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newSchoolWorks.com › VoTechWorks › Building & Construction › Carpentry › Rough & Finish Carpentry › Lesson 9
BLD-CARP03-02 · Lesson 9 · 15 clock hours · lab

Interior Finish: Baseboard and Door Casing

Objective. Students will measure, cut, and install baseboard and pre-made door casing with miter and coped joints that close to within 1/32 inch, fastened and filled to a paintable standard.

Interior Finish: Baseboard and Door Casing

Interior finish trim—baseboard and door casing—is among the last work a carpenter does before a room is painted. Because the work is visible at eye level, precision and clean joinery matter far more than they do in rough framing. A gap you can slip a business card into is a failing joint; a gap wider than 1/32 inch will show under paint and must be redone.

Why Trim Is Installed

Baseboard covers the gap between the finished floor and the drywall, hides fasteners, and protects the wall base from foot traffic and vacuums. Door casing covers the gap between the door jamb and the surrounding drywall (the "reveal") and ties the door unit visually to the wall. Neither piece carries structural load; their job is appearance and protection.

Reading the Room Before You Cut

Always measure the room before touching the saw. Walk the perimeter and note every inside corner, outside corner, and door opening. Record lengths on a cut list, not just in your head. Inside corners (two walls meeting inward) are handled with a coped joint on the second piece installed. Outside corners (wall corners that project into the room) use miter joints. Door casing uses miter joints at the top corners and straight cuts at the floor.

When building your cut list, add a few inches to each piece for fitting, and select stock lengths that minimize offcuts. For example, if two pieces measure 42 inches and 38 inches, a single 8-foot length (96 inches) covers both with material left for fitting.

The Coped Joint

A coped joint is the correct method for inside corners on baseboard. Here is why: walls are rarely perfectly square, so a miter cut that fits a true 90-degree corner will gap in a slightly out-of-square room. A cope, by contrast, lays one molding flat against the wall and cuts the second piece to overlap its profile exactly, so small angle variations disappear.

To cope a piece:

1. Cut the end of the second piece at 45 degrees with the miter saw, as if you were making an inside miter. This exposes the profile as a sharp line on the face.

2. Follow that profile line with a coping saw or jigsaw, cutting straight through (perpendicular to the face), removing the waste behind the profile.

3. Back-cut slightly—angle the saw a degree or two toward the back of the piece—so only the face edge contacts the adjoining molding. This makes it easier to close the joint tight.

4. Test-fit against a square-cut reference piece. The face of the coped piece should nestle flush against the face of the reference piece with no visible gap.

Repeat test cuts until the joint closes. Sand or use a rasp to fine-tune the cope before installing.

Miter Joints for Outside Corners and Door Casing

Outside corners on baseboard are mitered at 45 degrees each (on a square corner). Always test the actual corner angle with a digital or bevel gauge before cutting—true 90 degrees is rare. If the corner reads 92 degrees, each piece gets cut at 46 degrees.

Door casing is traditionally installed with a small reveal on the jamb edge—typically 3/16 inch set back from the face of the jamb. Use a combination square set to 3/16 inch to mark reveal lines on all three jamb faces before cutting any casing. The two side pieces (legs) are plumb-cut at the top to receive the head casing. The head casing is mitered at 45 degrees on both ends to meet the leg miters. Nail legs to the jamb and the rough framing; nail the head casing the same way.

Fastening and Finishing

Use a pneumatic finish nailer loaded with 2-inch finish nails. Drive nails into the wall plate or stud behind the drywall and into the door jamb on casing—not into drywall alone, which will not hold. Set every nail slightly below the surface with the nailer's depth adjustment or with a nail set. Fill every hole with wood filler, let it dry fully, and sand flush with 120-grit sandpaper. Remove all mill marks (machine-planed ridges) before inspection. The finished surface must accept paint without visible voids or proud spots.

