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When interior sills begin to show signs of rot, the problem often starts small—water seepage from condensation, plumbing leaks, or poor drainage around the sill. The first move is a careful inspection that includes looking for soft wood, dark staining, or cracking paint. Use a screwdriver to test for firmness along the length of the sill; if you depress the edge and hear a hollow sound, further attention is needed. Gather supplies: a utility knife, chisel, pry bar, wood epoxy or Portland cement for masonry, matching filler, a replacement sill, wood primer, and exterior-grade paint. Plan to address both surface damage and underlying moisture sources.

Before you remove the old sill, protect surrounding surfaces with painter’s tape and drop cloths. Photograph the unit from multiple angles to guide the replacement process, and record measurements precisely. Carefully remove the casing boards and any trim surrounding the window without damaging them, then extract the rotted sill. If the sill is fastened to a cavity or reveals hidden rot in the framing, you must decide whether to repair or replace the affected area. In many cases, stabilizing with a temporary brace allows you to continue while preparing the new sill.

Start by crafting a precise template of the existing sill from cardboard or scrap plywood. This ensures the new piece matches the window opening exactly, preventing drafts and water intrusion. If you find any mortar or masonry beneath the sill, remove loose portions with careful chiseling and add fresh filler or mortar to restore a solid ledge. The replacement sill should be of the same profile and thickness, treated for exterior exposure. Sand the edges to avoid sharp corners that could injure the casing. Apply wood preservative to all sides before installation to prolong life and resist future decay.

Fit the new sill into place, checking for level along its length and across the width. Use stainless-steel finishing nails or corrosion-resistant screws to secure it into framing members that are solid. If the original sill rested on a rotted support member, replace that support with a sturdy, properly sized block or sistering technique to ensure even load distribution. Seal the joints with a high-quality exterior caulk, smoothing it with a damp finger to create a uniform line. Allow the caulk to cure fully before reattaching the interior trim.

Once the new sill is secured, address the interior finish to restore appearance and functionality. Apply a high-adhesion primer to the sill faces and edges, then paint with an exterior-grade enamel in a color that matches adjacent trim. If you notice recurring moisture, consider installing a sill apron or micro-drainage feature that channels water away from the window. Check the sill for any gaps where air and moisture could penetrate, and reseal as needed. The interior trim should be reinstalled carefully after the paint has dried, preserving clean lines and a neat appearance.

Recheck the window casing to ensure smooth operation and a snug seal against the wall. Open and close the window several times to identify any binding or misalignment caused by the new sill. If the frame shifted slightly during replacement, you may need to re-square the window by adjusting shims behind the casing. Reinforce vulnerable corners with additional trim nails and a light coat of paint. Finally, inspect the exterior for paint flaking and repaint as necessary to maintain weather resistance and curb appeal.

It is common to encounter hidden rot within the framing that supports the sill. If you discover compromised studs or a hollow edge inside the wall cavity, you need to treat or replace this wood before completing the project. Using epoxy wood filler can stabilize minor damage, but larger areas require cut-and-replace methods. When removing problematic pieces, cut back to intact wood, then fit new lumber with precise joints. Seal all new wood with preservative before applying primer and finish coats. Keep the work area dry during treatment to prevent mold growth and ensure a strong bond for paint.

After major structural work, you may need to adjust interior trim to maintain a seamless look. Re-mount the casing so that it sits flush against the new sill, using shims if necessary. Re-seat any stops or stops blocks to keep the window aligned, and reinstall the baseboard or sill trim with fasteners that won’t split the wood. Touch up paint on the casing to hide any work that was done during replacement. Finally, clean the window area to remove dust and debris, restoring a neat, professional finish.

Once the structural work is complete, focus on moisture management and air sealing. Use exterior-grade caulk with a smooth, continuous bead at all joints between the sill and surrounding framing. Pay attention to the joint where the sill meets the wall to prevent capillary action that could draw water inside. If your climate is humid, consider adding a vented sill or a low-profile drip edge to encourage airflow and evaporation. Regularly inspect for signs of new moisture buildup and address promptly to avoid repeating rot issues.

In addition to caulking, you may install a weather-resistant membrane beneath the sill if the window location experiences heavy exposure. This extra layer acts as a shield against water ingress, especially where the window meets the exterior brick or siding. Ensure the membrane overlaps with the exterior flashing and is properly tucked behind the siding to avoid visible seams. By combining careful replacement with thoughtful sealing, you create a long-lasting barrier against moisture that preserves the interior space.

After completing the project, establish a simple maintenance routine to protect the sill for years to come. Inspect the interior window area every season for signs of moisture, paint wear, or small cracks in the caulk. Repaint and reseal as needed, using a breathable finish that resists cracking in temperature swings. Clean gutters and downspouts to prevent overflow near the window, and ensure any landscape grading directs water away from the foundation. Keeping the area dry is the best defense against future rot and the most reliable way to preserve the integrity of your newly replaced sill.

Finally, document the repair with notes and photos for future reference. Record the materials used, the exact measurements of the sill, and the condition of surrounding framing before and after replacement. This archive helps with any future renovations and can be valuable if you decide to rent or sell the home. Share the process with family members or neighbors who might face similar repairs, offering practical tips that come from hands-on experience. A well-documented project also provides peace of mind, confirming that you addressed rot thoroughly and correctly.