How to Repair Rotted Window Sills: A Step-by-Step Guide

Window sills are an essential part of a home’s exterior, providing a decorative and functional element that protects the window frame from water damage. However, over time, these sills can become weathered and rotted, compromising their appearance and structural integrity. Fortunately, repairing rotted window sills is a manageable task that can be completed with some basic tools and materials. In this comprehensive guide, we’ll walk you through the steps to repair your rotted window sills and restore them to their former glory.

Assessing the Damage

The first step in repairing rotted window sills is to assess the extent of the damage. Carefully inspect the sill for signs of rot, such as soft, spongy wood or visible decay. If the rot is confined to a small area, a simple epoxy repair may be sufficient. However, if more than 10% of the sill is rotted, it’s best to replace the entire sill.

Preparing the Surface

Once you’ve determined the extent of the damage, it’s time to prepare the surface for repair. Begin by removing any remaining paint and caulk from the affected area using a scraper or sandpaper. Next, use a wood hardener to tighten up the remaining wood and prevent further decay. If you need to fill in gaps or build up the surface, use a two-part epoxy wood filler, such as Minwax High Performance Wood Filler.

Cutting the Old Sill

If you need to replace the entire sill, use a circular saw with a beveled blade to make a plumb cut flush with the sheathing along the length of the rotted sill. For areas where the circular saw can’t reach, such as the sill ends, switch to a multitool equipped with an oscillating saw blade. Smooth the cut with a few strokes of a hand plane.

Selecting a Replacement Sill

When choosing a replacement sill, consider using a rot-resistant wood like cedar or a solid piece of PVC that matches the thickness of the existing sill. Cut the replacement sill to the desired length, making sure to match the angle of the sill slope along the back edge (usually 15%).

Attaching the New Sill

Before attaching the new sill, repair any remaining rotten spots on the old sill with epoxy. Trim the back edge of the new sill as needed so that its lower edge fits tightly against the siding and its back edge fits against the old wood. Drill pilot holes every 16 inches or so through the new sill’s front and back edges. Run a bead of waterproof, marine-grade adhesive along the old sill, and press the new sill into the adhesive. Immediately clamp the sill in place by driving 3½-inch deck screws through the plastic and into the wood, stopping when the heads are about ¼ inch below the sill surface. Wipe up any adhesive that squeezes out and fill any gaps between the old and new sill with adhesive.

Finishing Touches

To hide the screwheads, cover each one with a dollop of a white, two-part acrylic adhesive, such as Bond&Fill FastCure. Allow the adhesive to harden completely (about 30 minutes), then sand it flush. Finally, install the side casing by squeezing a bead of caulk beside the ends of the siding and nailing the new casing in place using cellular PVC trim boards and 8d stainless-steel ring-shank nails. Set the nailheads slightly below the surface and cover each one with a dab of the two-part adhesive. After the dabs harden, sand them flush.

Repairing rotted window sills requires some patience and attention to detail, but the results are well worth the effort. By following these steps and using the right materials, you can restore your window sills to their original condition and protect your home from water damage for years to come.