What an arch-top vent is
An arch-top gable vent is a rectangle with a rounded top, like an arched window. The sides are straight, the bottom is flat, and the head is a half circle or a segment of one. It is more formal than a plain rectangle and more upright than a half-round.
Spectis molds it in high-density polyurethane. It ships factory-primed white and is ready for paint. It does not rot, and it cuts and fastens much like wood trim.
Where it looks right
Put it on a gable that already has an arched window, an arched door, or a round-top pediment. The repeated curve is the whole point. Centered under the peak, it can also stand in for a round vent when you want more height than a circle of the same width.
Line the straight sides up with the casing below if there is a window under it. A vent that is wider than the window, with no shared centerline, looks accidental.
Open or closed
open arch-top gable vents ventilate. closed arch-top gable vents are decorative only, with a solid face behind the slats. Over living space, or on a gable that is already vented, the closed arch is often the right call. On an open attic wall, the open arch can do both the look and the air. Compare the two on the open versus closed page.
A short sizing note
LOA1818, an 18 by 18 inch arch-top gable vent is 18 by 18 inches overall. The arch uses part of that height, so the slat field is shorter than a square of the same size. Size the width to the gable and to the window it echoes. The sizing page has an appearance range only. It will not tell you the net free area. That number, when you need it, is on the product page.
Pair it with
Pair it with the arched window or door that suggested it. A keystone at the head of the door and a plain arch on the vent is enough. You do not have to add a keystone to the vent unless the rest of the house is already that formal. See the install overview for the difference between cutting an opening and mounting a closed vent on the surface.