How to Replace a Rotted Windowsill

If a windowsill hasn’t stood up to the weather especially well, cut a replacement modeled on the old sill. Here’s how to do it:

Materials and Tools:

workbench
sawhorses
table saw with a fence and miter gauge
carpenter’s pencil
push stick
carpenter’s square
power drill
rag
reciprocating saw
hammer
folding rule
bricks
caulk gun
5-in-1 tool
cat’s paw
circular saw
safety glasses
dust mask
pretreated lumber
2-by-6-inch board
1-by-6-inch board
screws
nails
caulk

Steps:

1. Raise the sash and remove the top plate. Pry away as much material as you can. Use the reciprocating saw to cut through solid material and clear it away.

2. Pry out the main part of the sill. Try to keep it as a whole piece for use as a template for cutting a replacement.

3. Use the folding rule to get inside measurements to cut a new top plate.

4. Place the old sill on a 2-by-6 and trace around the edge to mark cut lines. While you're at it, mark the measurements for the top plate on a 1-by-6.

5. Wearing a dust mask and safety glasses, cut the new pieces with a table saw.

6. Slide the new sill into place, and then pry away interior trim and slide the top plate into position. Drive galvanized screws down through both pieces and into studs below. Secure the bottom sill by driving screws through the outside edge.

7. Run a bead of caulk along the edge to seal any gaps. Reattach trim pieces if necessary.