How to Trim an Attic Dormer

Dormers stand out when they're left unfinished — raw drywall edges around the window, exposed corners where the knee walls meet the ceiling, joints that catch your eye for all the wrong reasons. Trimming a dormer isn't complicated carpentry, but it rewards precision. You're creating clean lines in a space where three planes converge: the window opening, the sloped ceiling, and the vertical knee walls. Done well, the trim disappears into the architecture. Done poorly, every gap and miscut angle announces itself. The work breaks into three zones: window casing first, then baseboard along the knee walls, and finally corner trim where walls meet ceiling. Most dormers use simple painted pine trim, which forgives minor gaps with caulk and paint. The real trick is cutting accurate angles where your baseboard transitions from vertical wall to sloped ceiling — that compound angle is where most first-timers stumble. Measure twice at every transition, and you'll spend less time backtracking to the miter saw.

1. Prepare Your Window Frame. Check that your window is set properly in the rough opening — it should sit flush with the finished drywall. If the window frame is recessed, rip jamb extensions from 1x material to bring the window frame even with the wall surface. Nail extensions to the window frame with 2-inch finish nails every 8 inches. These extensions give your casing something to land on and create the reveal line around the window.
2. Miter and Frame the Window. Measure and cut your window casing with 45-degree mitered corners for the head and side casings. Start with the head casing, then fit the side pieces to meet it. Nail casing through the face into the jamb extensions with 1.5-inch finish nails, and through the outer edge into the wall framing with 2-inch finish nails. Keep nails 3/4 inch from edges to prevent splitting.
3. Finish the Window Bottom. Cut the window stool to extend 3/4 inch beyond the side casings on each end, with a notch at each end to fit around the casing. Nail the stool to the window sill and wall framing below. Cut the apron to fit between the side casings and nail it beneath the stool, keeping the top edge tight against the stool's underside. This horizontal trim assembly finishes the window bottom.
4. Capture the Slope Angle. Place a piece of baseboard against the knee wall where it will sit, then mark where it meets the sloped ceiling. Use a T-bevel or digital angle finder to capture the exact angle of the slope. Most dormer ceilings run 35-45 degrees, but measure yours specifically — guessing costs you material. Mark this angle on your miter saw and make test cuts on scrap pieces.
5. Install Angled Wall Base. Cut your baseboard pieces with square cuts at the floor end and the angled bevel cut where they meet the slope. Nail baseboard to the wall studs with 2-inch finish nails every 16 inches, keeping the bottom edge tight to the floor. Where two baseboard pieces meet at inside corners, cope one piece to fit the profile of the other rather than mitering both.
6. Seal the Ceiling Joint. Cut corner molding or cove molding to fit where your knee walls meet the sloped ceiling. These pieces hide the drywall joint and create a finished edge. Miter the ends where molding pieces meet at corners, and nail into blocking or ceiling framing with 1.5-inch finish nails every 12 inches. If there's no solid backing, use construction adhesive to supplement the nails.
7. Fill Every Hole and Gap. Fill all nail holes with wood filler, slightly overfilling each hole. Let it dry completely, then sand flush with 150-grit paper. Run a thin bead of paintable caulk along every joint where trim meets wall, ceiling, or other trim pieces. Tool the caulk smooth with a wet finger, wiping excess on a damp rag.
8. Finish With Professional Paint. Apply one coat of primer to all bare wood trim, cutting in carefully where trim meets walls. Once primer dries, apply two coats of semi-gloss trim paint, allowing proper dry time between coats. Use a 2-inch angled brush for precision along edges and a small roller for wider casing faces. Keep a wet edge to avoid lap marks in your finish.