Tricks for getting tight-fitting joints on door and window casings and on base moldings.
6 Tips for Perfect Trim on Doors, Windows and Base Moldings
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 conform to these wacky conditions.
In this article, we’ll show you many tricks for cutting wood door trim molding and window casing and baseboard joints to fit perfectly, even when you have less-than-perfect walls and jambs.
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. Getting great results is much easier when you can hold the molding in place with one hand and drive the nails with the other.
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Shaving Technique for Tough Corners
How often 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 the trim molding. 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.
Close a gap on the top of a miter by placing a skinny (1/16-inch 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 inches from the blade’s path.
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.
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Tight Miters on Recessed Jambs
Occasionally, you’ll encounter a door or window frame that 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. This solves half the problem.
But a regular 45-degree miter won’t fit even now 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.
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 before cutting it.
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.
Lift the outside edge of the trim molding up with the shim. Cut the 45-degree miter on the molding. Repeat the shimming and cutting process for the opposite miter.
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Coping Tall Base
Coping rather than mitering inside corners is the best method to fit baseboards. However, 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. 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.
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-inch wide baseboards.
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. 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.
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Easier Adjustments for Inside Corners
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. You then file or plane to the line.
Check the fit against the square-cut piece of the 7base before nailing either. The straight sections rarely fit perfectly.
Close a gap at the bottom by removing the square molding base and driving a drywall screw into the wall about 1/2 inch from the floor. Test the cope and adjust the screw in or out until the cope fits tight. 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.
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Overcut Outside Corners
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. 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 at 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.
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 inch long. 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.
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Mitered Returns
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.
Cut the angle for a small mitered return on your miter saw, but don’t completely cut it off from the trim stock. Rather, cut through the remaining sliver of wood with a utility knife. Glue the mitered return in place with a fast-acting cyanoacrylate glue formulated for wood (Krazy Glue is one brand).
