Installing Window Sills

If you are looking for the basics of installing window sills you’ve come to the right place. If you did not have a sill there previously it is fairly straightforward. Just skip ahead past the repair part. On the other hand, if you are replacing one that has gone bad due to weather, then it’s a job of removing everything gone bad and then building from the ground floor (so to speak) back up. Let’s assume you are doing a repair.

The first thing you will want to do is remove all the old caulking from around the of the sill’s edge. It is easy to do this using a 5-in-1 painter’s tool/putty knife

Sill Template

Just for the sake of covering our bases, let us assume that this damaged sill was not truly square. This being the case, it will be more convenient and hassle-free to make a template of it using paper rather than trying to replicate it by using our measuring tape. So let’s place sheets of paper right along the sill and use tape to connect them together. This is how we will form our template.

Next, we will cut the old paint seam that is between the trim and the damaged sill. We can do this using a utility knife. For the next part of the operation we will need to put on safety glasses. Now that we have done that we can go ahead and cut the sill. We will cut it into two pieces and we will do it using a pull saw.

Loosening Sill

Now we can loosen the first piece of sill by getting our painter's tool underneath the sill and gently tapping it with a claw hammer. We will first work one piece of our sill up just enough to insert a small pry bar between the pieces. Now we will pry up one piece.

Once we have that first piece out we do the operation all over again to take the second piece out. Before going any further, examine the entire work area and remove any nails that stayed behind and any other debris that might get in the way when we install the new window sill.

This is a good time to look for any other damaged wood that we might have as a result of the same weather that ruined the old sill. If you find some, repair it before you continue with installing the new window sill.

New Sill

Now it’s time to get to work on the new sill. First, tape the paper template or pattern we made to the new board and clamp it to workbench. Did you remember to buy pressure treated wood so we won’t have to do this again for a while? Good, now trace around our pattern. With the outline as a guide, cut the new board to fit. Once again, use the pull saw.

Now dry fit the new sill. You might find it necessary to insert 2x4 blocks in between the drywall and wall studs. You want to have plenty to nail to. These blocks serve as mounting blocks. You will find it easy to secure the blocks using a cordless drill and screws.

Fitting and Locating

Fit the new sill into place one more time and check it for level. If it’s necessary, insert wooden shims to get everything level. To avoid cracking the sill, go ahead and pre drill holes right through the new sill. Now attach it with finish nails. Be careful that you do not to drive the nails home with the hammer. Sink them using a nail set.

Seal it in by running a bead of caulk along the seams, and then smooth the bead with your finger. Finally, spackle the nail holes with putty. Let the caulk and putty have a few days to set up, and then prime and paint the windowsill to match.