Perfect Trim on Doors, Windows and Base Moldings
Updated: Jul. 14, 2023
Tricks for getting tight-fitting joints on door and window casings and on base moldings.
These DIY tips will help you get tight-fitting joints on doors, windows and base moldings, even if your walls are less than perfect. We'll show you how to adjust your cuts so the trim fits together on out-of-square corners and wavy walls. Get out your miter saw and follow these steps, and you'll end up with professional looking trim every time.
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A full day
Tricks for tight-fitting joints
Base trim molding, outside corner trim pieces
A tight-fitting outside corner on base molding
Base molding, inside corner
A tight-fitting inside corner on base molding
Perfect trim on a window casing
Wood door trim casing
A tight-fitting joint on door casing
Miters rarely fit on the first try and for good reason. Corners are out of square, walls aren’t plumb and drywall has bumps.
The secret to tight-fitting joints is knowing how to adjust the cuts to make them conform to all these wacky conditions. In this article, we’ll show you a bunch of tricks you can use to cut wood door trim molding and window casing and baseboard joints to fit perfectly, even when you have less than perfect walls and jambs.
Shaving technique for tough corners
Photo 1: Shim the piece being cut
Close a gap on the top of a miter by placing a skinny (1/16-in. or less) shim against the portion of the fence farthest from the blade. Slide the trim molding tight to the shim and against the fence near the blade. Hold it in this position while you make the cut. Caution: Keep your fingers at least 6 in. from the path of the blade.
Photo 2: Shim the second piece too
Trim the other half of the miter using the same technique. Use the same shim and place it the same distance from the blade. Drop the blade slowly through the wood to shave thin slices.
How many times have you set your miter box exactly on 45 degrees and cut miters on a pair of moldings only to discover they don’t fit? Well, don’t worry. There’s nothing wrong with your miter box or your technique. Miters almost always have to be shaved to fit perfectly.
One method is to simply adjust the angle slightly on your miter box and recut both moldings. The trouble is that making tiny adjustments to the cutting angle is difficult on many power miter boxes. A quicker and easier method is to place a shim against the miter saw fence to slightly change the angle. Move the shim away from the blade for smaller adjustments and closer for larger ones, or vary the thickness of the shim. Remember, both pieces need the exact same cut to fit precisely.
Tools for Perfect Trim
Even the best carpenter can’t cut a tight-fitting joint with a dull saw blade. Invest in a good carbide trim blade for your power miter box. Read the labeling on the package and choose a blade designed for cross-cutting trim molding on a power miter box. A thin-kerf 60-tooth blade will make even the least expensive miter box perform like a champ.
Also rent or buy a power trim nailer and compressor. It’s much easier to get great results when you can hold the molding in place with one hand and drive the nails with the other.
Tight miters on recessed jambs
Photo 1: Cut a shim to size
Cut a shim just thick enough to slip under a straightedge spanning the drywall corner. Use this shim to elevate the outside edge of your trim molding (Photo 3) before cutting it.
Photo 2: Cut back the drywall
Trim back the drywall with a sharp utility knife until the trim molding no longer rocks when it’s set in place against the jamb and drywall. Use a hammer to mash and flatten the drywall if necessary.
Detail of shim under the molding
Lift the outside edge of the molding up with the shim.
Photo 3: Cut the miter
Cut the 45-degree miter on the molding. Repeat the shimming and cutting process for the opposite miter.
Occasionally you’ll run into a door or window frame that for whatever reason isn’t quite flush with the wall. The best solution is to fix the jamb by planing it off if it protrudes or, if it’s recessed, adding thin strips, called jamb extensions. But this isn’t always possible.
If the jamb is only slightly recessed and adding jamb extensions would be unsightly, there’s another solution. First remove enough drywall so the trim can span the jamb and wall without rocking (Photo 2). This solves half the problem. But even now a regular 45-degree miter won’t fit because the molding has to tilt down to meet the jamb. Correct this problem by tilting the trim on the bed of the miter box to match the angle at which it rests against the wall.
