Here’s the straight scoop on curved trim

Curves, whether in walls, windows, archways or other parts of the home, can make a real design impact.

Curved windows, for example, whether used alone or in combination with other windows, easily can become a focal point. A curved wall or a curving tray ceiling can really make a statement in any room.

The challenge with curves, however, isn’t so much the installation of the framing or the window. Instead, it’s how to trim it out afterward.

Here are some of the basic techniques, along with a modern, ah, twist.

Window jambs

Jambs are the pieces that are used to cover the inside surface of the wall framing, between the inside face of the window and the interior face of the wall. In most houses, the wall framing itself is not actually curved. Instead, the framing is simply a series of angled blocks that roughly fit the shape of the window. Then the jambs are curved to fit the contour of the window and finish off the installation.

Curved jambs usually are installed in one of two ways, kerfing or laminating. For a kerfed installation, you begin by ripping a piece of ¾-inch-thick wood to whatever the finished width of the jamb is. Then, a series of saw cuts, called kerfs, are made on the back of the board. The kerfs are cut across the width of the board to a depth of about ⅝ inch, and are typically spaced about ¼ inch apart.

The kerfs allow the board to bend, and make it possible for you to shape the board to the curve of the window; the sharper the curve of the window, the more cuts you’ll need in order to make the necessary bend. They also may need to be closer than ¼ inch apart. After kerfing, the board is then installed in the opening, using shims behind it as needed to smooth out the curve.

For a laminated installation, several thin strips of wood are used, typically 1⁄16 to ⅛ inch thick, depending on how tight the curve needs to be. The strips are ripped to the finished width, coated with glue on both faces, then stacked on top of one another to make up a thickness of

¾ inch.

While the glue is still wet, the boards are bent to the curvature of the window; the wet glue allows each board to slide over the surface of the next one. When the correct curvature is achieved, it’s held in that position with clamps or other means until it dries, and is then sanded smooth and installed.

Wood casings

The casings are the outer trim, installed flat on the wall and partially overlapping the exposed edge of the jambs. The casing is the final trim piece on any window or door installation, and hides the gap between the back, the jamb and the rough framing. In the case of either of the previously described jamb installations, it also covers the saw kerfs or most of the lamination lines.

In traditional installations, the casings are created by first cutting the ends of several short boards on an angle; the exact angle depends on the diameter of the window’s curve. The boards are then glue-jointed end to end so that they make up a rough curve. Once dry, a band saw or handheld jig saw is used to cut the assemblage of boards to the correct curvature of the window.

Flexible casings

Now enter modern technology. Several manufacturers now make trim that is flexible, and can be bent and shaped to match just about whatever curve you’d like. One of the most well known is Flex Trim, which currently offers about 35,000 different trim profiles.

Flex Trim is made from a polymer resin blend that allows it to bend easily — it’s literally flexible enough to tie in a knot. It’s paintable and, depending on the specific product, also stainable and can replicate specific wood grains.

Installation is simply a matter of cutting the molding to length, bending it by hand to the proper shape — it bends easily and smoothly without kinking — and then nailing it in place in the same manner as any wood molding.

It’s also an excellent choice for baseboards on curved walls. And since there are so many sizes and patterns available, it can be matched seamlessly to traditional wood moldings on the straight wall or window areas.

You can see examples and get more information, including ordering information, at www. flextrim.com.