What a half-round vent is
A half-round gable vent has a flat bottom and a semicircular top. It is wider than it is tall, so it belongs where a full circle would be too high or too large. The slats run level, and the arched top does the decorating.
The material is molded high-density polyurethane. It ships factory-primed white, ready for paint. It does not rot, and it cuts and fastens much like wood trim.
Where it looks right
Use it low in a wide gable, or centered over a window when you want the vent to echo an arched window below. Houses with fanlights and half-round windows already speak this shape. A tall, narrow peak is usually a better fit for a full round, an oval, or a peak top.
Keep the flat bottom level. If you set it tight under the rake, the arch can touch the roof trim on both ends. Drop it until there is siding visible around the curve.
Open or closed
open half-round gable vents move air through the slats. closed half-round gable vents are the same arch with a solid back, for looks only. A half-round over a window on living space is a common place for the closed version. A half-round high on an unfinished attic wall is a common place for the open version. The open versus closed page walks through that choice.
A short sizing note
The piece shown is LOHR2412, a 24 by 12 inch half-round gable vent. Width is the number that has to agree with the gable. The height is about half the width on a true half-round, so a vent that is wide enough can still look short. Start with the appearance range on the sizing page, then confirm both overall dimensions on the product drawing.
Pair it with
Pair it with an arched window, a sunburst pediment, or a half-round transom so the curve shows up more than once. On a house with only square windows, a rectangle or a round may look more at home. Installation for an open half-round still means a smaller opening than the vent, so the frame covers the cut. See the install overview.