The days of the back saw and miter box, I think, is a thing of the past. Today, we use compound power miter saws, also commonly called cutoff saws. Our discussion on how to cut crown molding inside and outside corners will be based on using these modern tools.
Think of the table on your cut-off saw as the floor or the ceiling. Baseboard is cut upright and crown molding is cut upside down. I usually have four sample pieces with me when I cut them, one for the inside right miter, one for the inside left, one for the outside right and one for the outside left. These are marked on them. It saves me cutting up expensive crown molding to get my head right every time. I'm referring, of course, to maximum 3 1/2" crown molding that will stand up vertically against the saw fence. Any larger crown molding would have to be cut on the flat, a very different procedure covered in my article How to Cut Wide Crown and Cove Moldings.
The first thing I do when cutting up to 3 1/2" crown molding is to install a plywood fence on my saw. Rip a piece of 1/2" plywood the length of the fence and 3 1/2" wide. Most brands of saws have little holes pre-drilled in the fence for this purpose. Attach the plywood fence using the right length of wood screw. Cut a 45 degree miter each way to cut out the fence. Put the saw at 90 again and place the crown molding in position, upside down. The fence is the wall and the saw table is the ceiling. Lean a 12" piece of crown molding up against the fence and table forming the same angle you wish to install the crown molding on. It will show its position with the 90 degree edges formed into the profile of the crown molding. Hold the crown molding tight to the fence and table while cutting a 45 on each end. I'm referring here to the miter angles of the saw, not the compound angles. That is, the blade is still 90 degrees with the table. The 45's cut on each end will match the inside corner miters. (I'm assuming you know what an inside miter cut is compared to an outside miter cut. See the following pictures.)
Cutting an inside Miter
Cutting an Outside Miter
Swing the saw back to a 90 and cut the piece of crown molding in half. The piece of crown molding on your left is the left inside and the piece of crown molding on your right is the right inside. Mark them accordingly. Take them to the ceiling and place them in position in their respective corners. Check to see that the correct miter is marked on the correct piece of crown molding. That is, when facing the inside corner the piece of crown molding on your left is actually the right hand miter of the wall on your left. The piece of crown molding on your right is actually the left hand miter of the wall on your right. Get this right in the old noggin or we are talking about different things. The idea is when you are installing a long length of crown molding, your sample piece of crown molding marked inside right will match the right hand end of the crown molding not the right side of the corner. Don't go any further with this article until this is clear in your mind.
Let's cut the sample pieces of crown molding for the outside miters. Take a 12" piece of molding and place it in position on the base and against the fence with the top of the crown molding resting on the table, just like before. Now swing the 45 on the saw for cutting an outside miter, this is opposite of the inside miter cut done previously. Cut each end of the crown molding with a left and right outside miter, then cut them in half as before. The piece of crown molding on your left is the left outside miter and the piece of crown molding on your right is the right outside miter. Mark them accordingly and check them at an outside corner against the ceiling. Fits good, right?
Now that we have our 4 pieces of crown molding, we have to relate these to the crown molding we are going to measure and install. On an inside miter, measure between the inside of each wall. Notice on the inside samples there is a 90 formed in the cut on the backside of the piece of crown molding on the bottom.
Here's the back side of a crown molding to show the 90 degree edge for measuring. Notice the small hole in the ends of my samples to string on a wire and hang in the shop for the next crown molding job.
Since it is on the bottom of the crown molding it will be against the fence on the top when we cut it. The crown molding is cut upside down with the top of the crown molding against the table of the saw. Cut the first 45 on one end of the crown molding and then measure from this point to the other end of the crown molding and mark your measurement on the bottom of the crown molding so when it is arranged in the saw you can see the mark at the top. If your tape measure doesn't hold onto the end of the miter you are not on the bottom edge of the crown molding. Swing the saw arm to the opposite 45 of your previous cut. The piece of crown molding should fit snug, but not too tight to cut into the drywall, or the miters won't fit nice. Adjust the length a bit, if too long.
When cutting crown molding for an outside miter, the place to measure is the 90 degree angle on the bottom of crown molding, like the inside miter, but you will notice that the tape won't hold here, you have to get a helper or hold the tape in position as you move down the crown molding with your tape and mark the opposite end. This measurement is the wall from the outside to the other end.
You'll notice I don't cope the inside corner joints for crown molding. I've done both, but I find that coping takes longer and doesn't make as nice a joint as cutting a miter.
Usually ceiling crown molding is painted, so I finish up with a paintable latex caulking with silicon added for adhesion and run a bead along the top to fill any voids in the crown molding especially in a rough ceiling and a bead in the corners. Have a damp cloth handy to remove any smudges right away, before the caulking dries. Don't fill the nail holes with caulking though, use a paste filler. Caulking can't be sanded very well, but has excellent properties for expansion and contraction, staying in the joint and not cracking. I usually glue the outside miters of crown molding to keep them tight when assembling them.
Good luck with your crown molding installation.
Dave(Ask Dave) (About Dave)
Hi, I'm Dave Osborne. With over 50 years experience as a journeyman carpenter, foreman and contractor in heavy construction I enjoyed working with apprentices and sharing the tricks of the trade that others shared with me. Now I get emails from Members all over the world and we include many of my answers in our Free Monthly Newsletters. Some of my answers include drawings and instructions specific to a project, but may also answer your questions. I use correct construction terminology, so you can confidently inform your building supply dealers or contractors exactly what you need.
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