Mark, don’t measure
Whenever possible, hold your trim in place and mark where the miter cut should be. It’s quicker and more accurate than measuring, and you’ll avoid measuring mistakes. A sharp pencil works fine for marking window and door moldings where the reveal (a thin strip of exposed jamb; see photo) allows you a bit of leeway. But for super-accurate marks where there’s no margin for error, such as for baseboards and crown moldings that go around outside corners, use a sharp utility knife.
Test the fit, then fine-tune the miters
Don’t settle for an okay cut—tweak the angle and recut until the gap closes.
Adjust the angle of the cut slightly to remove a fraction of an inch from the part of the miter that’s touching. Adjustments of 1/2 degree or less are usually all that’s needed.
Start by cutting your moldings a little long so you’ll have material to trim off if the fit is bad. Then hold the miter joint together, making sure the trim pieces are held parallel to the door or window jambs, and check the fit. If you’re lucky, the miter will fit perfectly and you can trim a bit using the same miter saw setting. If the miters have gaps, like the ones in the photo, adjust the miter saw slightly and recut the miters. Then check the fit again. You may have to repeat this process several times.
Use a shim to cut a back bevel
Cut a back bevel on miter joints that are open in front but touching at the back. To create a back-beveled cut on a standard miter saw, place a pencil under the molding. If you have a compound miter box, tilt the blade a degree or two to cut the back bevel.
Cut a back bevel by shimming the trim slightly or (on compound miter saws) by tilting the blade slightly.
Smash protruding drywall
Occasionally window and door jambs end up slightly recessed, which causes trouble when it comes time to install trim. Correct minor level differences by either bashing in or cutting out the drywall along the edge of the jamb. But be careful to avoid going beyond what will be covered by the trim. If the level difference is greater than about 3/16-in., nail thin strips of wood, called jamb extensions, to the jamb to bring it flush to the wall surface.
Use a brad gun for the best results
It’s hard to beat a nail gun for perfect miters, especially if you’re not skilled with a hammer. Trim nail guns allow you to hold the moldings in perfect alignment while you pin them in place. If you can afford only one trim gun, buy one that shoots thin 18-gauge nails up to 2 in. long. Fifteen- and 16-gauge nailers are good where more strength is needed, such as for nailing jambs, but the thicker brads make larger, more conspicuous holes and can crack thin moldings. Use shorter brads to nail the molding to the jamb, and long brads along the outside edges.
Don’t nail too close to ends or edges
Avoid weak grain
Even with an 18-gauge trim nailer, you can split the molding if you’re not careful. Avoid nailing less than 3/4 in. from the end of a trim piece or less than 1/4 in. from the edge.
Pin the miter before nailing the outside
Lock floppy corners
In a perfect world, you could nail the trim flat to the wall and the miter would look great. But in reality, minor variations in level between the jamb and the wall often interfere. To solve this problem, start by pinning the inside edge of the trim, making sure the miter joint is pressed tight together. Then, while the miter is still tight, drive a pair of brads through the outside corners at opposite angles to pin it. To deal with gaps between the molding and the wall, see the next tip.
Shim behind the miter
Trick of the trade
If there’s a slight gap between the molding and the wall, don’t press the trim tight to the wall and nail it; the miter joint might open up. Instead, slip a thin shim between the molding and the wall. Then nail the outside edge of the trim. If the gap and shim are visible, fill the crack with caulk before painting.
Required Tools for this Project
Have the necessary tools for this DIY project lined up before you start—you’ll save time and frustration.
- Air compressor
- Air hose
- Brad nail gun
- Combination square
- Hearing protection
- Miter saw
- Safety glasses
- Tape measure
- Utility knife
Required Materials for this Project
Avoid last-minute shopping trips by having all your materials ready ahead of time. Here’s a list.
- Brad nails
- Wood shims