklist spells out the numbers from R806.2:

• 1 square foot of net free vent area for every 150 square feet of attic floor, as the base rule.
• 1 for every 300 when the vents are split high and low: 40 to 50 percent of the required area high in the attic, within 3 feet of the ridge, with the balance low at the eaves. The county lists its own added conditions for this lower ratio, so confirm how your city reads it.
• A clear air space of at least 1 inch between the insulation and the roof sheathing, kept open at the eaves with baffles (R806.3).

"Net free area" is the open area a vent actually passes after its screens and louvers, which the Department of Energy's Building America guidance notes is less than the vent's overall size. Worked through for a 1,500-square-foot attic floor:

On a re-roof, St. Louis County requires the attic ventilation to be checked against R806 and brought into line if it falls short.

Ridge vents need intake to match

A ridge vent is only the exit. ARMA describes a balanced system as one where intake at the eaves equals exhaust at or near the ridge, so air sweeps the whole underside of the deck on its way up.

The common mistakes are easy to list:

• Blocked intake. Soffit vents painted shut or buried under blown-in insulation. ARMA's fix is clearing them and adding baffles.
• Mixed exhaust. A ridge vent plus gable louvers or a powered fan on the same attic. ARMA advises using only one type of exhaust on a common attic, because a second one can short-circuit the airflow.
• Gaps in coverage. DOE recommends venting every rafter or truss bay at the intake side.
• No soffit at all. Some houses have nothing to vent at the eaves. Intake can come from the roof edge instead; Owens Corning's InFlow vent, for one, is sold for soffit-less homes.

On every replacement, we pair the ridge vent with intake to match, as shown in the cut-away on our roof replacement page.

Ice dams and the St. Louis freeze-thaw

Winter here is a cycle more than a season. Lambert's normals show the low dipping to 32 or colder on about 85 days a year, while the high stays at freezing or below on only about 19, so most freezes are followed by a thaw. Snow averages 16.6 inches a year, and the local ground snow load used for design is 20 pounds per square foot (the roofing code guide lists the other design figures).

The Department of Energy's Building America guidance treats ice dams as typical where ground snow loads pass 60 pounds per square foot and likely where they pass 30. By that measure, St. Louis is below the line, and the design tables published locally, in St. Peters and St. Louis County, do not call for an ice barrier at all. When a dam does form here, the house is usually helping it along: heat leaking through light fixtures and exhaust fans, ducts or equipment in the attic, or thin insulation over the outside walls at the eaves.

DOE puts air sealing first, then insulation, then ventilation. For the vents, its guidance is to keep the intake at least equal to the exhaust, vent every bay, and leave 2 inches of air space above the insulation even though the code asks for 1. We still put ice-and-water membrane at the eaves and valleys of every new roof, and the roof repair page covers winter leaks.

Bath fans, dryers and kitchen ducts

Every exhaust fan in the house is supposed to end outdoors. When a bath fan's duct stops in the attic, or lies loose on the insulation, each shower sends a cloud of warm, wet air straight to some of the coldest surfaces in the house: the nails and the underside of the deck.

The rules agree. The Department of Energy's code brief on attic upgrades says bath fans venting into the attic must be changed to vent outside. ARMA tells homeowners to make sure nothing from inside the house discharges into the attic. St. Louis County goes further for re-roofs: its checklist has any duct that ends in an enclosed attic or rafter space extended to discharge outdoors. The one allowance it makes is for bathroom exhaust, which may end at a gable or soffit vent if the duct is secured within 6 inches of it.

Look for these in your attic: flexible duct with an open end, a duct aimed at the soffit but not attached to anything, or a stained patch of deck near where a fan duct ends. Dryer vents and kitchen hoods belong outside for the same reason, and their lint and grease make it worse.

Ventilation and your shingle warranty

Shingle makers treat ventilation as part of the installation, not an option. Owens Corning's current standard limited warranty lists "damage caused by improper or inadequate roof ventilation or roof drainage, unvented attics, or enclosed roof rafter assemblies" among the things it does not cover, and its installation instructions say ventilation must comply with local building codes.

That matters most in three situations:

• Re-roofing an older house whose vents were never sized to the code, which ARMA says is the time to evaluate the attic.
• Attics converted to unvented space, such as foam sprayed on the underside of the deck. Read the shingle maker's terms before you choose shingles for that roof.
• Cathedral ceilings and finished attics, where each rafter bay needs its own path from eave to ridge.

If your next roof is a shingle roof, the roof shingles page compares lines; whichever you pick, the vent plan should be written into the quote.

Signs your attic is under-ventilated

Most of these show up from inside the house or the attic, not from the yard:

• Winter: frost or droplets on nail tips and the underside of the deck, stains on the deck with no roof leak above them, damp or matted insulation, a musty smell.
• Summer: upstairs rooms that stay hot in the evening while the air conditioner runs and runs.
• On the roof: curled or cupped shingles, and ice dams or heavy icicles at the eaves after a snow.
• At the vents: no daylight visible at the ridge vent from inside the attic, which ARMA uses as a quick check, or soffit vents clogged with paint, dirt or insulation.

Our free roof inspection photographs every vent and penetration along with the slopes. If the attic is the problem, ARMA notes the fix can be as simple as unblocking soffit vents, and any of it is easier to sort out before a re-roof than after one.

Common questions about roof ventilation in St. Louis

Where we work

Roof ventilation across greater St. Louis, from the city and St. Louis County out to St. Charles, Jefferson and Franklin Counties and across the river into the Metro East. Each city has its own page with the permit rule and code edition that apply there: