How to Cut Crown Moulding

The following information is from “Pro Tips For Installing Crown Molding” by Popular Mechanics. Click here to read the full article.

(1) Trace the crown’s rear to plan a backer board — installed between wall and ceiling, this board acts as a mount for the crown.
SITE PREP
Crown molding is usually nailed to the wall studs along the bottom edge and into the ceiling joists above—a lot of stud-finder work. I skip all that by installing a plywood backer board to the top plate (the horizontal framing member above the wall studs). That method allowed me to nail this project’s 4½-inch crown to any point along every wall. To determine the width of the backer board, I held a piece of crown molding against the inside corner of a framing square and drew a line along the crown’s back (1) The diagonal line, minus 1/8-inch for clearance, is the width of the backer board. I used a table saw with the blade set to 45 degrees to rip the backer boards from 3/4-inch plywood, then fastened each board to the top plate with 3-inch drywall screws spaced 16 to 24 inches apart.
SAW SHORTCUT
The power miter saw and stand provide a safe, quick and accurate way to execute precise cuts. Choose a 10- or12-inch saw with a dust-collection bag or exhaust port for attaching a wet/dry vacuum. Select a stand with an integral power strip and extendable arms to support lengths of molding. The Craftsman rig that I used included a 12-inch dual-bevel compound miter saw ($350) and a pro-duty saw stand ($250). There are two ways to cut crown:Either lay it flat beneath the blade, or set it against the saw at the angle,the way it will be installed between the wall and ceiling. I prefer the latter.The flat method requires adjusting the saw blade to make both a bevel and miter cut; my way, the saw table acts as the ceiling, the fence is the wall, and an upside-down piece of crown can be cut at a compound angle with a simple 45-degree vertical chop.To hold the crown in place, it helps to clamp a cleat to the saw table. To set this up, I first clamped a length of molding with its edges flush against the saw’s vertical fence and horizontal table. I pressed a 30-inch, straight-edged board tight against the crown and clamped it to the table (2).This piece, the cleat, stayed in place between cuts, so I could easily drop each piece of crown into position. I cut away the center of the cleat by making a left-hand and a right-hand 45-degree miter cut, opening a channel for the blade to pass through (3).
(2) Clamp a board to the miter saw to create a simple, consistent way to hold crown in place as each piece is cut.
(3) The cleat, with its middle cut away, holds inverted crown in the saw at an angle. Placing the molding in the saw this way allows simple vertical chops to create the compound angles necessary for the installation.
(4) Connect straight runs with scarf joints, which mateside-by-side pieces in a discreet seam. Fasten it by firing 2-inch finish nailsor hand-nail 2-inch 6d nails.
STRAIGHT RUNS
Most crown comes in 16-foot lengths, so unless you’re trimming out a gymnasium, a single piece can usually span each wall. When it can’t, two lengths join end to end in a scarf joint (4). This combines opposing compound-angle miters in a clean, nearly undetectable seam. Molding can shrink or shift out of position slightly — with a scarf joint, as opposed to a square-edged butt joint, a gap won’t appear at the seam. To form a scarf joint,I made a compound-angle miter cut on the end of one length of crown. I nailed the crown to the backer board and then made an opposing compound-angle miter cut onto the end of the mating length of crown. After applying a little wood glue to the joint, I slid the second piece of crown into position and nailed it to the backer board.
INSIDE CORNERS
A coped joint connects two pieces of crown molding at an inside room corner. I prefer this type of joint over a miter joint because wall corners are rarely perfectly 90 degrees. A coped joint, in which a piece of crown is tailored to fit an adjacent profile’s curves, makes a tight-fitting seam even if the inside corner is out of square — as many are.I cut the first length of crown with a square end, pushed it tight into the corner and nailed it in place.Then, I cut a compound-angle miter into the end of the mating length of crown (1). Next, I used a coping saw to back-cut the molding along its contoured profile (2). The idea is to saw away enough wood to allow the coped cut to fit tight against the profile of the first piece of crown.After cutting, the coped piece needed a little fine-tuning before it fit snugly. I smoothed the coped joint with rat-tail, half-round and flat files, as well as a wood dowel wrapped in80-grit sandpaper (3).
(1) Cut a square-ended section on the miter saw and slide it into the corner. Cut the second piece at a 45-degree angle.
(2) Use a coping saw to cut away the back of the second piece of molding, so that it can conform to the profile of the square-ended piece.
(3) Fine-tune the coped molding by sanding it with a dowel rod wrapped in 80-grit paper.
OUTSIDE CORNERS
Outside wall corners are seldom perfectly square, so simply cutting both crown pieces to 45 degrees usually won’t cause them to meet snugly at the corner. A technique that I’ve used fory ears measures perfect outside miter joints, regardless of the wall angle. I hold two overlapping 20-inch-long 1 x 4s against the ceiling at the corner. I trace both edges of the bottom board onto the top board (4), then draw a diagonal line to connect the two marks.Then I stack the 1 x 4s on the miter-saw table, adjust the saw blade angle to match the diagonal line and cut the 1 x 4s (5). I test-fit the 1 x 4s by holding them against the outside corner and checking the seam. If their edges don’t make tight, even contact, I adjust the saw for a second cut. When the 1 x 4s fit on the wall, I lock the saw-blade angle and make a cut into one of the lengths of crown. ThenI adjust the saw to make an opposing miter cut–at the same angle–into the end of the mating piece of crown. With the first piece of the crown’s end sitting flush with the wall corner, I drill a pilot hole and hand-nail it to the backer board with 1½-inch 4d finish nails (fired nails can deflect and break through the face of the molding).
(4) To make accurate cuts on out-of-square walls, start by tracing the edges of two 1 x 4s. Connect the lines.
(5) Cut through both pieces along the connecting line. Once you get the boards to fit flush against each other on the corner, the saw is set to the correct angle.
(6) Two mirrored miter cuts will yield a perfect outside corner — the pieces will meet neatly.