Installing Window Trim the Easy Way: MDF Trim Molding

The World’s Easiest Trim?

You’ll find a classic trim style that looks similar to this in many century-old houses. But that’s where the similarity ends. Old-school carpenters carefully cut and fit mitered returns at the ends of the window aprons and crown moldings to create the classic profiles.

You could reproduce this look with wood moldings and a good bit of patience. But if you’re going to paint, why not follow our lead and make your own moldings from medium-density fiberboard? MDF is easy to cut and shape, and it paints up beautifully. And even better, by cutting the parts to length first and routing the shapes around the corners, you eliminate all the fussy miters.

In this article, we’ll show you how to make classic trim using three router bits. You can follow our pattern exactly, or create your own using router bits you may already have. If you’ve never used a router, this is a great introduction to the tool. Since MDF is relatively cheap, a goof here or there won’t cost much.

We’ll show you how to make trim for windows and doors. Since most home centers and lumberyards stock at least one choice of primed, wide baseboard, we won’t show how to make base. But if you prefer, you can use the techniques we show here to make your own baseboard, too.

Time and Materials

For this project, you’ll need 1/2-in.- and 3/4-in.-thick MDF, and some paintable 1×3 lumber for the stools. We recommend wood for the stool because unlike MDF, it’ll withstand a little moisture from window condensation. We used pine, but poplar, maple and birch would also work well.

You can figure out how much material you need using your cutting list, but as a rough estimate you can cut parts for about four or five windows from one 4 x 8-ft. sheet of MDF. MDF is heavy, and full sheets are hard to handle. Ask to have the sheets cut in half lengthwise at the home center.

Step 1: Getting started

Remove the old window and door trim

Pry off the old trim and pull out any nails remaining in the doorjambs and window jambs. If there’s a ridge of paint or finish on the face of the jambs, sand it smooth to prepare for the new trim.

Measure between the jambs

Make a list of the windows and doors that need new trim. Then measure the width between the jambs and write these measurements on your list.

After you pry off the old moldings, the first step is to measure all the openings and create a cutting list and a shopping list. If you’re careful to do the math right, it’s easy to create the cutting list by simply adding to the width of the openings.

In the Cutting List we provided for a window, we show how much to add to the width inside the jambs for each part of the apron, stool and head casing assemblies. Since you’ll be working with fractions, the math can get time consuming. If you have several openings to trim out, consider downloading a construction calculator app for your smartphone to help with the math.

Step 2: Cut the parts

Cut strips of MDF

Use the list of parts you created to calculate how many feet of each trim piece you’ll need. Then cut the sheet of MDF into the correct number of strips. It’s a good idea to add about 20 percent to your totals to allow for waste and mistakes. Use a miter saw to cut these strips to the correct lengths. Make sure to label all your parts with a corresponding window or door number.

Using your cutting list as a guide, rip strips of MDF on a table saw. Cutting MDF creates tons of fine dust, so wear a mask and if possible, move your cutting and routing operations outdoors. If you must cut indoors, put an exhaust fan in the window and connect a shop vac to your saw. Our router included a vacuum cleaner hose attachment port. If yours doesn’t, you may be able to buy an attachment that fits your router.

After ripping the parts, cut them to length on a miter saw. Label the parts as you go to avoid confusion later. If your table saw leaves saw marks along the edge of the MDF strips, be sure to sand them smooth. The guide bearings on the router bits follow along the edges, and if they are bumpy or rough, the shape you rout won’t be smooth either.

Step 3: Shape the edges

Rout the long edges

Find a large, flat work surface that you can screw into. Attach a stop and a backer board to the work surface as shown above. Make sure the trim overhangs the work surface slightly to allow room for the router bit guide bearing.

Rout the ends

Sandwich the trim piece between strips of MDF to hold the trim in place and provide a level surface for the router. Carefully align the ends of the trim piece and two strips of MDF, making sure the ends protrude past the work surface slightly to allow room for the router bit guide bearing. Screw the strips to the work surface. Then attach a stop behind the trim piece to hold it in place. Run the router from left to right across the trim to shape the end. Then flip the trim end-for-end and rout the shape on the opposite end.

Rout the aprons

Rip two lengths of 2×4 to the same width as the lower window apron (3 in.) and screw them together. Toe-screw the 2x4s to the work surface and clamp the lower window apron part to the 2x4s. Now you can easily shape both ends and the long edge with the router. The 2x4s provide the support needed to hold the router level as you rout the narrow ends.

We’re using a handheld router with router bits that include a guide bearing. With this method, you move the router over the pieces to shape the edges. You could also mount your router in a router table. For information on how to build your own router table, check out our router table plans. And don’t miss these router table tips and techniques!

Shaping the long edges of your trim pieces with a router is straightforward. But you need a way to hold the piece steady as you shape it. And the edge has to overhang your work surface slightly to allow room for the guide bearing. Step 3 shows one easy setup to achieve these two goals.

