Roof Vent Leaks: Why Box, Ridge and Turbine Vents Let Water In

Every vent on a shingle roof works like any other roof penetration: the shingles above it lap over the top of its flange, and its flange laps over the shingles below it. Water running down the roof should pass over the vent and keep going. When that lapping is wrong, the vent leaks around its edges, usually in any steady rain.

The second path is through the opening. A vent has to breathe, so it can never be fully sealed. Its hood, baffle or filter is what keeps weather out, and strong wind can push rain or fine snow past all of them. These leaks are picky: they show up in one storm direction, in blowing snow, or not at all for months.

The third path is the vent body. Plastic hoods get brittle from sun and cold and crack from hail. Metal hoods dent, rust at the seams and get nail holes where somebody fastened them through the top. Any crack above the roof line is a direct path into the attic.

A ridge vent runs along the peak over a slot cut through the roof deck, and it is capped with shingles so it nearly disappears. It is the most common exhaust vent on newer Michigan roofs, and when it leaks the cause is usually one of four things: no external baffle, a slot cut too wide, cap shingles nailed wrong, or the gap between the vent and textured shingles left open.

The baffle matters most in snow country. Baffled, filtered ridge vents such as CertainTeed's filtered ridge vent and Air Vent's ShingleVent II use an external wind baffle to deflect wind and weather over the vent and an internal filter to help keep wind-driven rain, snow, dust and insects out. Older unbaffled ridge vents, including some rolled mesh products, lack that baffle and are far more likely to let fine, powdery lake-effect snow sift into the attic on a windy night. When it melts, it looks exactly like a roof leak.

Slot width is set by the manufacturer, not by the installer's saw. For ShingleVent II, Air Vent's instructions and its ICC-ES evaluation report call for a 1 1/2 inch slot centered on the ridge on truss roofs, or 3/4 inch on each side of a ridge board, stopped 6 inches short of the end walls and 12 inches short of a chimney or another ridge. The instructions also say the cap shingle should not overhang the edge of the air slot. A slot cut wider than the vent can cover gives rain a straight drop into the attic.

The 2015 Michigan Residential Code, still the code in force for houses, writes the leak problem into the ventilation rule itself. Section R806.1 requires attics to have cross ventilation "by ventilating openings protected against the entrance of rain or snow." Openings must be between 1/16 and 1/4 inch at their smallest dimension, or be covered with corrosion-resistant screen or hardware cloth with openings in that range. Section R806.4 then says ventilators shall be installed according to the manufacturer's instructions and the roof weather-protection rules in Section R903. A vent that lets snow drift into the attic is not just a nuisance; it falls short of the rule it was installed to meet.

The amount of venting is set by Section R806.2: net free area of at least 1/150 of the vented space, reduced to 1/300 under either of two conditions. One is that 40 to 50 percent of the required area sits in the upper part of the attic, no more than 3 feet below the ridge, with the rest at the eaves. The other is a Class I or II vapor retarder on the warm-in-winter side of the ceiling, but only in Climate Zones 6, 7 and 8.

Michigan's version of the energy chapter assigns every county a zone in Table N1101.10, and that splits Stellar's service area. Kent, Ottawa, Allegan, Kalamazoo and Muskegon counties are Zone 5A, so the vapor-retarder shortcut is not available there. Grand Traverse, Antrim and Charlevoix counties, around Traverse City, Elk Rapids, Charlevoix and Boyne City, are Zone 6A, where it is. In practice, a West Michigan attic sized at 1/300 should have its exhaust high and its intake at the eaves; adding more vents somewhere else on the roof is more holes, not a fix. On a reroof, Section R908.5 also requires existing vents that are rusted, damaged or deteriorated to be replaced rather than reinstalled.

A vent is one of the cheapest parts on the roof; the labor is in lifting the shingles around it and laying them back correctly. So replacement is the honest answer far more often than sealing. Caulk over a cracked hood or a reverse-lapped flange buys a season at best, and then the leak comes back with more wet wood under it.

Done properly, a vent replacement means pulling the old vent and its nails, checking the deck around the hole for rot, setting a new vent with the flange lapped the way the manufacturer specifies, and nailing and sealing it to the instructions. If the leak was weather blowing in rather than water running under, the right answer may be a different vent: a baffled, filtered ridge vent instead of an unbaffled one, or a single exhaust type instead of two that fight each other.

At Stellar, a vent problem found during a free inspection is written up with the cause and the options, and if a pipe boot or a few shingles nearby need work too, handling them in the same visit costs less than separate trips.