Gable Vent Louvers and Screens Belong in the Roof Spec

A gable vent sits in the triangle of wall above the ceiling line at each end of the house, and most roof bids treat it as a box to tick. In St. Louis that box matters. The attic under a shingled roof has to move air in the cold months to keep moisture from condensing on the roof deck, and in the warm months to keep the attic from running hot enough to bake the shingles from below. A gable vent that is undersized, screened with the wrong mesh, or louvered with blades that let rain through does not help the roof, and the spec is where that gets decided.

Name the vent by its free area, not its size

The size of a gable vent is the outside frame, but the air it moves is the free area through the opening after the louvers and the screen take their share. Manufacturers usually publish a net free ventilation area in square inches. Ask for that number in the spec, written beside the vent model, because a vent that measures 24 by 12 inches on the outside may pass far less air than the frame suggests.

The common residential rule in the International Residential Code calls for one square foot of net free vent area for every 150 square feet of attic floor. The rule can drop to one to 300 under certain conditions, such as a vapor retarder on the warm side of the ceiling. Local permit desks in St. Louis County and St. Charles County adopt the code in different editions, so your permit desk or your roofer can tell you which count applies. The spec should show the calculation, not just the word vent.

A gable vent also does not work alone. It pulls air out of the attic only when there is intake air coming in at the soffit or the eave. If your house has a ridge vent, the gable vents may be working against it. Ask the roofer to show, on the spec, how the intake and the exhaust balance across the roof, and whether any gable vent is being added to a roof that already has a ridge vent.

Louver style and the rain that comes in sideways

A standard louvered gable vent has angled blades. The blade angle and depth decide how much rain and snow come through in a driving storm. Spring fronts that move through the metro often bring wind-driven rain that pushes water flat against a gable. Ask whether the louver is a drainable style with a sill that sends water out, and whether the blade angle is named. A louver with a flat sill and shallow blades will show a wet stain on the gable board after the first hard rain from that direction, and that stain will look like a roof leak even when the shingles are fine.

Hail matters here too. The blades of an aluminum or vinyl louver can dent at the same time as the gutters and the soffit. If you have hail damage on the house, photograph the louver blades and the sill before the roofer touches the gable, and note the blade material on the photo log. A dented louver belongs in the same photo record as the roof, not in a separate note that gets lost. Keys documents damage and puts the written scope and photos in your hands, and the insurer conversation stays with you.

Screen mesh is a restriction, so name it

Insect screen keeps bees, wasps and birds out of the attic, and every gable vent in a neighborhood with mature trees will attract something. The mesh is also a restriction on airflow. Fine aluminum mesh, the kind that keeps out gnats, cuts airflow more than a coarser bird screen does. A spec that says "screen" without a mesh size or a product name leaves the crew to choose, and the crew will choose whatever is in the truck.

Ask for the mesh opening size, the frame material, and whether the screen comes out from inside for cleaning. A screen that is painted over, or packed with cottonwood fluff and seed from the nearby trees, loses area through the season. Plan to check it in the spring and again in late fall, before the first hard freeze, when a blocked vent can hold moisture in the attic all winter. Moisture that stays in an attic through a freeze and thaw cycle shows up later as frost on the nail tips and stains on the underside of the deck.

Fasteners and the wall around the vent

The vent frame sits in the gable wall, usually over sheathing and house wrap. The spec should name how it is fastened and sealed. Stainless or hot-dipped galvanized screws are the usual choice for an exterior vent, and the spec should say which one. A vent set with bare steel fasteners will show rust tracks on the siding within a few seasons. Ask whether the vent is caulked, gasketed or set in a flashing flange, and whether the wall behind it is flashed so that water running down the wall goes outside the vent frame and not into the wall cavity.

On a brick gable, the frame is set into the mortar joints or onto a block, and the weep holes and counterflashing decide whether rain stays out. If the gable is brick, say so in the spec and name who sets the vent into it. A vent that is set into a joint with no flashing behind it can look fine from the yard for years while the wall behind it stays wet.

What to ask before you sign

• The vent model number and the net free area in square inches, written on the spec.
• The total attic floor area and the code edition used for the count, with the math shown.
• The louver style, the blade angle and whether the sill drains outward.
• The screen mesh size, the frame material and whether the screen comes out for cleaning.
• The fastener material and whether the frame is gasketed, caulked or flanged.
• Whether the old vent is reused or replaced when the roof comes off, and who checks its condition on tear-off day.

A roofer who hands you that list with real numbers beside each line has named the vent the way a spec should. A roofer who says the gable vents are included has left the one part of the attic airflow that you can see from the yard unspecified. If you want a second read on your gable spec before you pick a crew, call Keys Roofing at (314) 220-2333 and ask for the vent line in writing.