External frame deterioration comprises the majority of wooden window damage. Cracked caulking, disintegrated weather stripping, and ignored mold are the primary causes, yet the possibility of termites should also be addressed. Aside from broken panes, inside wood window damage is usually limited to gouged sills and frames or cracked caulking. Though unsightly cosmetics, these internal problems seldom result in difficult repair. Therefore this article will focus on replacing the rotted wood portions of an external wood window.
By default, we are assuming a rotted windowsill. For tools, you will need a level cutting surface, a carpenter’s pencil, a quality circular power saw, a tape measure, a Speed Square, a caulk gun, several tubes of exterior window caulking, a tube of wood glue, a small can of wood putty, a length of 2*6 pressure treated lumber, a hammer, a good handful of 3” 8d nails, a ¾” wide wood chisel, a small sheet rock sanding block, a small paint brush, and a sufficient supply of color matched exterior paint. Though detailed instructions as to the use of any of the included carpenter tools is readily Internet searchable, I warn you that the following procedures are dangerous to the inexperienced.
Now for example purposes, assume damage along the outer edge of the wood windowsill. Such decay is often limited to a single corner, and seldom penetrates beneath the base of the sliding window. Replacing the entire wood sill would require a complete removal of the wood window frame. So we repair only the damaged materials.
Along the existing wood windowsill, mark out a triangle that extends from the widest point of the rotted wood to an exit point long ways of the good wood. Be sure to thoroughly overlap so as to remove all traces of the rotted wood. Cutting from the new supply of pressure treated lumber, mark and remove an identical triangle layout. On the wood window to be repaired, use the hammer and chisel to cut out the bad wood. Be careful that the chisel cut remains on the throwaway portion of wood.
Sand the planned joint surface edge free of paint to a measure of one-quarter inch or better. Apply wood glue to the joint portions of both the new wood and the remaining section of the existing wooden windowsill. Along the planned contact joint of the replacement wood, near the upper edge, apply a one-quarter inch bead of wood putty.
Press the new wood triangle into place. Nail to secure. Now press a second layer of wood putty into the joint.
Wait several hours, even into the next day if possible. Sand the joint to a smooth finish. Brush down to remove dust, and then paint as needed.