Your window sill is the bottom portion of your window, running horizontally within the jamb, resting against the outside of the window sash. Wooden sills have a tough time standing up to the elements year in and year out. If your sill is rotting or falling apart, replace it with these simple steps.
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Use a utility knife and, if necessary, a hammer to carefully break and remove any caulking or sealant that's binding the old sill to the window frame. Use the crow bar or chisel to slowly pry the windowsill off the window. Proceed with care: Keep the sill in one piece so you can use it as a model when cutting the new sill. You may also need to pry away trim material, known as side casing. Set it aside for later reattachment.
Some sills are sloped downward so rainwater runs away from the window. If your sill is sloped, you'll also need to remove the wooden supports that are lifting it up. Newer homes may use beveled wood sections, which should come off in one piece. Any other piece that interferes with removing the sill must go, including decorative pieces under the window.
Use the rag and utility knife to clean away any debris left on the window frame. If parts of the sill or caulk remain, use sandpaper and solvents to clean the area. Remove any leftover screws or nails. If rot is your problem, remove all rotted wood you can find because it will continue spreading unless you chip it out. A little rot in the frame can be replaced with an epoxy mixture. These readily available wood fillers are a type of plastic putty that settle and harden, properly sealing the frame and bonding it to the existing wood.
For major rot problems, contact an expert for advice.
Measure the empty sill space to determine if you need a 2-by-4 pressure-treated board or a larger 2-by-6 board. The board needs to be larger than the sill space. Place your old sill over the selected board and draw its outline with your carpenter's pencil.
If you can't salvage the old sill for a template, measure the sill space and draw a new sill template on your board. Buy more than one board if you think you'll need an extra attempt.
Substitute cedar boards if you're unable to purchase pressure-treated lumber.
Use the table saw to cut your new sill. A jigsaw is useful for secondary cuts and shaping the sill if you have a more decorative design in mind. Cut wide — you can shave off the ends of the sill to fit in the frame later, but gaps encourage leaks and rot problems if you cut too short.
If your original sill was sloped, you'll need to measure the slope of the old sill and mimic it, another reason the old sill can be an invaluable template. Otherwise, measure the angle the original side casing creates to find your proper angle with a protractor (14- to 15-degree angles are common for window sills). Make the required angle cuts so the sill fits tight against any existing weather proofing material.
Nail or screw the new sill down in place. Make any necessary adjustments and add shims if needed before the final placement to ensure the new sill is level. Drill slowly to avoid cracking or damaging the new sill.
Place the screws along on the outside edge of the sill, drilling them in where your old screws were placed. If you're not sure where the old screws were, space the new screws 6 inches apart for a strong fit. If you want, add caulk on the screw heads to help hide them from view when you're finished.
If you use pressure treated boards for the new sill, use fasteners labeled for treated lumber.
Use a caulk gun to apply a layer of caulk along the edges of the sill. Choose an outdoor caulk designed to repel water. If you removed trim, wait until the caulk is fully cured (about 24 hours) before replacing the trim.
Wear protective gloves, glasses / goggles and mask while cutting wood.