Remove the Side Casings
Before tackling the rotted windowsill, start by carefully removing the side casings. This step is crucial to access the sill for removal and replacement. If you find that the casings themselves are also affected by rot, it’s best to replace them. Use a utility knife to cut through the caulk bead that seals the casing to the siding, and then use a flat pry bar to gently remove each casing. Keep the removed casings as they can serve as templates for cutting new ones. Also, ensure that the window’s flashing behind the casing is intact and correctly installed, as this helps prevent future water damage.
Saw Off the Old Sill
To remove the rotted sill, use a circular saw to make a straight, plumb cut along its length, ensuring the cut is flush with the window sheathing. For areas where the circular saw can’t reach, such as the ends of the sill, switch to a multitool equipped with an oscillating saw blade. After cutting, use a hand plane to smooth the cut edges, preparing the area for the new sill installation.
Apply the Adhesive
Before installing the new sill, address any remaining rot on the old sill with epoxy. This step is crucial to prevent further deterioration. Once the epoxy is set, trim the new sill to ensure a snug fit against both the siding and the old wood. Drill pilot holes along the front and back edges of the new sill at regular intervals. Then, apply a waterproof, marine-grade adhesive along the old sill, which will provide a strong bond for the new sill.
Attach the New Sill
Carefully position the new sill onto the adhesive layer, pressing it down firmly. To secure the sill in place, use long deck screws, driving them through the new sill and into the underlying wood. Ensure the screw heads are slightly below the surface of the sill for a clean finish. Wipe away any excess adhesive that squeezes out during this process, and fill any gaps between the new and old sill with additional adhesive.
Hide the Screwheads
Once the new sill is attached, cover each exposed screw head with a white, two-part acrylic adhesive. Apply a slight overfill to each screw head, and allow the adhesive to harden completely. Once set, sand down the adhesive to make it flush with the surface of the sill, creating a smooth, finished appearance.
Install the Side Casing
The final step is to reinstall the side casing. First, apply a bead of caulk along the ends of the siding where the new casing will be attached. Use cellular PVC trim boards for the new casing, as they offer durability and resistance to rot. Nail the casing in place using stainless-steel nails, ensuring each nail head is slightly countersunk. Cover the nail heads with the same two-part adhesive used earlier, and once hardened, sand them flush for a neat, professional look.
