How to Replace a Rotted Window Sill

Window sills are the frontline defense against the elements, constantly bearing the brunt of wind-driven rain and intense sunlight. When you notice soft, spongy wood or peeling paint localized at the sill, it is not just a cosmetic issue; it is a clear sign that moisture has breached the sealant and is working its way into the wall assembly. Ignoring this will inevitably lead to structural rot in the surrounding framing and siding. Successfully replacing a sill requires patience, particularly during the demolition phase where you must avoid damaging the existing window frame or the surrounding siding. Done well, the new sill will be properly pitched to shed water away from the house and sealed with high-quality flashing, effectively resetting the clock on the window's lifespan. Precision in your measurements and a commitment to sealing every gap are the difference between a permanent fix and a recurring headache.

1. Clear the Work Area. Remove any existing storm windows or screens attached to the sill area. Carefully score the caulk between the sill and the siding using a sharp utility knife to prevent tearing the siding when you remove the wood.
2. Extract the Rotted Sill. Use a reciprocating saw or a hammer and a sharp chisel to cut and pry the rotted sill out in sections. Be extremely careful not to cut into the interior stool or the structural wall studs located directly beneath the sill.
3. Check for Hidden Damage. Once the wood is out, inspect the wall cavity for water damage or mold. If the underlying framing is sound, apply a wood preservative or a high-quality fungicide to the area before installing the new material.
4. Cut with Proper Pitch. Measure the width of your window opening precisely and transfer those marks to your new pressure-treated lumber. Use a table saw or circular saw to cut a slight downward angle—roughly 5 to 8 degrees—on the exterior edge so that water drips off the sill rather than soaking into the wall.
5. Secure the New Sill. Apply a generous bead of high-quality exterior construction adhesive to the sub-sill framing. Press the new sill into place and secure it using galvanized finish nails driven into the window frame and the exterior studs.
6. Waterproof the Perimeter. Install self-adhering flashing tape over the joint where the sill meets the side casings, ensuring the tape overlaps the house wrap below. Seal all exterior joints with high-grade exterior silicone caulk, leaving the bottom edge unsealed to allow any trapped moisture to escape.