Wooden window sills will rot if exposed to moisture and insects, and they will eventually require replacement. You can replace a rotting window sill with a few basic hand and power tools, and it won't cost much at all. Below is how to remove an old sill and install a new one that will last for many years:
Tools and materials needed
- Cedar board
- Paper and pencil
- Circular saw or hand saw
- Measuring tape
- Electric drill
- Pry bar
- Reciprocating saw
- Caulk gun and silicone caulk
- Wood screws
- Wood chisel
- Carpenters hammer
- Rubber mallet
- Medium-grit sandpaper
- Fine-grit sandpaper
- Exterior latex house paint in color of choice
1. Remove old sill - to remove the rotting sill, you will need to make a couple of strategic cuts to make the process easier and prevent damage to the window frame and other components. Measure one inch from the inside of the window frame on both sides and make perpendicular cuts through the sill with a reciprocating saw until you reach the bottom of the sill. This will separate the sill into three parts.
Once you have cut through the sill on both sides, drive 2-inch wood screws into the face of the sill about one-half inch from each cut line. Next, hook a pry bar on the wood screws and use it to pull the sill from its location. It may stick due to caulk, wood swelling or nails, but continue to apply force on both sides, and the sill will eventually pull free.
After the central portion of the sill has been removed, use a wood chisel and hammer to chip out the remaining sides of the sill. Be careful not to damage the window frame or surrounding wood while chiseling. Clean out rotting debris with the chisel and remove any nail ends that might be protruding into the space.
2. Measure and cut new sill - once the rotting sill has been removed, measure the side-to-side distance of the vacant space. In addition, measure the depth of the sill space from its face to the window frame as well as the thickness from top-to-bottom. Next, measure the offset required for the window sill on both ends so that it will neatly abut the existing wall studs.
After obtaining these measurements, make a simple sketch with all the dimensions and use this to help you purchase an appropriately-sized cedar board. If you can't locate a cedar board of adequate size, you can also use treated pine lumber as a substitute.
When you have the board in hand, mark the dimensions obtained above and make necessary cuts with a circular saw or hand saw to get it to size. Sand off rough edges with a piece of medium-grit sandpaper and smooth the ends of the wood with medium grit and fine-grit sandpaper. Test fit the sill by pushing into position and make sure that it does not need additional trimming or sanding. Once the sill has been cut to size and you verify a fit, adjust your circular saw to a ¼-inch cutting depth and make a single cut along the bottom of the sill from end-to-end. This "drip line" will aid in allowing rainwater to exit the sill and prevent it from flowing back down the sill into the house's siding.
3. Install the new sill - after the sill has been cut to shape and size, insert a one-quarter inch line of silicone caulk along the bottom and each end of the vacant sill space. Push the new sill into the vacant space and use a rubber mallet, if needed, to hammer it into its final position. Check the sill once more for fit, and apply a line of caulk along all edges of the sill to seal it from water intrusion. After the caulk has dried completely, paint the new sill in an exterior latex paint of your choosing.
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