The ice at the eaves in February starts with warm air leaking up through the ceiling. No vent stops that, and the fault the roof can fix is a different one: exhaust at the peak with nothing feeding it at the eaves. Wisconsin sets the amount by ceiling area and the split by height, and the split decides whether a ranch needs 720 square inches or twice that.
How much attic ventilation a 30 by 50 ranch needs
Take an example house: a 30 by 50 single-story ranch, one 50-foot ridge and two 50-foot eaves, the shape of the mid-century ranches on the North Side and around Mount Simon. The ceiling under that attic is 1,500 square feet, and that horizontal ceiling area is what the ratio runs against. At 1/300 that is 5 square feet of net free ventilating area, or 720 square inches. Net free area is the opening left after louvers and screen, and a labeled vent has it printed on.
The 1/300 ratio holds only when more than half but less than three-quarters of that area sits high on the roof. Split 60 high and 40 low, the 720 is 432 square inches at the ridge against 288 at the eaves. That is a floor to clear, and building exactly to it is a mistake. GAF prints the opposite instruction on the Cobra product page: in no case should the exhaust at the ridge exceed the soffit intake. Air Vent’s booklet says the same. So 432 high over 288 low passes the state ratio and still leaves the attic under suction, and the buildable minimum lands above the floor.
Put three-quarters or more high, which is what a ridge vent with nothing at the eaves does, and the requirement becomes 1/150: 1,440 square inches here, 80 feet of ridge vent on a 50-foot ridge, which cannot be built. The eaves have to open.
What a full-length ridge vent and open soffit intake add up to
Run the ridge full length and match it at both eaves:
- Ridge vent the full 50 feet. GAF Cobra Rigid Vent 3 is rated 18 square inches per linear foot, so the ridge gives 900 square inches of exhaust.
- Intake to match the 900. Thirteen 8-foot pieces of Lomanco 105 continuous soffit strip at 72 square inches each, running both eaves end to end; or fourteen Lomanco C816 undereave vents at 65 apiece, seven per eave. Round the count up.
- Total 1,800 square inches, 12.5 square feet, one square foot of vent per 120 of ceiling.
That clears both code ratios and Owens Corning’s stricter 1 per 150 guidance. Ridge cap shingles come in 10-inch and 12-inch widths and the vent is sold to match: the VentSure 4-Foot Strip is 20 square inches per foot in the 12-inch version and 18 in the narrower ones. Thirteen strips on the same ridge give 1,000 square inches, more than 100 feet of soffit strip can feed, so the intake goes to sixteen C816 vents. Ask which width a bid quotes.
A house with no soffit takes a vented drip edge, which replaces the metal edge at the eave during the tear-off. Lomanco’s SV-10 is 63 square inches per 10-foot piece, so 100 feet along both eaves is 630 square inches, enough for about 35 feet of ridge vent rather than the full 50.
This is step seven of a roof replacement. A bid can say ridge vent and stay silent about the eaves, so ask every bidder for the intake figure in square inches of net free area and how it is achieved: cut-in soffit vents, a vented drip edge, baffles in the rafter bays.
Gable vents, buried soffits, and foamed decks read against the ratio
The old frame house with a louver in each gable. Take a 30 by 30 house, 900 square feet of ceiling, a 12 by 18 louver in each gable and nothing at the eaves. The gable-louver attic is still standard on the pre-war frame houses here, and that is the attic a lot of them have. Unlabeled, with insect screen behind metal louvers, the state counts each at half its 216-square-inch opening, so the pair is 216, all of it high. All high is the 75-percent case, which sets the requirement at 1/150, 864 square inches: the pair supplies a quarter of that, and only 14 percent if the vents are Air Vent’s labeled aluminum at 59.4. Once a ridge vent goes on and the soffit intake is open, the louvers should come out and the openings be patched. John Nessen’s point in Roofing Contractor is that a second exhaust type becomes the ridge vent’s intake, air in at the gable and out at the peak while the eaves sit still.
The ranch with a ridge vent and buried soffits. Where the soffit has vents at all, decades of blown insulation have crept over the top plate at the wall and buried them from behind. The code’s answer is one sentence, insulation shall not block the free flow of air, and the state’s commentary adds that vent chutes may be required at the eaves. The code puts no inch figure on it. The Department of Energy describes the fix: a baffle, a rigid chute 2 to 4 feet long, set in every rafter bay (the space between two rafters) with a soffit vent, holding a 2-inch gap under the deck past the insulation. Without them the intake exists on paper and the count goes back to all high: 1,440 square inches, more ridge than the house has. The roof vents; the attic floor does not.
Opening the eaves, cutting in soffit vents and setting baffles stands on its own while the shingles are sound, and on a house that does not need a roof yet we correct the intake as a roof repair rather than a replacement. The ridge vent usually waits for the tear-off, since it is cut into the deck. Both in one mobilization costs less than either one twice.
The foamed attic. SPS 322.39(3) is the code’s exception: no venting is required where air-impermeable insulation, closed-cell spray foam, is attached directly to the underside of the deck, provided no interior vapor retarder sits between the living space and the attic and nothing condenses on the foam. The note puts that at 45°F on the insulation surface with the house at 70°F and 45 percent humidity, and that condition still has to be met.
Why a new roof still gets ice dams
A ridge vent on a leaky ceiling can make the leak worse. An attic short on intake pulls makeup air from the easiest opening, and with recessed can lights, an unsealed hatch, or gaps at the plumbing stack and chimney chase, that opening is the house. The harder the ridge vent pulls, the more house air comes up through those holes, and it is that air, not the venting, that holds the deck above freezing under the snow with the outdoor air below it. That is the dam.
Which is why ventilation is third in Joseph Lstiburek’s order at Building Science Corporation, and why the intake correction does little until the ceiling is sealed: air leakage into an attic matters more than the insulation level. Focus on Energy, the statewide program run with Wisconsin’s utilities, recommends R-49 or higher for a Wisconsin attic, about 18 inches of fiberglass, and its Trade Ally locator lists the contractors who do the air sealing. Do that first or alongside the re-roof, and the venting has a cold attic to keep cold.
A bathroom fan that blows into the attic
Look in the attic for a flex duct that ends near a gable vent, points at the soffit, or lies on the insulation. SPS 323.02(3)(a)1 is one sentence, all exhaust vents shall terminate outside the structure, and the state’s commentary, asked whether a fan can terminate in an attic near a vent, answers “No.” It does allow a termination through the soffit of an overhang, with the soffit blanked off for 2 feet either side so the intake cannot pull the exhaust back in.
The code requires 50 cubic feet per minute of intermittent exhaust from a room with a tub or shower, and Michael Maines ran the numbers in Fine Homebuilding: a 110 cfm fan moving 80°F shower air at 80 percent humidity makes about three-quarters of a cup of condensate over a 10-minute shower with 25°F outside. That lands as frost on the sheathing and drips onto the insulation when it thaws, which the decking replacement page picks up. Run the duct to a cap outside and insulate it, R-3 to R-8 in Maines’ guidance and the last 6 feet at minimum, so the moisture leaves the house as vapor.