Window Sill Trim: Profiles and Install

Window sill trim — the stool that caps the interior sill and the apron beneath it — does more visual work than any other four feet of molding in a room. Get the window sill trim proportions right and the whole window looks intentional; get them wrong and even expensive casing reads as builder-grade. After trimming out a few thousand openings, I can tell you the difference comes down to three numbers: the horn overhang, the side reveal, and the apron length. This guide covers the two main profile systems, material choices with real prices, and the install sequence that produces tight joints.

Stool and Apron vs. Picture Frame

Interior window trim resolves into two schools. Traditional stool-and-apron puts a horizontal shelf — the stool, which everyone calls the sill — across the bottom of the opening, projecting past the casing with returned ends called horns. Below it, an apron board covers the gap to the drywall and visually supports the stool. Side and head casings die into the stool’s top surface. Every American house from 1850 to 1960 used this system, and it remains the default for colonial, craftsman, and farmhouse interiors.

Picture-frame trim runs casing on all four sides with mitered corners — no stool, no apron, just a frame. It uses about 15 percent less material, installs faster, and suits modern and mid-century interiors. The trade-off is losing the shelf: no place for a plant, a candle, or a cat. Drywall-returned openings with only a small stool at the bottom are a third, minimalist option that has taken over new construction; even there, builders usually keep a stool because a drywall bottom edge gets destroyed by condensation and cleaning.

Stool Profiles and the Numbers That Matter

A stock stool is milled from 5/4 material — a true 1 to 1-1/16 inch thick — in widths from 2-1/4 to 5-1/4 inches, with a rabbeted back edge that laps over the sill frame and a rounded or eased front nose. Common profiles at the yard: WM 1021 (rabbeted, 14-degree bevel to match sloped sash sills) and flat-bottom versions for modern windows that sit flush. The proportions I hold to:

• Nose projection: 3/4 to 1-1/4 inch past the apron face — enough for shadow line, not enough to catch hips in a hallway
• Horn overhang: stool extends 3/4 inch beyond the outside edge of each side casing
• Casing reveal: side casings set back 1/4 inch from the jamb face (3/16 inch on narrow 2-1/4-inch casing)
• Apron length: cut to the outside-of-casing to outside-of-casing dimension, so it aligns with the casing edges, not the horns
• Thickness step: apron should be thinner than the stool projection so the stool nose shadows over it

Material Options and Cost per Linear Foot

• Primed finger-joint pine: $1.50 to $3 per linear foot for stool stock. Paints beautifully, easiest to cope and sand. Weak point is moisture — a sweating window will swell and peel it inside five years.
• Clear pine or poplar: $3 to $5 per foot. Poplar is my default for painted work: stable, takes nail heads without splitting, machines clean.
• Oak, maple, cherry: $5 to $12 per foot for stain-grade stools. Match species to the floor or the sash, never approximately.
• Cellular PVC (Azek, Royal, Kleer): $4 to $7 per foot. Immune to water, so it is the only thing I put on bathroom, kitchen-sink, and basement windows. Cut it with standard carbide blades; glue joints with PVC cement; paint is optional but light colors only, since dark paint over PVC can heat-distort.
• MDF: $1 to $2 per foot and dead flat, but a single condensation season turns the nose into oatmeal. I no longer install MDF stools anywhere, period.

A typical 36-inch window needs about 5 feet of stool stock and 4 feet of apron; with casing, figure $25 to $60 in materials per opening for paint-grade, double for hardwood.

Tools for a Clean Job

Miter saw with an 80-tooth finish blade, jigsaw or handsaw for the horn notches, 18-gauge brad nailer with 1-1/2 and 2-inch brads, block plane, combination square, scribe or compass, wood glue, painter’s caulk, and a shim assortment. A pocket-hole jig is handy for site-joining long stools on mulled double windows. Nothing exotic — the skill is in the layout.

Installing the Stool Step by Step

1. Lay out the length. Stool length = jamb-to-jamb width + two casing widths + two reveals + two 3/4-inch horn overhangs. For a 36-inch opening with 2-1/2-inch casing: 36 + 5 + 1/2 + 1-1/2 = 43 inches.
2. Mark the notches. Center the stool on the opening, and mark where each jamb face lands. The material between marks stays full depth into the opening; outboard of the marks, notch the back edge away so only the horn projects past the wall.
3. Scribe to the sash. Hold the notched stool level in the opening and scribe the back edge to the window; walls are never flat and sashes never parallel. Plane or jigsaw to the line until the back edge sits within 1/16 inch of the sash — that gap gets caulk, anything bigger gets seen.
4. Fasten. Bed the stool level (shim beneath if the rough sill slopes), then nail down through the stool into the rough sill with 2-inch brads, two per foot, plus a brad through each horn angled into the framing. On PVC, add a bead of construction adhesive since brads alone let it creep.
5. Return the horn ends on stain-grade work: miter a small return piece so end grain never shows. On paint-grade, a sanded eased end is acceptable.

Casing and Apron

Side casings sit on the stool and run up at your chosen reveal; head casing miters or butts across the top depending on profile. Then the apron: cut it to align with the outside casing edges, and return or bevel its ends — a square-cut apron end showing raw profile is the number one giveaway of a rushed job. Nail the apron into the framing below the sill with 2-inch brads, and clamp it tight up under the stool while nailing so no shadow gap opens. Finally, run a fine bead of caulk where stool meets sash and where trim meets wall, set nails, fill, sand with 180 grit, prime any bare wood, and paint. Two topcoats of a scuff-resistant enamel — Benjamin Moore Advance and Sherwin-Williams Emerald urethane are the two I trust on sills that get wiped weekly.

Fixing Common Sill Trim Problems

Peeling paint and black staining at the stool’s back edge mean condensation, not bad paint — fix humidity first, then replace the stool in PVC or back-primed poplar. A stool cupped upward can sometimes be flattened with a saw kerf underneath and screws drawn into the rough sill; past 1/8 inch of cup, replace it. Gaps opening at the horn miters each winter indicate the trim was installed at high summer humidity; re-glue, re-caulk, and expect stability once indoor humidity is controlled to 30 to 50 percent. And when replacing only a damaged stool under existing casing, cut the old one out in pieces with an oscillating multitool rather than prying — you will save the casings and an hour of patching.

Matching Existing Trim in an Older House

Adding or repairing one window in a 1920s house means matching a profile the molding aisle stopped carrying decades ago. Three routes, in order of cost: check a specialty millwork supplier’s historic catalog first, since WM-numbered patterns cover more old profiles than people expect; have a lumberyard with a molder grind a knife to a sample — setup runs $75 to $150 plus footage, painless when you need trim for three or more openings; or fake it with a built-up profile, stacking a stock stool, a piece of base cap, and a parting strip to approximate the original shadow lines. Take a clean cross-section photo with a ruler in frame, or better, cut a 2-inch sample from a closet window where the loss will never show. Paint hides a near-match; stain-grade forgives nothing.

Exterior Sill Trim Is a Different Animal

One clarification that saves confusion at the lumber desk: the exterior sloped sill is part of the window unit or the sill nosing on the frame, not trim you buy off the molding rack. Exterior sill repairs use sill nosing stock, historic sill replacement blanks in 8/4 fir or mahogany, or PVC sill covers — different products, different install, and always pitched 5 to 14 degrees to shed water. Everything in this guide addresses the interior side, where the stool’s only jobs are to look right, hold a coffee cup, and survive the occasional wet windowsill morning.