Then make standard 45-degree miter cuts. Photo 1 shows how to determine the correct thickness for the shim used in Photo 3 to tilt the trim molding.
Coping tall base
Photo 1: Cut a bevel in the baseboard
Cut a 45-degree bevel on the baseboard piece to be coped. This 12-in. compound miter saw allows us to cut up to 8-in. wide baseboards.
Photo 2: Flip the base over and make another cut
Turn the mitered baseboard upside down in the miter box. Adjust the angle to about 15 degrees and saw down along the straight section of the beveled cut. Keep the blade slightly to the outside of the line. Let the blade stop before lifting it from the cut.
Photo 3: Cut the profile with a coping saw
Saw out the remaining profiled section with a coping saw. Tilt the saw to at least a 30-degree angle to create a back bevel for easier fitting.
Coping rather than mitering inside corners is the best method to fit baseboards. But on tall baseboards, cutting the long straight section of the cope with a coping saw is difficult, and the cut is usually wavy. Instead, start the cope as usual (Photo 1). Then tip the molding upside down in the miter box and saw straight down to the profiled section. Finally, complete the cope by sawing out the profile (Photo 3).
Easier adjustments for inside corners
Photo 1: Test fit the wall corner trim
Check the fit against the square-cut piece of the 7base before nailing either. The straight sections rarely fit perfectly.
Photo 2: Fix a gap at the bottom
Close a gap at the bottom by removing the square molding base and driving a drywall screw into the wall about 1/2 in. from the floor. Test the cope and adjust the screw in or out until the cope fits tight.
Photo 3: Fix a gap at the top
Close a gap at the top by scribing the gap with a small compass to mark the wood to be removed. Then file or plane to the line.
Floors that are out of level can cause even perfectly coped inside corners to look lousy. Check the fit of your cope before you nail in either base molding. That way you’ll still have the option to shim out the bottom of the square-cut (uncoped) piece to close a gap at the bottom of the cope (Photo 2). Photo 3 shows marking a cope that’s open at the top. You then file or plane to the line.
Overcut outside corners
Photo 1: Use a knife to mark outside corners
Mark outside corners with a sharp utility knife. Repeat the marking process on the opposite baseboard. Cut 45-1/2 degree angles on both boards, leaving each an extra 1/8 in. long.
Photo 2: Test fit the corner trim pieces
Hold the boards in place to check the fit. If the miter is open on the front, increase the cutting angle to about 46 degrees and recut both sides. Be careful to remove only a hair’s width from each board. Reduce the angle if the cut is open at the back. When the angle is correct, recut each board just to the outside of the marks before nailing them into place.
Getting outside corners to fit tight is trickier than it looks. The key is to make accurate marks with the baseboard in place rather than relying on measurements. And then cut the piece a little long so you still have the option to shave a little from the angle if it doesn’t fit. Since gaps on the back side of the corner are barely noticeable, while gaps on the front are glaring, it’s a good idea to start by cutting slightly steeper 45-1/2 degree angles first. Then if there’s still a gap in the front, cut a slightly steeper angle on both pieces.
You’ll need a compound miter saw or sliding compound miter saw to easily cut tight-fitting miters on a wide baseboard.
Photo 1: Complete the cut with a knife
Cut the angle for a small mitered return on your miter saw, but don’t completely cut if off from the trim stock. Rather, cut through the remaining sliver of wood with a utility knife.
Photo 2: Attach the return with glue
Glue the mitered return in place with a fast-acting cyanoacrylate glue formulated for wood (Krazy Glue is one brand).
Return miters are an elegant way to finish the end of moldings. But cutting the small return can be tricky. The tiny mitered pieces of molding tend to catch on the spinning blade and launch into space. Photos 1 and 2 below show one solution.
Required Tools for this Project
Have the necessary tools for this DIY door molding project lined up before you start—you’ll save time and frustration.
Required Materials for this door molding Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
Originally Published: January 07, 2019