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Baseboard and Crown Moulding: Measuring, Mitering and Nailing

Trim looks wrong when the corners are not 90 degrees. Check each corner first, adjust the miter angle to match, nail into wood and fill what is left.

Published 4 min read

The corner has a gap you could slide a coin into, and it is the third try. Trim corners open up because the walls are not truly 90 degrees, so a 45 degree cut is wrong before you start. Check the corner, then cut to what it actually is.

Check this first: how square is the corner?

  • Test pieces. The Vancouver Carpenter cuts a couple of spare pieces at 45 degrees and holds them in the corner to see how far out of square it is.
  • Angle finder. For crown, the Lowe’s video measured the outside corner at 92 degrees with an angle finder, split it in half, and cut at 46.
  • Why 44 and 46. Both baseboard videos cut inside mitres at 44 degrees and outside mitres at 46, because joint compound builds up on the corners and they aren’t truly square. The inside corner has a build-up of tape and mud, and the outside corner is built out.

Safety

  • Brad nails. Don’t hold your hand close to the nail. A brad that hits another nail can spike out. One video keeps hands at least 2 in. from brad nails on outside corners.
  • Know what is behind the wall. Pipes can sit very close to the drywall. Where water lines may run behind the wall, one video uses dabs of construction adhesive and threaded nails at an angle, so a 2 in. nail cannot go far enough to puncture a pipe. Avoid nailing through bi-fold doors.
  • Saws. Safety glasses and hearing protection at the miter saw.

Tools and materials

  • Miter saw (a compound miter saw is the best tool for crown, says Lowe’s), angle finder, tape measure, stud finder, nail gun (an 18 gauge brad nailer with 2 in. nails), hammer
  • Baseboard (primed pine 1×4 or 5 1/2 in. by 3/4 in. MDF in the videos), wood glue, caulk, filler, nail set

Baseboard: step by step

  1. Find the studs. Nail into the bottom plate first for a good grab. Studs are usually near the door jamb and in the top corner. Screw pops in the drywall are a telltale sign. From a known stud, lay out 16 in. centres. A stud sensor also works. See How to Find a Stud in a Wall.
  2. Measure and write a cut list. Measuring and cutting in bulk saves switching the saw angle. Check tape and mud build-up in a coved bottom corner, which can sit about an inch above the floor.
  3. Cut. Butt joints on inside corners, with no glue needed, if the stock is flat with no profile. Mitre outside corners. Mitring inside corners is easier to measure and mark, says the first video. A flat bar can pull a board out slightly if a gap shows.
  4. Glue and nail. Glue goes on the mitres. Leave the corners un-nailed at first so they can still be adjusted; nails can pull a corner in and open it. Nail into studs or the bottom plate.
  5. Where there is nothing to nail to, one video uses caulk as an adhesive on steel stud because it wipes clean and does not squash out like construction adhesive.
  6. Fix a gap. A corner that opens can be glued and have sawdust worked in, then nailed, and two coats of paint hide it. Overhanging mitres can be sanded while the glue is wet, so the dust turns into filler.
  7. Seal and paint. Caulk the top edge and gaps. On MDF, caulk on the unfinished edge seals it, so two coats of finish are enough without priming (his claim).

Crown moulding: step by step

  1. Measure from the bottom, because that part touches the corner.
  2. Cut upside down against the saw fence. The fence acts like the wall and the saw base like the ceiling, so the bottom goes on the wall and the top on the ceiling.
  3. Which way? The bottom of the moulding is shorter than the top on outside corners and longer than the top on inside corners. This holds even when cutting upside down.
  4. Make a stop. Tape a couple of inches from the fence on the saw, set the piece, and draw a line where it sits so every piece matches.
  5. Overlapping cuts join two or more boards when a wall is longer than one board, so the joint looks seamless.
  6. Secure with a nail gun.

Mistakes to avoid

  • Cutting every corner at exactly 45 degrees.
  • Nailing the corners before they are fitted.
  • Nailing without knowing where studs and pipes are.
  • Moving trim around a finished room, which dings walls (as one carpenter notes).

If that didn’t fix it

If a mitre still opens, the corner is probably badly out of square. Cut the pieces square and butt them, or trim the back of one piece and test again. If the walls are wavy, a coat of caulk or filler and two coats of paint hide a lot.

See also

Sources

  1. Shown on this site, source of the corner check, mitre angles and glue advice. Mitered Baseboard Install: Cutting 44 and 46 Degree Corners in a Rental (video, Vancouver Carpenter) Source
  2. Shown on this site, source of the stud layout, butt joints and nail safety. Simple Baseboard Installation: Butt Joints, Miters and Stud Layout (video, Vancouver Carpenter) Source
  3. Shown on this site, source of the crown cutting method. Crown Moulding: Inside, Outside and Overlapping Miter Cuts (video, Lowe’s Home Improvement) Source

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