Working with historic homes can be difficult. Over time, historic homes often settle, twist, or bow in some way and become static in that position. For carpenters working with irregularities, it’s all about finding installation techniques that result in a look that is refined and expertly hides them.

In this project, a dormer has an intersecting roof at the sill line of some windows. We discovered that the dormer had put pressure on the wall and bowed it in. Although it’s static now, we needed to deal with the bow and make it disappear to the eye by installing our window stool in a straight line to catch the eye hiding the bow that’s still there. Once the stool was installed, we installed custom trim to match the original millwork in the existing historic home. Today, I'm walking you through how we did it so you can apply the process to your own job. 

Use Measuring Techniques to Find Irregularities

To get started, apply measuring techniques to identify where any problems might be with bows in walls or floors, out-of-plumb walls, or varying wall thicknesses. This can be done using a straightedge, string lines, or even a laser if used correctly. The end result will be millwork that remains pleasing to the eye when finished, hiding irregularities that still exist. When it comes to working with historic homes, remember: a level can actually get you in trouble. Take the advice the old boat builder once gave to a new employee and “leave your level at the door!” 

string line on window in historic home

Think Through the Problem and Installation Technique

It’s important to understand that if a wall is bowed, the windows won't line up because they are angled out of alignment with each other. Each window is straight with itself, so the stool rip-cut intersecting each window is straight. However, each rip at each window has to be done separately. (We can’t rip the material in a table saw, notch the window dividers, and install the piece, because it would just follow the wall rendering a hideous bow to our stool.) Instead, we will need to preinstall the piece, secure it straight, and scribe it to fit each window.

Step 1: Set Up a Reference String Line, Measure, and Pre-Cut 

Because the stool will be scribed and cut at the window side, we need to create a reference point where the front of the stool will be. I find that a string line is perfect for this application. Set up the string line with a spacer block at each end attached to the wall (the spacer should be exactly the thickness of the stool horns that come out from the wall). In this case the thickness is 1 1/4".

Set the string so it’s a little higher than the top of the stool, ensuring it isn’t in the way when scribing or installing the piece. Once that’s done, measure back to the windows and find the widest point along the whole section. Then rip the stool stock to that measurement. This is also a good time to cut the stock to length. This will clarify where the butt of the casing is going to sit on either end, and how far past that point the stool should extend. It’s simply measuring that length and cutting the stock.

spacer for window replacement in historic home

Step 2: Mark and Cut the Dividers and Horns

Now hold the stool in place and mark where all the cuts will be made at each side of the dividers. Then, mark the end cuts that go back to the window. Find the depth of each cut by measuring from the string line to the wall at each cut out. Then, take the stool back to the workstation, and mark and make your cuts.

Step 3: Preinstall the Stool and Scribe the Cuts Intersecting the Windows

Preinstall the stool by securing it with a few finish nails, using the string line as a reference point to keep the stool straight. Once secured, scribe each window. The trick here is to cut a shim at exactly the distance of the widest gap seen after the stool is temporarily secured before using that shim to scribe each window onto the stool. Since the windows are straight, it’s really only necessary to mark the ends of each rip-cut.

scribing windows for millwork

Step 4: Rip Each Window Intersection Cut Using a Track Saw

Back at your workstation, make each cut separately, setting up and breaking down the track saw for each cut. The windows follow a bow in the wall, so every cut is out of alignment with the others. It’s important to ensure the stock is secured straight when making each cut, so check it with a straightedge. If it’s bowed a little, then your cut will be off when trying to fit it against the window. Secure the stock straight when using a track saw in this type of application. One little trick here is to back bevel each cut by setting the saw a degree or two so the top edge of the stool is the long point and will fit tighter.