Safety

Eye and hearing protection are required any time the miter saw, jigsaw, coping saw, or finish nailer is in use. Molding offcuts can fly from a miter saw at speed—never reach past the blade to retrieve a piece while the blade is spinning. Wait for the blade to stop completely before moving your hands near the cut zone.

With the finish nailer: treat it like a firearm. Keep your finger off the trigger until the nose is pressed against the workpiece. Never point it at another person. Disconnect the air hose before clearing a jam. Hearing protection is mandatory when the compressor is running nearby.

Keep the floor clear of offcuts—sawdust and scrap wood underfoot are a trip hazard in a workspace where you are carrying long molding lengths.

Quality Standard

NCCER performance profiles require that all miter and cope joints close to within 1/32 inch, nails are set and filled, and no mill marks are visible. Inspect every joint before moving on; it is faster to recut one piece now than to re-do finished trim after paint.

Key terms9

BaseboardA horizontal trim molding installed at the base of a wall to cover the joint between the finished floor and the drywall and to protect the wall surface.
Door casingThe decorative trim molding applied around a door opening on both faces of the wall to cover the gap between the door jamb and the surrounding drywall.
Coped jointAn inside-corner joint in which the end of the second piece of molding is cut to follow the profile of the first piece, allowing the face of the second to overlap the face of the first and close tight regardless of small corner-angle variations.
Miter jointA joint formed by cutting each of two mating pieces at an equal angle (typically 45 degrees on a square corner) so the cut faces meet and the outside face shows no end grain.
RevealA deliberate setback—commonly 3/16 inch—between the face of the door jamb and the inside edge of the door casing, creating a small visual step that gives the trim a finished appearance.
Back-cutA slight angle cut toward the back face of a coped piece so that only the profile edge contacts the adjoining molding, making the joint easier to close tightly.
Nail setA hardened steel punch used to drive a finish nail slightly below the wood surface so the hole can be filled; also refers to the depth-adjustment feature on a pneumatic finish nailer that sets nails automatically.
Mill markA shallow ridge or wave on a molding surface left by the moulder blades during manufacture; mill marks must be sanded smooth before painting.
Cut listA written record of every piece of trim needed for a job, including length, profile, and joint type at each end, used to select stock and minimize waste before cutting begins.

Worked examples4

1. A wall runs 12 feet 5 inches from an inside corner to a door opening. The door opening is 2 feet 10 inches wide. How long is the baseboard piece that fills the wall from the inside corner to the start of the door opening, leaving nothing extra (exact measured length)?
  1. Convert the wall length to inches: 12 ft 5 in = (12 × 12) + 5 = 144 + 5 = 149 inches.
  2. Convert the door opening width to inches: 2 ft 10 in = (2 × 12) + 10 = 24 + 10 = 34 inches.
  3. The baseboard fills the wall from the corner to the edge of the opening, so subtract the opening: 149 − 34 = 115 inches.
  4. Convert back to feet and inches: 115 ÷ 12 = 9 remainder 7, so 9 feet 7 inches.
Answer: The baseboard piece must be 9 feet 7 inches long (exact measured length, before adding a fitting allowance).
2. You are installing door casing with a 3/16-inch reveal on a jamb that is 2 inches wide (face dimension). Where do you mark the reveal line from the inside edge of the jamb opening?
  1. The reveal is the setback from the face of the jamb, not from the rough opening.
  2. Set a combination square to 3/16 inch (0.1875 inch).
  3. Hold the square's fence against the face of the jamb and run a pencil along the blade to mark a line 3/16 inch back from the jamb face on all three jamb faces (two legs and the head).
  4. The inside edge of the casing will align to this line, not to the raw edge of the jamb.
Answer: Mark a line 3/16 inch (0.1875 in) back from the face of the jamb on all three jamb faces; the casing's inside edge aligns to that line.
3. A corner reads 94 degrees with a bevel gauge instead of a true 90 degrees. At what angle should each miter cut be set on the miter saw to make the outside corner joint close properly?
  1. For an outside corner, the two miter angles must add up to equal the corner angle: 94 degrees.
  2. Divide evenly between the two pieces: 94 ÷ 2 = 47 degrees.
  3. Set the miter saw to 47 degrees for each piece.
  4. Verify: 47° + 47° = 94°, which matches the measured corner angle.
Answer: Set the miter saw to 47 degrees for each piece; the two cuts will sum to 94 degrees and close the joint at that corner.
4. You need to cover three walls of a rectangular closet with baseboard. The two side walls each measure 4 feet 3 inches and the back wall measures 6 feet 8 inches. All corners are inside corners. How many linear feet of baseboard do you need at minimum (exact total, no waste allowance)?
  1. Convert each measurement to inches: 4 ft 3 in = 51 in; 4 ft 3 in = 51 in; 6 ft 8 in = 80 in.
  2. Add the three lengths: 51 + 51 + 80 = 182 inches.
  3. Convert to feet: 182 ÷ 12 = 15 remainder 2, so 15 feet 2 inches.
  4. Express as a decimal for ordering: 15 ft 2 in = 15 + 2/12 = approximately 15.17 linear feet.
Answer: The minimum total is 15 feet 2 inches (about 15.17 linear feet) of baseboard; always add a waste allowance when ordering stock.

