Window Casing with a Fillet
Adding a protruding bead under the top piece of casing adds a distinctive touch to traditional window trim.
This traditional-looking style features a thin protruding horizontal piece just under the top casing and on top of the side casing pieces. Variously called a “fillet,” “parting bead,” or even “popsicle stick” because of its thinness, this piece adds a distinctive touch for a small outlay of money and labor. A fillet may be a simple piece of lath; in the example shown here, we added some detail by using a piece of upside-down Colonial stop.
Fillets are usually used along with side casings that are not tapered along their widths, as most mitered casings are. The side casings could be plain, without a profile (sometimes called “sanitary casing”). In our example, we add a decorative touch to the side casings by attaching outside corner molding to each side; we left the head casing piece plain.
| TIP: It’s important that both the side casing pieces be precisely tall enough to create the desired reveal on the top jamb. Before fastening the side pieces, set a scrap piece of wood on top, spanning both pieces, to be sure the reveal will be consistent. |
Making Plinth BlocksIf you cannot buy plinth blocks that suit you, it’s not difficult to make your own. In our example, none of the available blocks came close to matching plinth blocks elsewhere in the house. You may have to go to a hardwood supplier to buy some 1-in.-thick or 11⁄8-in.-thick stock. Find a router bit (1) that creates a similar profile; you probably won’t get an exact match, but the difference will not be noticeable if the blocks are in different rooms. Cut the blocks to length and width (2). Use a power sander to smooth any rough edges, then rout the top and side front edges (3). 
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 TIP: If the baseboard is in place, hold a plinth block (this one is scheduled for edge routing) in place to mark for cutting the baseboard. Note that the block is wider than the casing, and so nearly covers the entire thickness of the jamb. |
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| Modifying a Fillet for an Imperfect Wall
If your wall is wavy or otherwise imperfect (a not unusual situation, especially in an older home), you may need to shave one end of a fillet so it can come tight against the top jamb. Use a plane for a small discrepancy, or make a cutout with a tablesaw and handsaw to close a gap of more than 1⁄8 in.
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Excerpted from Trim (The Taunton Press, 2015) by Steve Cory.
