Adding Detail With Built-Up Molding
The final result looks like custom trim, but it's painless and affordable to make by combining stock profiles.
Synopsis: A professional trim carpenter explains how to install window and door casing step by step. At the same time, he offers a half-dozen design ideas for changing the look of the trim by combining different moldings.
When I’m asked for ideas to upgrade the interior of a house, the first thing I say is “Change the trim.” The dramatic difference that built-up trim makes easily justifies the extra cost. As a carpenter, installing an interesting three-piece casing is always more fun than working with boring old clamshell or colonial casing. Because it consists of multiple pieces, built-up trim is actually much more forgiving than single piece trim. One piece follows the window or door jamb, and a second follows the wall. Then a third piece joins the two, concealing any gaps. As my kids would say, sweet.
A few companies offer architecturally correct built-up trim arrangements. But much the same effect can be achieved with a little imagination and some stock trim from a lumberyard or building supply store. To test trim combinations, make up small sections with all the details, as I’ve done in the top photo on the facing page.
Window trim starts with the stool
For built-up trim, I prefer a thicker stool with bullnose edges. To find the length of the stool, I assemble a short section of the built-up side casing. Then I set it in place near the bottom of the window, making sure to leave a 3/16-in. reveal on the inside of the window jamb. I make a light pencil mark on the wall along the outside edge of the trim section, then I repeat the process on the other side. I make the marks low enough to use as a reference later when installing the apron.
Next, I measure the distance between the pencil lines and add 3 in. The extra length allows the ends or “ears” of the stool to extend 1-1/2 in. past the edge of the trim, rather than the 3/4 in. typically used with conventional molding. The extra length also accommodates the decorative trim that will be applied to the apron.
I make sure the finished stool is deep enough for the built-up trim to land without overhanging. Ideally, the stool should extend 3/4 in. to 1 in. beyond the outermost edge of the trim. To maintain the profile of the stool on all three faces, I cut 45° miters on the ends and install small return pieces that fit between the miters and the wall. I glue and nail the return pieces to the stool before nailing the stool into place.
For more photos and details on built-up molding, click the View PDF button below.