The math in this lesson2

Learners subtract door widths from wall runs, mark a 3/16 in reveal, and halve a 94° corner for miter angles.
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Brush up in the math halls at newSchoolWorks.com:
In class: Room measuring and cut list: students measure a mock room corner (two walls meeting at 90 degrees with a door opening), record inside measurements, calculate miter and cope cuts with a cut list worksheet, then select stock lengths to minimize waste. · Coped joint practice: students use a jigsaw and coping saw to cope the end of a base profile on three sample pieces, fitting each to a square-cut reference piece until the joint closes tight; they then back-cut and test fit. · Full baseboard and casing installation: working in pairs, students install baseboard around the mock room and door casing on both sides of a pre-hung door rough opening, nail with a finish nailer, set nails, and fill holes with wood filler, sanding smooth when dry.
Materials: Pre-made base molding (colonial or ranch profile), pre-made door casing, miter saw, jigsaw, coping saw, finish nailer and compressor, 2-inch finish nails, wood filler, 120-grit sandpaper, tape measure, combination square, pencil, safety glasses, hearing protection, NCCER Carpentry Level 2 Trainee Guide
Check: Instructor evaluates each installed joint: miter or cope gap must not exceed 1/32 inch on outside corners and must be tight at wall on inside copes; nails set and filled; no visible mill marks; satisfactory rating required on NCCER performance profile checklist.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Carpentry — Interior Finish Carpentry
1. Why is a coped joint preferred over a mitered joint at interior wall corners for baseboard installation?
NCCER Carpentry — Interior Finish Carpentry
2. What is the standard purpose of a reveal when installing door casing?
NCCER Carpentry — Measurement and Layout
3. A student measures a corner and finds it is 88 degrees instead of 90 degrees. What miter angle should be cut on each piece of baseboard to close an outside corner at that corner?
OSHA 10 — Construction Safety and Health
4. Which of the following is the correct procedure when a finish nailer jams?
NCCER Carpentry — Material Estimation and Planning
5. When building a cut list for baseboard, why should each piece be measured a few inches longer than the exact finished dimension?
NCCER Carpentry — Interior Finish Carpentry
6. After driving finish nails with a pneumatic nailer, what are the correct next steps before the surface is ready for paint?
OSHA 10 — Construction Safety and Health
7. Which personal protective equipment is required at a minimum when operating a miter saw to cut molding?
NCCER Carpentry — Interior Finish Carpentry
8. A student installs a coped baseboard joint and holds a light at a low angle to inspect it. A gap of approximately 1/16 inch is visible at the cope. According to the lesson quality standard, what should the student do?
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