Step 5: Install the Window Stool 

Secure the stool so that it will fit snug to the windows, but make sure it doesn’t have any humps or dips along the top surface. To accomplish this, I like to use shims under the stool. To get the stool to snug to the windows I also like to install screws at the dividers first, and then nail down into the framing sill. As the stool is being secured, use a spacer block between the string and the top surface of the stool to check for continual straightness. Adjust as needed. Use a combination square to check that the top surface of the stool is horizontally square with the window.

installing window stool for millwork in historic home

Step 6: Install the Window Apron

Sometimes it’s advantageous to install the apron under the stool before the extension jambs or the rest of the casing are installed. The apron provides more of a ledge for the stool to sit in, which is especially helpful in circumstances where there isn’t much of a framing sill for the stool to be nailed into. In this way, the apron can be used to support the stool if needed, and keep it horizontally square with the windows. The apron is cut in length to line up with the outside edge of the window casing above the stool. Miter cut the ends, and install the returns on the apron using blue tape to clamp the ends until the glue dries. Then install the apron (fat-side up) tight against the underside of the sill.

Step 7: Rip, Cut, and Install the Extension Jambs

This process is similar to any other extension jamb that will make up the area between the window and the outside surface of the drywall. Because the measurement is fairly consistent all around the windows, rip the extensions at the same depth. There are two important factors when it comes to installing the extensions.

Tip 1:

Because the top casing is a continuous piece, and it’s important that the reveals are consistent across the top of the window liners. This string line trick is a great way to double-check all the reveals are consistent. Set up a string line end-to-end at the top edge cut of each vertical piece. Then use a square from the string to the window frame and make a mark on each window frame to note the top of each vertical extension jamb.

Tip 2:

Be sure the vertical extension jambs at the framing dividers between the windows are exactly parallel and that all of them are exactly the same width. This will result in a consistent reveal, and ensure all the divider or “mullion” trim are exactly the same (which saves you time). Once measured and cut, this trim is easily nailed directly into the windows. Use glue where needed.

Step 8: Make the Window Casing Trim Between the Windows

With a flat stock style of casing, the trim between the windows is really easy to install, because there isn’t any profile to match, as all the surfaces are flat. (It’s less easy with profiled trim.) The vertical divider trim will need to be modified to match up the vertical profile with a continuous head trim when the intersection joint is cut at the top. Use two pieces of trim ripped down equally so that when glued together, they form a piece that is exactly the width between the windows. Once measured, ripped, and after the glue has dried, sand the resulting profile down to the flat surface on the trim. This flat surface will be the same thickness as the flat part on the head trim. Now it’s time to install the trim.

First, install the trim on each end. Next, measure and miter cut the continuous head piece. Then install the divider trim.

sanding millwork for window casing trim

Step 9: Mark and Cut the Head Trim for the Divider Trim

The head piece is temporarily installed with a few finish nails and fitted with the tops of the end trim. Take a cut off from the divider trim and set it against the head trim at each divider, making sure the reveals line up. Mark the head trim where the miter portion of the cut will be made. Then, remove the head trim, and mark and cut it at the workstation.

Refer to the video to see this process in real time. Make a 45 degree miter cut on either side with a horizontal rip-cut between the two. This is done in such a way as to miter the profile section of the trim. Flat-cut where the two pieces of trim meet at the flat surfaces. As I’m cutting the head piece, I like to fit each joint with a precut top of a divider trim as I go along, just to be sure my cuts will fit well. Once I’m happy with that, I install the head piece permanently.

cutting head trim for divider trim

Step 10: Install the Divider Trim

The best way to install the divider trim is to cut the top first and leave the bottom a bit long. This way the top fits first, so the cuts at the top can be adjusted if need be (and still have material to work with). Once I’m happy with the top fit, it’s simply a matter of cutting the bottoms and securing with glue and finish nails.

installing divider trim on window millwork

Once painted, this series of new windows will look really great, and will match the existing historic home exactly. Your diligent work will be well worth it. What happened to the bow? It’s still there. If you were to take a string line and stretch it along the bottoms of the trim just above the stool, it would show about 1/4” from end to end. But if you were to stand at one end of the wall and try to sight it in, you would be hard-pressed to see it.

windows with new millwork
how to install millwork on windows in historic homes