Wooden window sills are among the parts of a house most vulnerable to the weather. If they are not well protected with paint or varnish they will rot, then window sill repair and replacement is required. Small patches of rot are easy to repair. Deal with them instantly to prevent them spreading. Test for rot by pushing a sharp knife or other tool into the sill. The tool will meet resistance in sound timber, but pass easily through rotted wood. Begin window sill repair by cutting out the rolled section. Gut well into sound wood on each side to form a wedge-shaped section. Burn the rotted timbers. To repair modern sills, you may be able to buy a matching piece of standard window sill molding. For older window sills, it might be necessary to plane down piece of square timber to match. Remember that drip channels will need to be cut into the section. Brush wood preservative well into the cleaned edge of the sill and soak the replacement piece in it thoroughly, Fix the new timber into place with dowels driven into the sill. You need two dowels, no matter how small the window sill repair is, and extra dowels at 100 mm (4 inches) intervals for longer pieces. To strengthen the repair, fix steel strips to the underneath of the sill. Corners can be repaired similarly. Concrete sills can also be repaired, too. Fill small cracks with a Decorators filler suitable for outdoor use. Paint the sill with either masonry or gloss paint to conceal the repair.
Large holes and cracks should be filled with cement; buy a small bag from a DIY store. First, though, treat the crack with a pva building adhesive, and add some of the adhesive to the mortar to make it more workable, as recommended in the adhesive instructions. A deep hole is best filled in two stages. Smooth the finishing coat down flush with the edge of the sill, and then disguise the window sill repairs with paint. A framework is required if a concrete sill needs extensive repair. Make it of stout timber at least 25 mm thick. Hold the formwork in place by nailing it to blocks of wood fixed to the wall with masonry nails. Besides, drip channels prevent rainwater from running over and under a sill back on to the wall under the window opening. The drenching that results could cause the foundations or the interior of the house to become damp. Drenching is to be avoided especially under a window opening, where it could be conducted to the frames and eventually cause them to rot. One or more grooves formed, during manufacture, into the underside of a timber sill or molded into a concrete one will prevent the rainwater reaching the wall, causing it to drip harmlessly off on to the ground instead. Replace window sill when required. It is important to keep drip channels clean in window sill replacement; otherwise they will not function effectively. Dirt and spider webs tend to gather there, so scrape or brush them out regularly. If there are no drip channels under your sill, it is worth fitting them. Using a router, you could cut out a couple of grooves in a timber sill while it is In situ. This, however, is not an easy task. An alternative that works just as well as a groove is to glue and pin a couple of lengths of half-round molding underneath the sill. A gap between the moldings forms an effective groove. On concrete, merely glue the molding into place. Window sill repair or replacement would not cost too much though, but you can still save some when you do it yourself.