Mount your router bit in the router and set the depth. Test the depth setting on a scrap of MDF and adjust it if needed. Using the setup shown in step 3, move the router left to right. Relocate the backer board for the different width pieces and switch to a 1/2-in.-thick MDF support board for shaping the fillet. Be sure to recess the screws that hold the support boards so they don’t interfere with the router base.

Routing the shapes on the ends of the fillet, crown, stool and upper apron pieces is tricky because the narrow trim pieces don’t provide enough support for the router. You can solve this problem by sandwiching the piece you’re routing between MDF strips. You’ll have to relocate the support strips and the stop for pieces of different widths and lengths.

Use a cove bit for the lower apron pieces and top cap. Switch to the classic ogee bit to shape the lower cap and upper apron. And use the round-over bit to shape the top and bottom edges of the fillet and the stool and the long edges of the side casings.

Step 3 shows a method for supporting the router to cut the cove in the lower apron piece. Rip two 2×4 pieces to 3 in. and screw them together to use as the support pieces.

Basic Router Bits

Basic router bits are the key tools for this job. Using just a cove, round-over and classic ogee bit mounted in a router, you can shape the edges of inexpensive MDF to make the parts for a classic trim makeover.

Step 4: Assemble the parts

Build the head casing assembly

Collect the four parts for the head casing and arrange them on your work surface. Spread glue along meeting edges and pin them together with 1-1/2-in. trim nails. Before nailing them together, make sure to align the parts so the end reveals (overhangs) are even.

Assemble the window stool and apron

Glue and nail the upper window apron to the lower window apron. Then glue and nail the stool to the apron assembly. Make sure to keep the end overhangs even.

Step 4 shows how to build the head casing assembly and the stool and apron. Spread a layer of glue between the parts before you pin them together. For super-tight joints, clamp the pieces after gluing them together. You can remove the clamps in about 20 minutes and safely install the assemblies an hour after gluing them up.

Priming and applying the first coat of paint before you install the window and door trim will result in a neater job and save you time. Sand lightly after the primer dries, especially along the cut edges that tend to get a little fuzzy. Then apply the first coat of paint. Install the trim, then fill the nail holes and paint a second coat.

Step 5: Install the new trim

Nail on the stool and apron assembly

Make pencil marks 3/16 in. inside the jamb at each corner of the window to indicate the edge of the trim and establish a 3/16-in. reveal. Also mark the center of the window and the center of the stool assembly on short pieces of masking tape. Line up the center marks and align the stool with the reveal marks. Then nail through the apron into the wall framing under the window to hold the assembly in place.

Mark the side casing length

Cut a casing piece about an inch or two longer than needed. Rest it on the stool, and mark the top at the reveal mark. Cut the side casing to length on a miter saw and nail it into place, being careful to keep the reveal consistent from top to bottom. Mark, cut and install the casing on the opposite side using the same process.

Nail the head casing assembly into place

Center the head casing assembly over the window, making sure the fillet overhangs the side casings evenly. Then nail through the head casing assembly into the wall framing above the window.

Make base plinths for the doors

Complete the classic look by adding plinth blocks under the side casings on doors and other openings. Make the 1-1/4-in.-thick plinths by gluing strips of 1/2-in.- and 3/4-in.-thick MDF together. Then rout a cove on the top edge. Make the plinths about 1/4 in. wider than the casing to create a small offset where the casing rests on the blocks.

You’ll be surprised how easily the trim goes up. There are no miters to cut. All you have to do is cut the side casings to length. Start by marking 3/16-in. reveals at every corner. A combination square set to 3/16 in. works great for this. Center the stool and apron assembly and nail it on. Then mark and cut the sides. And finally, install the head casing.

If the drywall around your window or door protrudes a bit past the jamb, trim it back at an angle with a utility knife, or crush it with a hammer until it’s flush to the jamb. Otherwise there may be a gap between the moldings and the jamb. If you do end up with a small crack, fill it with good-quality latex caulk before brushing on the final coat of paint.

Start trimming door or other openings by nailing on the base plinth first. We made these plinth blocks by gluing 1/2-in. MDF to 3/4-in. MDF and then routing a cove shape on the top edge. If you have several doors to do, make a bunch of blocks by first cutting strips of 1/2-in. and 3/4-in. MDF a little wider than the height of the blocks (we made ours 7-1/2 in. tall) and gluing them together. When the glue has set, clean up both edges by running the strip through the table saw until it’s the right width. Then rout the cove along one edge. Finally, cut the blocks to 4-1/4 in. wide.

Finish the door openings just like the windows by cutting and installing the side casings and nailing on the head assembly. When that’s done, all that’s left is to install the baseboard and finish up the painting. Read these instructions on how to install baseboard molding to complete your project.

Classic Trim

Simple Craftsman Trim

The trim shown on the cover of our June 2016 issue is even easier to put together than the classic trim in this article. There are no profiles to route; the pieces are just square-edged. Use the information in the attached ‘Simple Craftsman Trim Diagram’ to determine the length of the pieces.

Additional Information

• Cutting List & Materials List
• Simple Craftsman Trim Diagram