This story shows you how to replace a windowsill.
A windowsill is the part that extends outside the house; what is commonly called an interior sill is technically the stool. A wood sill is one of the few exterior surfaces that presents a horizontal face to theelements, making it vulnerable to rot.
Check the extent of rot by poking with a screwdriver; where you can easily push in, the wood is rotted. If the sill is rotted near the exterio casing, the casing may be rotted as well. If the sill is rotted all the way thorugh, the house's framing or siding may be rotted as well.
If the rot is localized, use wood hardener and two-part epoxy filler to repair it. Replacing a sill is tricky but only requires basic carpentry skills. If the old sill is so rotted that you cannot use it as a template, measuring and cutting will be more difficult.
Buy special sill stock or cut the sill our of 2x6 (1 1/2 x 5 1/2 inches). If the sill is 1 inch thick, use 5/4 decking or buy special sill stock. Use pressure-treated lumber to resist rot.
About half a day to remove an old sill and replace it.
Hammer, tape measure, flat pry bar, circular saw or poer mitersaw, handsaw, small hacksaw, large slip-joint pliers, reciprocating saw, drill, sawhorses
Basic carpentry skills
Spread a drop cloth on the floor near the window and protect the plants outside. Set up a pair of sawhorses.
Lumber to match the sill in thickness and width (its OK if it's wider), shims, galvanized nails or decking screws, primer and paint.
Start by removing the stool. You may need to remove the interior casing molding first. Some stools are rabbeted and overlap the sill. Others are straight and simply butt up against the sill. To remove a rabbeted stool, pry it up from the outside. Pry out a straight stool from the inside.