Trimming a Window With a Custom MDF Sill and Box

A finish carpenter trims out a window in a house where the exterior windows were already done, and the interior detail calls for a custom sill built from 1 in. MDF. The reveal he is after is 1 1/2 in. on every side, and the whole job turns on measuring for that.

Measuring for the sill

The window measured 18 1/2 by 18 3/8. Adding 1 1/2 in. on each side makes the opening 21 1/2 in. in one direction and 21 3/8 in. in the other. He also notes the distance from the window back to the drywall, about 3 13/16 in. at one spot and 3 3/4 in. at another, and writes these on a drawing of the sill. He makes the rough-opening cut 1/2 in. smaller so it fits in nicely, at 23 1/2 in.

The trim is 3 1/2 in. wide, with a 1/2 in. lip from trim to sill and a 1/4 in. jamb reveal, giving 4 1/4 in. on each side. The sill length comes to 29 7/8 in. He cautions that these numbers depend on the reveals and are a general guide, not a bible.

The stock is not truly 3/4 in.; it is 11/16 in. So a 1/2 in. lip plus the stock gives 1 3/16 in., the setting for his combination square when marking.

Shaping and priming

His daughter, in the second week of her apprenticeship, routs the square edges with a 1/8 in. round-over bit backed off slightly so it is sharper, in one smooth counterclockwise motion, followed by a quick sand. A 10° back bevel is cut with a jigsaw to clear shims and caulk. Edges that meet the window and wall are primed with latex primer sealer and a roller, so painters do not need two coats of finish paint.

The box

The box is ripped to 21 3/8 in. inside, plus 11/16 in. for the top piece, making 22 3/16 in. The sides taper from 3 3/4 in. at the top to 3 13/16 in. at the bottom, cut with a track saw. Small pieces can kick out if pushed too fast.

The first cut came out at 3 11/16, and the top had to be recut. A fix came from the track's small holes: cabinet hinge screws pinned the track so it could not move. He nails the top first, then adds one screw in the middle; MDF splits with two.

Setting it

He uses a 1 1/2 in. block to check the reveal, shims to adjust, and checks level and the window's mitre welds. He nails only after checking the sides, cuts shims with a multi-tool, and leaves slack because gravity will settle the piece.

Casing

A 1/4 in. reveal is marked with the combination square. Casing is marked in place by eye and with a knife, which he says is more accurate than transferring measurements. Shims behind the top casing let him bring the miter flush. The apron is the same 29 in. length, and a glazier's bar is run over the nails to check none stick up.