Adding Detail with Built-Up Molding

Synopsis: A professional trim carpenter explains how to install window and door trim 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.

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 (see photo), 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.

Start with the stool  After mocking up the trim to double-check the look, scribe the casing width on the wall to determine the length of the stool and apron. Once cut and returned on the ends, the stool is nailed through the top into the windowsill. Don’t forget the reveal To get the length to the short point of the first side piece, measure from the stool to the inside edge of the frame, then add 3⁄16 in. for the reveal. Nail the side pieces only to the jamb at this point. Sides first, then top Measure between the sides to get the distance between the long points of the top piece. After fitting the top piece, glue the biscuits and miters and fasten the top piece to the jamb edge. Start adding layers Measure along the outside edge of the flat casing to find the length of the outside edgeband at its short point. Press the edgeband against the wall, and nail it to the edge of the flat stock.  The art of nailing trim Although the primary goal is to secure the pieces firmly in place, also think about hiding the nails whenever possible and spacing any visible nails as neatly as possible. It’s also a good idea to locate the framing in the wall first, typically by probing through the drywall with a finish nail, but only in the areas that will be covered by trim. What size nails? Rick uses nail guns and typically shoots three different sizes of nails: 1 1⁄4-in. 18-ga. nails for small moldings, 2-in. 15-ga. nails for wood-to-wood nailing, or 2 1⁄2-in. 15-ga. nails for nailing through drywall into framing. The standard nail equivalents are #4, #6, and #8, respectively. Apron wraps up the job With the decorative trim already attached, the apron lines up with the pencil marks made when the stool is laid out. Holding the apron tight to the underside of the stool, nail it to the framing.

Flat casing first

The first part of the built-up molding that I install is the flat casing. I measure from the top of the stool to the inside edge of the top jamb of the window, then add 3⁄16 in. for the reveal. After squaring one end of a piece of stock, I mark the length along the inside edge (in this case, the beaded edge). I cut a 45° angle using the mark as the target for the short point of the angle.

I repeat the same step for the opposite side of the window, making sure to reverse the direction of the cut. If I’m trimming a lot of windows that are the same height, I check a few to make sure they are exactly the same, then cut all the pieces at once rather than completing one window at a time. 

Before nailing in the pieces, I use a biscuit joiner to cut a slot into the mitered ends. Miter joints that are reinforced with biscuits are less likely to come apart over time. Taking care to keep an even reveal, I nail the side pieces into the edge of the window jamb using 11⁄4-in. nails. 

I find the length for the top piece of casing by measuring from long point to long point on the two side pieces. After marking the top piece, I cut it just a bit long, usually by the thickness of my marks. Then I test-fit the piece, shave it if necessary, and cut the biscuit slots. To shave a hair off the miter, I place it tight against the chopsaw blade, raise the blade, turn on the saw, and bring the blade back down. By the way, I start with the wider windows first so that if I cut a top piece too short, it still can be used on a narrower window. 

I glue and insert the biscuits into the side pieces with a little more glue on the mitered edges of both top and sides. I wipe the glue with my finger to ensure a thin, even coat. Next, I push the top piece into place and rock the side pieces back and forth until the miters align. If I had nailed the outside edges of the side pieces, I wouldn’t be able to adjust the fit so easily. Finally, I nail the top piece into the edge of the jamb and drive a nail through the edge into the side pieces near the long points. 

Three-piece casing Combining off-the-shelf moldings creates a complex look without custom milling. The three-piece casing used in this article is from the Greek Revival series by Windsor One (888-229-7900; www.windsorone.com). On the following pages, you’ll find designs composed of more common moldings. More casing options

Add decorative layers

Trim pieces are added to build up the molding, beginning with the edgeband. I determine the length of the outside edgeband by measuring the outside edge of each flat side piece, from the stool to the long point. After marking that length on a piece of the edgeband, I cut the 45° angle, using the mark as the target of the short point. 

The edgeband is flexible enough that it can follow all but the most severe irregularities in the wall. So as I install each piece, I press it hard against the wall. I nail the edgeband into the edge of the flat stock only. Just as with the flat casing, I measure for the top piece of edgeband between the long points, cut it a touch long, fit it, and then glue and nail it into place.

Next, I go back and nail the flat casing into the stud framing. If the framing allows, I nail as close to the outside edge of the casing as possible so that the nail holes will be covered by the final filler trim. It’s a good idea to find the edge of the framing beneath the wallboard before running any trim. At this point, I also nail up through the stool into the bottom edge of the flat casing.

To complete the built-up molding, I mark and cut the filler trim that sits just inside the outside edgeband. I use the same measurements that I took for the edgeband, only this time the measurement is to the long point of the 45° miters on the filler trim. Instead of installing the sides first and then the top, I work my way around each window (see photo). I make the pieces slightly long so that they spring tightly into place. I secure them to both the flat casing and the edgeband using 1-1⁄4-in. nails.

Trim the apron before it goes on

For the look I prefer, the length of the apron is the same as the distance between the first pencil marks that I made on the wall. The apron is constructed out of the same flat stock as the window casing. Just as I did with the stool, I bevel both ends of the apron and then cut, glue, and pin the small return pieces in place. Next, I cut and install the decorative trim that runs along the top of the apron. If

I have a lot of windows to trim, I can work more efficiently by making all of the aprons at one time. 

Finally, I hold the assembled apron hard against the underside of the stool and then fasten the apron to the stud framing . Also, I carefully shoot a few nails down into the apron through the top of the stool. 

Trimming a matching door? Height is the key The process of installing the built-up molding around doors is the same as it is for windows, only there is neither a stool nor an apron. If the finished flooring is in place, I simply measure from the flooring as if it were the stool. In this case, the finished floor had not been installed, so I set pieces of the flooring in place temporarily and used them to gauge the length and height of the door casing. After I’ve finished trimming the door, I pull out the flooring pieces.

Fine Homebuilding contributing editor Rick Arnold lives and works in Wickford, R.I. Photos by Roe A. Osborn.

From Fine Homebuilding #172