Do I Have to Replace Siding if the Inside Window Sill Is Rotting?

A Stitch In Time

• If the rot is noticed before it takes an aggressive hold on the sill, it may be possible to repair rather than replace the wood and save the siding. If a thin-blade screwdriver cannot be pressed more than 1/2-inch into the sill, this is worth a try. Use whatever tools will suit the purpose to dig away the soft wood until it is all removed and only sound wood remains. Use a rot-killing preparation to ensure the problem does not reoccur then build the shape of the sill back up to its original dimensions with two-part elastomeric auto body filler, otherwise known as bondo. If this process is successful and the rot did not transfer to the siding, there is no need to disturb or replace it.

Preventative Maintenance

• Determine why the rot occurred in the first place. If the cause was nothing more than poor maintenance, such as pooling condensation on the inside of the window, then there may be no further projects to undertake. If, however, water was allowed to reach the interior sill from a remote area, such as by running down the back of the siding from a leak higher up the outside wall, the causes as well as the symptoms must be addressed. If the siding is leaking because of large-scale deterioration, rather than because of one ruptured seam or a puncture, it should be replaced. Siding that is softened but still complete can often be saved with two-part epoxy rot-blocker and reformer. Such products are available from home improvement stores and, once mixed to the manufacturer’s instructions, can be brushed onto the deteriorated wood.

Sill Replacement

• Traditional wood window sills can be purchased at home improvement centers and lumberyards. Vinyl and PVC replacements can often be bought from companies that specialize in double-glazed windows. For certain applications, such as the restoration of an historic building, it may be necessary to fabricate a sill that matches others in the room from a particular wood type; most lumberyards can accommodate this request.

Window Sill Removal

• To replace the window sill, you will need to remove the window to protect the glazing then remove the moldings from around the inside of the window frame. If the sill is located inside the frame liners -- meaning they were installed first then the sill was simply fastened in place between them afterward -- it should be possible to pry it up and out until it is free; if it is located beneath the liners, they must be removed first. Retract the sill fasteners then pull the sill into the room. It should be possible to do this without disturbing the siding outside the window so long as it is sound. Install the replacement sill then reverse the removal process to reinstall the window. Alternatively, you may be able to draw the sill out from beneath the window without removing it. Use a reciprocating saw to make a cut through the center of the sill then a rotary tool to cut through any fasteners. If the halves can be pried upward and extracted from the wall, it should be possible to clean out the resulting hole then press a replacement sill into place. Again, this project can be carried out without disturbing the siding.