Cut the length of the header (short point to short point of miters) to be ½” longer than the inside measurement of the door jamb. That way we will end up with 1/4″ reveal between the inside edge of the jamb and the casing. Cut both miters to 45°. Apply the header by first driving one nail into the jamb at the center. With it tacked in the center, line up the reveal along the top so it’s even and then use more nails to secure it in place.
How to Get Picture Perfect Miters When Installing Door Casing
If your house is cursed with small builder grade door casing, upgrading to something bigger and more interesting is an easy DIY project anyone can tackle. Here’s the process I use to get perfect mitered corners, even if your walls are less than perfect.
The best and most accurate way to figure out the length of the casing legs is to mark them in place. Set the bottom on the finished floor and with the leg overlapping the header, use a utility knife to make a mark on the inside edge of the casing leg.
This is the mark made by the knife that shows us the exact length of the short point of the miter. You can darken it in with a pencil to make it easier to see. Cut the leg a freckle long (leave the mark) with the miter saw set to 45°.
I cut the leg a little long so I can test the fit. If the miter looks perfect, just cut it again with the same 45° setting on the saw right at the mark and you are good to go. If, however it’s not perfect, like in this example, we can adjust the cut to make it perfect. You can see that the 45° angles line up, but there is a small gap on the front because the pieces are touching first along the back. We need to make a compound cut on the leg so it will have a slight bevel on it while keeping the miter angle at 45°.
Making the compound back bevel cut sounds a lot more complicated than it really is. Since we want to cut a little more off the back than the front, we can simply shim the casing up closer to the saw. Here I’m using a carpenter’s pencil to change the bevel angle on the casing. Because of the nature of compound angles, we’ll need to adjust the miter angle slightly to end up with the true 45° angle that we verified works well with the header. To set the angle, keep the casing angled and line up the saw blade with the miter cut. For a small amount of bevel like this, it ends up being about 44 1/2°.
Now that the miter is perfect, apply some yellow glue to the joint because that will keep the miter from ever opening up down the road.
I like to pin the corner together to make sure it stays lined up until the glue dries.
To read a more detailed tutorial, head over to my blog https://plumbandlined.com/how-to-install-door-casing/
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