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Window Sill Replacement

A Window Sill Replacement Can Help Repair Rotted Sills. This Page Will Help You Learn the Basic Process to Replace

A window sill replacement is a good solution to fix a rotting or broken window sill on an older wood window. Sometimes it can be a cheaper alternative than removing and replacing the entire window. Sometimes it is necessary to maintain the historical appearance of the house. Sometimes the sill has simply rotted and needs to be repaired. Whatever the reason, it is best to understand how to perform a window sill replacement before you tackle the project.

Unfortunately there is no single method that is the "accepted" procedure for window sill replacement. No, this activity is performed on a "what's best for the present situation" basis. This is because the reason for each window sill replacement is different from window to window. This page will touch on several different options for a window sill replacement so that you are armed with the best method for your project.

First Things First...An Ounce of Prevention

Chances are the reason you are replacing a window sill is that it has rotted due to water infiltration. Over time the windows probably have not been maintained with regular exterior washing (glass as well as frame). Most likely they are not getting a new coat of paint every five to seven years. In addition, the caulk joints around and on the windows probably have not seen the nose of a caulk gun in a long, long time.

A lack of attention to any and all of these things can lead to a rotten window sill. If you are having to perform a window sill replacement because of rot, make sure you work to prevent more rot by performing regularly scheduled maintenance on all of your windows. You should also take the time to make sure the new sill you are installing is properly caulked and sealed at all the edges to prevent additional water infiltration.

CAULK AND PAINT, CAULK AND PAINT, CAULK AND PAINT... Nothing is more important to old wood windows. If you maintain the caulk and paint on the window, you will not have to go through the challenge of a window sill replacement. But enough of the soap box speech, let's discuss window sill replacement.

To Remove the Window or not to Remove, That is the Question

Old wood windows were built at a time when true craftsmen performed the work. The jambs of the windows are built with angles and cuts to fit the profile of the wood window sill and then secured to the window sill, usually from the bottom into the feet. The sills themselves have several different cuts and slopes that are difficult to duplicate. Some would say the best way to replace the window sills is to remove the entire window, frame and all and remove and replace the sill like it was originally constructed.

This is a very labor intensive method and opens the door to other issues like having to reinstall the entire window and reseal it. However, it is the most accurate way and insures a good seal around the new window sill.

The other school of thought is to remove the sill and perform a window sill replacement with the window still intact in the wall. This method is a lot simpler and can be done quickly, but you must pay attention to the details to insure that it is replaced and resealed properly when the job is done.

Either way, a good window sill replacement can be achieved by either method. However, the information on this page will focus on ways to replace the sill without removing the entire window since it is most likely what most people will choose to do when performing a window sill replacement.

Exterior sill removal

One way to replace a window sill is by cutting it out from the exterior without disturbing the interior window stool. This is probably the easiest way to replace a sill because it does not disturb the interior trim, but it is also the most "patchwork" method because it does not completely replace the window sill.

Begin by cutting a 2 to 3 inch "chunk" out of the middle of the sill. This is so you have a little room to work the remainder of the sill out from underneath the jambs on either side. You will need to use a saws all for this cut. You will start the cut where the interior stool meets the sill and cut back to the outer edge of the sill. Be sure to only cut as deep as the window sill itself and not cut into the framing or exterior trim around the window.

Once you have made this cut in two places 2-3 inches apart, you will need to remove the chunk of wood from the sill. Before you can remove it you will need a chisel to separate it at the end where the sill meets the interior stool. Start by chiseling a line across the seam where the sill meets the stool. Once you have a guideline started, chisel down into the chunk until you have enough removed to free the piece from the window.

Before you start to remove the rest of the sill from the opening you will want to remove the exterior brickmould or trim from the jambs. This will make it easier to work the sill out from the jambs when it comes time to remove them. Chances are this trim has probably rotted at the bottom along with the sill and will need to be replaced anyway.

After the center piece and trim has been removed you will begin to remove the two parts of the sill that remain. Begin by cutting a line along the seam of the sill and interior stool the same way you did with the chisel to remove the center piece. You can use the saws all for this, but be sure to keep your line straight and don't cut into the interior stool. Cut from the center gap back towards the jambs on the side. You probably won't get the saws all all the way to the edge of the jamb so you will need to chisel the last bit to get it separated.

Once you have this line cut, you will remove the remaining pieces of the sill from the opening. This will be done by slowly working the pieces loose from the jambs at the end. Chances are the sill is rotted at the end of the jambs so it should be fairly easy to work these pieces loose. You do want to be careful because the jambs may be fastened to the sill and jerking them loose may damage the jambs.

(To this point, keep in mind this description of a window sill replacement is if all goes well. This entire process is done on a case by case basis and you may encounter a challenge that this process is not taking into account. Regardless of what you encounter, you ultimate goal is to get the pieces of the sill removed from underneath the jambs on either side. Use whatever creative measures you need to get all of the old sill and rotted wood out in order to put in the new piece.)

After you have the main pieces of the sill removed, you will want to go back and clean up any little pieces that remain. Make sure you get the line where the sill and the interior stool meet nice and straight for the replacement piece to fit in. After the opening is prepped, you will need to fabricate a new sill to match the old one. This will require you to create a template using the old sill pieces you removed in order to get the correct slope of the wood and cut the piece to the correct depth.

Fabricating the new sill does take some carpentry skills as well as the proper tools such as a table saw, jigsaw, and circular saw. If your carpentry skills are lacking you may want to find someone who can make the new sill piece for you. Have it fabricated in a length twice what you need in case something goes wrong with the first piece.

Once you have the new sill ready, you should be able to slide it into the slot where the old sill used to be. Before pushing it all the way in, apply a bead of wood glue across the edge of the sill that will butt to the window sill side. Tap the new sill into the slot until it sits in its final position.

Once the sill is in place you will want to secure it to the jambs on the side. You can do this by screwing through the bottom of the new sill and up into the jamb. You might need to do this at an angle to get the screw to catch. The goal is to get the jamb to pull tight to the new sill to reduce any gap between the pieces. After everything is secured you will want to generously caulk all seams around the new sill. Reapply the trim on the jambs and caulk all seams and nail holes. When the caulk has cured, prime and paint the window sill and trim.

Interior Trim Removal

A window sill replacement can also be done by removing the interior window stool and apron before removing the window sill. This actually makes the sill removal easier because you can simply cut the entire sill out and replace it instead of having to cut the line along the seam of the sill and the stool as detailed above. However, some people avoid this for fear of damaging the interior trim and wall. If this is not a concern for you, this method of window sill replacement is a good one.

To begin, you will remove the interior window stool and apron from the window frame. Once these are removed you will be looking at the entire window sill. At this point you will cut your 2-3 block out of the middle, cutting all the way across the window sill instead of stopping at the trim. After the block is removed, you will want to follow the steps detailed above for removing and replacing the sill piece.


Performing a window sill replacement can be a somewhat complex project. If you are comfortable with using hand tools and have some basic carpentry skills, you should be able to tackle this project. However, if you do not have a lot of experience with woodworking and demolition, you could run into some problems with this project.

If you do decide to contract a professional to perform the window sill replacement, you can use the instructions above to interview the contractor doing the work. With a basic understanding of the process you can find out if the contractor knows what he is doing. Be sure to have the contractor walk you through their process for a window sill replacement before you allow them to do the work.

A properly executed window sill replacement can extend the life of your older windows by preventing further rot in and around the windows. Be sure to properly seal and paint your windows on a regular basis and you will not have to worry about another window sill replacement for a long time.

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