Window pediments are designed to shed water away from the sash while adding a decorative embellishment. This classic 18th-century Georgian design, a shed roof with cavetto cove and thumb bead base, can be readily reproduced by a handy DIYer. An existing window pediment will yield measurements and the molding profile, but it’s also possible to determine dimensions by examining paint shadow lines on the clapboards.
Before You Start
Select a block of wood that’s larger than your original window pediment to make your reproduction. In New England, white pine is a local, available, and traditional wood. Other wood species that work well are mahogany and cedar. Their dense growth rings afford longevity and a high resistance to decay. Be sure to select a premium, quality piece of wood.
- Measuring tape
- Bevel square
- English bead or rabbet plane
- Smoothing plane
- Gutter plane
- Table saw
- Band saw
- Profile gauge
- 100-grit sandpaper
- Dust mask
Trace a cross section of the original pediment’s profile on a clean end of block as a guide. If you don’t have the original, use a profile gauge to capture the dimensions from another pediment on the building, or make a cardboard cutout as your guide. Start with a short waste block to check all setup alignments and cuts on your table saw before you move on to the full-length board.
Rough cut the block on a table saw, using the bevel square to rip the roof angle: Place the bevel square on the original pediment to find the roof angle, and duplicate this angle on the new piece. Make another pass along the table saw to remove most of the cavetto’s angle. Next, using an angled fence, pass the piece of wood through the saw to finish the cavetto (shown). Do this slowly, 1/8" at a time. (The angle will be determined by the original piece; each will be different.)
Use a molding plane to make the pediment’s bead. There are different ways to do this, but my preferred methods are to use an English bead plane (shown) or a rabbet plane and sandpaper. A bead plane will make the process quick—you’ll only need maybe five or six passes. The rabbet plane is slower, and you’ll need to use more care to form the bead, but it’s doable if you don’t own a bead plane. You also can use a mechanical molding wheel or a router.
Cut the pediment to length, using the cut pieces as a template to trace and finish your returns. Cut returns out on a band saw (left), using measurements from the original or a copy. To finish, refine surfaces with planes—a smoothing plane to clean up flat edges and the roof of the return, a gutter plane to refine the cavetto’s shape (right), and a hand chisel on your bead return. Smooth edges with sandpaper. To install, face-nail the corners to the building, then toe-nail down through the pediment’s roof. Cover the roof with lead flashing.