Baseboards look simple. They’re just flat boards nailed around the bottom of a wall, right? Then you cut your first inside corner, hold the two pieces up, and stare at a gap wide enough to slide a coin into. The problem is almost never your saw. It’s the method — and the fact that walls are almost never square, straight, or flat.
Cutting baseboards well comes down to three joints: the coped inside corner, the mitered outside corner, and the scarf joint for long runs. Get those three right and your trim work jumps from “good enough from across the room” to genuinely professional. Here’s how each one works, and the mistakes that cause the gaps.
Start With the Right Setup
Before you cut anything, check your walls. Hold a long level or straightedge against the wall where the baseboard will sit. You’ll often find bows and dips of a few millimetres, especially in older homes. Baseboard is flexible enough to follow gentle curves, but it won’t bend around a sharp high spot. Mark high spots with a pencil and either knock them down with a block plane or plan to fill small gaps with caulk later — caulk hides a lot, but it can’t fix a joint that doesn’t meet.
Measure each wall run twice, and always measure from the actual corner, not from a previous cut. Walls that look square are routinely out by a degree or two, and those errors compound if you carry measurements from one piece to the next.
For tools, a miter saw is the workhorse — a sliding compound miter saw handles wide baseboards best. You’ll also want a coping saw (or a jigsaw with a fine blade) for coped joints, a tape measure, a pencil, a utility knife, and a small block plane or sanding block for fine-tuning.
Inside Corners: Cope, Don’t Miter
The classic beginner mistake is mitering inside corners at 45 degrees. It looks right on paper, but when the wall angle is 88 degrees instead of 90 — which it usually is — you get a gap at the front or the back of the joint, and the gap opens wider as the wood moves with the seasons.
A coped joint avoids all of that. One board butts square into the corner. The second board is cut so its profile matches the face of the first board, overlapping it like a puzzle piece. Because the two boards overlap rather than meeting edge to edge, small wall irregularities disappear, and the joint stays tight even as the wood shrinks and swells.
Here’s the process. Cut the first board square and fit it tight into the corner. Then take the second board and cut a 45-degree miter on its end — the same miter you’d cut for a mitered corner. This miter isn’t the joint; it’s a guide. It reveals the profile of the baseboard on the cut face, giving you a line to follow.
Now cut along that profile line with a coping saw, holding the saw at a slight back angle — tilt it so the back of the cut is slightly undercut. That undercut means only the very front edge of the profile touches the other board, which makes the joint close cleanly even if your cut is slightly imperfect. Take your time on curves and details, and test-fit often. A few passes with a sanding block or a utility knife will fine-tune the fit until the joint closes with light pressure.
One practical tip: cope the piece that runs along the wall you see least often first, or cope the longer run into the shorter one — whichever makes the visible side look cleanest. And always cut coped joints a hair long, then shave to fit. A board that’s a millimetre too long can be trimmed; one that’s too short is firewood.
Outside Corners: Miter and Adjust
Outside corners are the opposite — you must miter them, because both faces are visible and there’s no board to cope into. The challenge is that outside corners are rarely exactly 90 degrees either. If you blindly cut two 45-degree miters, the joint may close at the front but gap at the wall, or vice versa.
The fix is simple: measure the actual corner angle. A bevel gauge or a digital angle finder tells you the truth in seconds. Divide that angle in half and set your saw to that number. If the corner is 92 degrees, you cut each side at 46. If it’s 88, you cut at 44. This one habit eliminates most outside-corner gaps.
Even with the right angle, dry-fit before you glue or nail. Hold both pieces in place and check the joint from eye level and from below. If there’s a small gap at the front, shave a whisker off the back of one miter with a block plane — this “back-bevel” pulls the front edges together. If the gap is at the wall side, do the reverse.
Glue the miter before nailing. A dab of wood glue on both miter faces, held tight for a minute, locks the joint so it can’t open later. Then nail each side into the studs, not just into the drywall, and keep nails at least a few centimetres from the miter tip so you don’t split it. For painted baseboards, a tiny bead of paintable caulk over the miter line finishes it invisibly.
Long Runs: The Scarf Joint
Walls longer than your baseboard stock need a joint in the middle of the run, and a plain butt joint is the weakest option — it shows a visible seam and tends to open as the boards move. A scarf joint is far better. You cut both board ends at a shallow angle, usually 30 to 45 degrees, so they overlap along a long diagonal face. The overlap hides the seam, and the long glue surface makes it strong.
Cut both pieces with the same saw setting so the angles match. When you install them, the overlap should face away from the main viewpoint of the room — if you walk in through a door, the seam should point away from you. Glue the joint, then nail through the overlap into a stud if one is nearby. If there’s no stud, a pair of nails angled in opposite directions (toe-nailed) will hold it, and construction adhesive behind the joint adds insurance.
Plan scarf joints so they land on a stud when possible, and stagger them if you’re running baseboard around a room — two scarf joints right next to each other on adjacent walls draw the eye.
Common Mistakes That Cause Gaps
Cutting all pieces first, then installing. Walls vary, and a board measured from the plan rather than the wall will be off. Cut and fit one wall at a time, working around the room in order.
Ignoring the floor. Baseboards sit on the floor, and floors slope. If the floor drops, the top edge of the baseboard won’t line up with your pencil line. Scribe the bottom edge to the floor on uneven floors, or plan to cover the gap with quarter-round shoe molding — which is what it’s for.
Nailing before checking the joint. Once a board is nailed, adjusting a joint means prying and patching nail holes. Always dry-fit the full corner or joint, check it from several angles, and only then glue and nail.
Forgetting to prime cut ends. On painted baseboard, raw end grain soaks up paint unevenly and can show through as a dark line at the joint. A quick brush of primer on mitered and coped ends before installation keeps the finish even.
Finishing Touches
Once everything is nailed, fill nail holes with lightweight filler, let it dry, and sand flush. Run a thin bead of paintable caulk along the top edge where the baseboard meets the wall — this hides the inevitable small gaps from wavy drywall and gives a crisp painted line. Caulk the bottom edge too if there’s no shoe molding and the floor is uneven.
Paint or finish after installation for the cleanest result, and always do two coats on the filled areas. What separates decent trim work from great trim work is rarely the saw — it’s the patience to dry-fit, adjust, and finish properly.
Baseboards are one of the most visible details in any room, and they’re also one of the most forgiving projects to learn on. Master the coped inside corner, the measured outside miter, and the scarf joint, and every room you trim from here on will look like it was done by someone who charges for it.
