Easy Way To Trim a Door

This article represents my own opinion and may contain affiliate links. Please read my disclosures for more information.

Here's an easy way to cut and install trim around a door that doesn't need a lot of measuring and gets you tight, accurate mitered corners. This is a little different from how most people I've seen measure and install door casing.

I was finishing up a little work in my laundry room and I thought it would be helpful to show others how I install door casing around a door. The way I install door casing doesn't create a lot of mess, is easy to install on your own and gets you good miters.

Door casing is the trim that you install around the outside of a door. It finishes the installation and covers any gaps or rough edges from where the drywall meets the door jamb.

It's available in a variety of styles, widths and grades. You can get expensive stain-grade hardwood molding or affordable composite or MDF molding. I went somewhere in the middle with this 5 pack of 11/16" x 2-1/4" Primed, Finger-Jointed Pine Colonial Door Casing.5 sticks of trim is enough to trim both sides of most single doors and the style matched my existing trim.

Finger-jointed trim is made up of smaller pieces of wood that have fingers cut in the ends that provide a strong bond when they are glued together to make longer lengths. This makes it cheaper to produce compared to whole pieces of trim. Since I was going to be painting the trim I didn't need more expensive stain grade molding but I still wanted to use solid wood, not composite.

You'll need to learn how to attach door trim if you're installing a new door, repairing damaged trim or changing the look of the trim in your home.

What You'll Need

Materials

• 5 pieces of door casing
• Finish nails
• Paintable latex caulk (white)

Tools

• Miter box or power miter saw
• Hammer and nail set or power finish nailer
• (at least 3) 6" or greater bar clamps
• Caulking gun
• Tape measure
• Pencil

Step 1: Measure Opening Width

This is the only measurement I take of the door. Measure the width of the top of the opening where the top piece of door trim will be installed.

To this measurement add 3/8". This will leave a slight (3/16") reveal around the door frame. This is the reveal I like to use. If there is a requirement to have a different reveal make the necessary adjustments.

If you want to be more precise you can set a scribe (compass) to 3/16" and draw a line around the door jambs to help line up the pieces.

Step 2: Cut Top Door Casing

Unlike other methods I've seen, I start with the top piece of trim first. I find this makes it easier to get tight corners and a good fit.

The top section of door casing has a 45 degree cut on each end. The width of the bottom portion is the width we determined in Step 1. In my case I had a 28" opening so I needed to cut at 28-3/8". When cut it will look like this.

Start by trimming one end of the casing stock to give it the appropriate 45 degree angle.  You don't need to measure anything for this first cut, just cut near the end of a piece to reduce waste.

Next, measure from the bottom of the 45 degree cut and mark for the length of the top (28-3/8" for me.) This is where it gets a little tricky because you can't just hook the end of your tape measure to the bottom of the miter. You have to hold it in place or what I usually do is use a rigid measure (drywall square or whatever is handy) instead of measuring tape. You also have to be a little more careful when making the cut because the mark is going to be on the outside.

I own a power miter saw but I decided to use my manual miter box which I discussed in my article How To Make Square Cuts In Dimensional Lumber. It takes a little longer to make the cut but the cuts are still accurate and clean up is a lot quicker because I don't have sawdust blowing all over the room. It's faster to make the cuts closer to the work area and all I needed was a small sheet of plastic to catch all the sawdust that falls straight down from the saw. No need to vacuum!

The door casing is positioned with the back down and the thick side of the trim against the fence. I used some small clamps to hold the casing tight in the miter box. One hand operated the saw, the other held the miter box in place. If you have a work table nearby you can clamp the miter box down. I'm using some of the Irwin Mini and Micro clamps that came in a set (Irwin Quick-Grip Clamp Set from Home Depot) which is an affordable way to get enough clamps for this project.

Repeat the above steps for the other side of the door. Use the first top piece as a template for the second top peice after you're sure it's the right size. Save the second top trim for later.

Step 2: Dry Fit The Top Trim

With the top piece of trim cut to size, test to see if it fits. Leave a 3/16" gap from the bottom of the door frame to create a reveal and center it over the opening. You don't need to be exact with your measurements. Just eyeball it and temporarily clamp it in place using at least a 6" clamp.

As you're dry fitting the trim, you might come across some high spots in the drywall edges or compound. Take the time to trim or sand them down to get a good fit with the trim.

Step 3: Side Trim

For the side trim we don't need to take any measurements. Take a long piece of trim and flip it over so the back faces out and the thick side presses up against the top trim with the bottom resting on the floor. Hold it square with the top piece. Using a pencil, mark where it meets the top of the top trim piece.

Transfer your mark to the front and mark the direction of the miter too so you don't make a mistake. Then make the cut.

Test fit the side trim. Now you can start making more precise adjustments to the top and side so the miter is tight and everything is lining up with a good reveal. Once you get it positioned, clamp the side trim in place.

Repeat for the other side.

Step 4: Adjust and Nail Door Casing

With all 3 pieces of door casing cut to size, make final adjustments to get the corners tight and the right reveal around the door frame. Clamp everything down tight and begin nailing.

I used a pneumatic finish nailer which made things go quickly but since everything is clamped in place it's not too hard to use a hammer and finish nails. Then follow up with a nail set to recess the head of the nail below the surface of the trim.

Use 18 gauge (or 5d if hammering) finish nails along the inside to secure the trim to the door jamb. Use 16 gauge (8d) nails to secure the thicker, outer edge of the trim to the wall framing.

Step 5: Other Side

Repeat the steps above to trim out the other side of the wall.

Step 6: Caulk and Paint

When you're done with all the trim you need to install,  use a paintable latex caulk along all the edges where the trim meets the wall or door jamb. Fill in all the nail holes and any gaps you might have in the miters if you didn't get a good fit. When the caulk dries you're ready to paint. I like using Benjamin Moore Waterborne Satin Impervo mixed with a bit of Floetrol paint conditioner for trim.