A vinyl window sill is one of those parts that quietly does a lot of work. It sheds water on the outside, gives the interior a finished ledge, and takes daily abuse from sunlight, condensation, and the occasional flowerpot. When it cracks, warps, or discolors, replacing it is a satisfying afternoon project that instantly refreshes the look of a window. The trick is knowing which sill you are actually dealing with, because the interior stool and the exterior sill are two different components with two different jobs.
In this guide I will clear up the terminology, walk through measuring and cutting, and cover installation for both the interior and exterior pieces. Vinyl is a forgiving material to work with, but a clean, water-tight result still comes down to careful fitting and proper sealing.
Interior Stool Versus Exterior Sill
Homeowners often call the flat interior ledge below a window the sill, but in trade terms that piece is the stool. It caps the rough opening on the inside, projects slightly into the room, and gets an apron trim beneath it. The true sill is the sloped exterior member that sits at the bottom of the window and directs rainwater away from the wall. Vinyl versions of both exist, and modern vinyl windows frequently include an integrated sloped exterior sill as part of the frame extrusion.
Knowing which piece you are replacing matters because the exterior sill is a weatherproofing component and the interior stool is largely cosmetic. If water is getting into the wall, the exterior sill or its flashing is the culprit, not the interior ledge. If you simply want a fresh, bright surface inside the room, you are replacing or capping the stool. Sort this out first and the rest of the job is straightforward.
Why Choose Vinyl
Vinyl earns its place at the window because it does not rot, does not need paint, and shrugs off the moisture that destroys wood sills over time. A wood stool under a window that sweats in winter will eventually stain, peel, and cup. A vinyl or cellular PVC sill in the same spot stays dimensionally stable and wipes clean with a damp cloth. For kitchens, bathrooms, and any window prone to condensation, vinyl is the practical answer.
The trade-off is appearance and repair. Vinyl reads as slightly more utilitarian than a painted hardwood stool, though cellular PVC sills can be primed and painted to match trim if you want a seamless look. Vinyl also cannot be sanded and refinished the way wood can; a deep gouge means replacement rather than repair. For most homeowners the low-maintenance benefit far outweighs these minor limitations.
Measuring and Ordering
For an interior stool, measure the width of the window opening between the jambs, then add the length of the horns on each side. The horns are the small ears that extend past the opening and rest on the wall so the apron can tuck beneath them. A typical stool projects into the room by roughly an inch beyond the face of the wall or the casing, so account for that depth as well. Measure the depth from the window frame to the front edge you want.
For an exterior sill replacement, measure the full width of the opening and note the slope, or pitch, that carries water outward. Vinyl sill stock and PVC boards come in standard widths that you can rip down. Buy a little extra length so you can trim clean, square ends on site. Whenever possible, take the old sill out first and use it as a template, tracing its profile and horn cutouts directly onto the new stock.
Removing the Old Sill
Start by scoring the caulk and paint lines with a utility knife so you do not tear adjacent trim when the sill comes free. For an interior stool, remove the apron below it, then pry the stool up gently; it is usually nailed to the rough sill framing and to the jamb ends. Work slowly with a flat bar and a shim to protect the wall, and pull any remaining nails rather than hammering them over.
For an exterior sill, expect it to be sealed and possibly flashed. Cut through the perimeter sealant, remove any fasteners, and ease the sill out while noting how the flashing and any sill pan are arranged. This is your chance to inspect the framing below for water damage. If you find soft, dark, or crumbling wood, address the rot before installing the new sill, because a fresh vinyl sill over damaged framing only masks a problem that will resurface.
Cutting and Fitting the New Piece
Transfer your measurements or trace the old sill onto the new vinyl stock, then cut with a fine-tooth blade to avoid chipping. A miter saw with a blade suited to plastics gives the cleanest cuts, but a sharp jigsaw handles the horn notches and any curved profiles. Dry-fit the piece before you commit to sealant or fasteners. It should drop into place with even reveals and sit flat without rocking.
Trim in small increments; vinyl removes fast and you cannot add material back. For the interior stool, the front edge should be parallel to the wall and the horns should sit tight against the casing. For an exterior sill, confirm the outward slope is preserved so water runs off rather than pooling against the window frame. A few minutes of patient fitting here is what separates a professional-looking result from an obvious repair.
Sealing and Securing
Set the interior stool with a bead of construction adhesive and a few finish fasteners into the framing, keeping fasteners where the apron or a bead of caulk will conceal them. Reinstall or replace the apron, then caulk the joints between the stool, the jambs, and the wall for a crisp painted-trim look. Because the interior stool is not a water barrier, standard interior caulk is fine here.
The exterior sill is where sealing really counts. Bed the new vinyl window sill in a continuous bead of quality exterior polyurethane or hybrid sealant, set it into place, and fasten it according to the manufacturer’s pattern. Seal the ends and the joint where the sill meets the window frame, but leave the underside drainage edge open so any water that finds its way behind the sill can escape. Done this way, the sill will keep the wall dry and the window operating for decades.
Common Problems and Fixes
Even a straightforward sill swap can throw a few curveballs. The most common is discovering hidden water damage once the old sill is out, particularly under an exterior sill where a failed seal has let rain into the framing for years. If you find punky, discolored wood, do not simply cap it with a new vinyl sill; cut back to sound framing, replace the damaged wood, and correct the flashing that let the water in, or the problem returns. Catching this early is one of the quiet benefits of a sill replacement.
Another frequent issue is a stool that rocks or a sill that does not sit flat, almost always caused by a high spot in the framing or leftover caulk and paint that was not fully cleaned off. Take the piece back out, find the high spot, and dress it down rather than forcing the sill and hoping fasteners will pull it flat, which only stresses the vinyl. Finally, if condensation keeps pooling on an interior stool, that points to a window efficiency or humidity problem worth addressing, since the new sill will handle the moisture but the underlying condensation is worth solving at its source.
Maintenance and Longevity
The beauty of a vinyl window sill is how little it asks of you. Wipe it down when it gets dusty, and inspect the exterior sealant once a year, touching up any spots where the caulk has cracked or pulled away. Keep the exterior weep areas clear of debris so water can drain freely. With that minimal attention, a vinyl sill will stay bright and functional far longer than the wood it replaced, and you will not be repainting or repairing it season after season. It is a small upgrade that pays off every single day the sun hits your window.
