Trim Carpentry

Dry wood first

The handbook describes the problem that shows up in so many new houses: a few months after move-in, cracks open between floorboards, mitered trim joints gap, doors warp, and stair treads creak ¹. The cause is almost always moisture. Millwork leaves the factory dry, but an unheated house under construction is humid, so trim and flooring absorb moisture on site, get nailed up swollen, then shrink once the heat comes on ¹.

The fix is to dry the house before the wood arrives: hang the outside doors and windows, get the heating plant running, and keep the house warmer than outdoors. In warm weather the handbook says to hold it about 15 °F above the average outdoor temperature overnight; in cold weather, about 60 °F ¹. Then let the trim sit inside for some days before cutting it. Interior millwork should be at roughly 6 to 11 percent moisture depending on the region ¹.

Doors and casing

A door frame is two side jambs and a head jamb, with separate stops the door closes against ¹. Most interior doors today come prehung, but the parts are the same.

Casing frames the opening and covers the gap between jamb and drywall. Molded casing needs miter joints where the head meets the sides ¹. Set the casing back from the jamb’s inside edge by a small, even reveal (about an eighth of an inch is common); a reveal hides small irregularities that a flush edge would show. Nail into the jamb’s edge with small finish nails and into the framing with larger ones, and glue and pin the miter so it can’t open from the corner.

Base molding

Base finishes the joint between wall and floor. A two-piece base adds a small cap molding that follows the wall even where it isn’t straight, and a shoe molding at the floor covers the gap below ¹.

• Nail the base at the studs ¹, keeping it slightly clear of the finish floor.
• Inside corners: cope. Run the first piece square into the corner. Cut the second piece’s end to the first one’s profile with a coping saw, so it fits against it like a puzzle piece ¹. A coped joint stays tight when the corner isn’t square and when the wood shrinks; an inside miter opens.
• Outside corners: miter, glued and pinned.
• Long walls: join lengths with a scarf (two overlapping bevels) over a stud.
• Shoe: nail it down into the floor, not into the base ¹, so the floor and the wall can move without opening a gap.

Why miters open

Wood shrinks across its width as it dries, much more than along its length (the wood-drying and woodworking courses). A miter joins two pieces at 45°, so when each shrinks across its width, the joint pulls apart at the inside (heel) of the miter and the gap opens there. Trim that was nailed up damp in a humid summer and then dried by a winter’s heat shows it worst. Dry, acclimated trim, glued and pinned miters, and coped inside corners avoid nearly all of it.

What to take from this

Heat and dry the house before trim arrives (15 °F above outdoors in warm weather, about 60 °F in cold), and let the wood acclimate. Set casing back to a small reveal with glued, pinned miters at the head. Nail base at the studs, cope inside corners, miter outside ones, scarf long runs, and nail the shoe into the floor. Miters open on the inside when damp trim dries; dry wood prevents it.