But not inexpensive. "Anytime you build something that breaks away from a straight line," says This Old House general contractor Tom Silva, "it costs more." Most window companies routinely make true half-round windows, but a more traditional eyebrow — a semi-oval or a small segment of a circle — will have to be custom made. Depending on its size and whether the dormer is being retrofitted or planned as part of a new house, Tom says, an eyebrow can cost upward of $10,000. Bensley agrees. For comparison purposes, he tells his clients that a simple shed dormer will cost twice the price of a roof skylight and an eyebrow window three times as much. Along with the expense of a curve-topped window, the high cost of an eyebrow comes from the complicated carpentry it requires. Finishing the inside of a curved wall, be it drywall or plaster, adds to the cost.
Roofing an eyebrow is also expensive. According to Tom, one common, and quick, method is to cover the curve with malleable copper and to treat it as a separate roof from the other material, whether it's asphalt, wood, or slate. Tom's approach is more traditional, challenging, and time-consuming: He likes to continue courses from the main roof up and over an eyebrow. "Any roofing will work, but you need an experienced contractor," says Tom. "Because of the curve's geometry, courses of roofing shingles get shorter as they go up the eyebrow from each side of the main roof. It's a little tricky to figure out."
There are less expensive ways to bring light and air into a top-floor room, but few are as dramatic as an eyebrow dormer. "They give a whole new meaning to the phrase 'to throw a curve' at something," Bensley says.