Baseboard corners are where amateur trim work shows. Mitred corners with gaps, coped joints that don't fit, scuffs at the floor — every corner shows. The two methods (mitre and cope) each have their place: mitre for outside corners, cope for inside corners. Done well, corners look seamless and stay tight even as wood expands.
This guide covers both inside and outside corner techniques, plus the gap-management tricks pros use.
Before you start
Three things to settle:
1. Inside corners: cope; outside corners: mitre. Coped joint at inside corners: one piece runs straight to the wall; second piece is profile-cut to fit against the first. Closes tight even as wood shrinks. Mitred at inside corners: 45° cuts meet — looks fine but opens up over time. Outside corners: 45° mitre is standard.
2. Verify wall is square first. Walls aren't perfectly 90 degrees — they're 88-92 degrees often. A perfect 45° mitre on an 88° wall leaves a 4° gap. Test cut on scrap; adjust mitre saw 1-2 degrees to compensate.
3. Prefinish before cutting. Prime and paint baseboard before installing. Touch-ups are 5 minutes vs hours of in-place painting around already-installed trim.
What you'll need
Tools
- Mitre saw
- Coping saw
- Tape measure
- Speed square
- Brad nailer (18-gauge)
- Caulking gun
- Stud finder
Materials
- Baseboard (matching room style)
- 18-gauge brad nails (1.5-2 inch)
- Wood glue
- Wood filler
- Caulk (paintable latex)
- Sandpaper (220 grit)
Coped joint flexes; mitred joint opens
Wood expands and contracts seasonally. A coped joint stays tight because one piece runs full to the wall — gaps don't open up.
A mitred inside corner can open up to 1/16+ inch over years.
Cope inside corners; mitre outside corners only.
Step-by-step instructions
Test wall corner angles
Use a speed square or angle finder to measure each corner.
True 90°: mitre at 45° each side.
Out of square: adjust mitre angle accordingly.
Note each corner's angle; don't assume identical.
Cut first piece (full-length to wall)
First piece in any corner: cut square to wall length (or with a slight bevel for the back to fit tight).
This piece runs the full distance to the corner.
Position against wall, mark length where corner meets.
Inside corner: cope second piece
Second piece needs profile-matched cut.
Step 1: cut piece at 45° (mitred face cut).
Step 2: with coping saw, cut along the profile (the mitred face's profile shape).
Result: piece's end matches the contour of the first piece's profile.
When pressed against the first piece, the cope tucks tight against the profile.
Test fit the cope
Hold the coped piece against the first piece.
Should sit flush with no gap.
Adjust with file or sandpaper if needed.
When tight: install.
Outside corner: mitre both pieces
Both pieces cut at 45° (or angle compensated for non-square wall).
Mitres meet at the outside corner.
Apply wood glue to mitred surfaces before joining.
Glue holds joint tight as wood expands.
Pre-drill if hardwood (prevent splitting)
Hardwood baseboard: pre-drill brad nail locations near ends.
1/16 inch bit for 18-gauge brads.
Prevents splitting at vulnerable corner.
Locate studs and mark
Studs every 16 inches OC.
Stud finder + pencil mark on baseboard before installing.
Brad nails go into studs (or at least into bottom plate).
Nail baseboard to wall
Brad nailer with 18-gauge 1.5-2 inch brad nails.
Two nails per stud minimum (top and bottom of baseboard).
Don't drive nails near corners — split risk.
Nail near corners after pre-drilling.
Fill nail holes and small gaps
Wood filler in nail holes.
Sand smooth.
Caulk any small corner gaps with paintable latex caulk.
Touch up paint
Paint over filled nail holes.
Caulk over any visible joints.
Light sanding if needed.
Two thin coats better than one thick.
Common mistakes that cost you
- Mitred inside corners. Open up over time as wood shrinks. Cope inside corners.
- Skipping wall angle check. Mitre on out-of-square wall = visible gap. Test angle first.
- Painting in-place. Drips on floor, finish picks up dust. Pre-finish baseboard.
- Splitting at corners. Brad nails split hardwood. Pre-drill near corners.
- No glue on outside mitres. Joint opens. Always glue mitred surfaces.
Frequently asked questions
Can't I just mitre inside corners?
Yes, but they open up as wood shrinks.
Coping is the pro standard; mitred inside is acceptable for budget work.
Caulk closes small gaps if mitres open up.
Hand-cope or power-cope?
Hand cope (coping saw): traditional, reliable, inexpensive tool.
Power cope (Crown Stool & Cope, jigsaw with profile blade): faster for production runs.
DIY: hand cope works fine.
Can I run baseboard before flooring?
Best practice: flooring first, then baseboard on top.
Baseboard hides the expansion gap of flooring.
If baseboard already in place: 1/4 round shoe moulding covers expansion gap.
When to call us instead
Coped corners and crisp baseboard work are part of every renovation we run. Our finish carpenters know the techniques.
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