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DIY Window Sill Repair

Updated July 20, 2017

Weather-damaged wood window sills are common in regions with severe weather. Damage to a window sill is typically caused by exposure to the elements, usually occurring when there is too much time between maintenance periods. In severe cases, the window sill rots and must be repaired or replaced.

Repair Options for a Weather-Checked or Rotted Window Sill

There are two repair options for a wood window sill. Although not recommended, the sill may be treated and covered with an aluminium or vinyl extrusion. These extruded products typically will not exactly match the original sills, and if not fitted perfectly may trap moisture and cause additional problems with the wood. The preferred method is to utilise two epoxy treatments that will leave the sill looking like the original.

Cleaning and Preparing the Sill

The wood sill typically requires repair due to weather checking of the wood or, in extreme cases, due to rot. A chisel, wire brush and sharp-pointed probe may be required to clean the weather checks or remove any rotted wood. Following the complete cleaning of the sill and removal of rot, the sill must be allowed to dry completely prior to application of the epoxies. If necessary, cover with plastic to prevent the accumulation of moisture. Depending on the climate and weather, it may be several days or even weeks before the wood is completely dry.

Treating the Sill with Wood Consolidant

Wood consolidant is applied to any weather checks with the use of a squeeze bottle with a very narrow spout. Any areas that were pitted or gouged during the removal of the rot should also be treated with the consolidant. The consolidant, which should be deep penetrating, will waterproof and regenerate rotted, dried out or spongy wood.

Filler of Wood Putty Replacement Compound

After the consolidant has been absorbed and dried thoroughly, a filler of shrink-free adhesive putty wood replacement compound should be applied. This application should be forced into any checks, pits or crevices, and smoothed with a flat wooden applicator like a tongue depressor. As the filler begins to set up, it should be smoothed again and allowed to dry. If the filler slumps during cure, a second application may be required. The filler should be sanded smooth and painted with two coats of exterior paint. Regular maintenance of all exterior millwork, including windows, should be performed every two years and should include scraping, caulking and painting.

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About the Author

William Frick is a full-time writer. Frick began his professional career as an engineer working in aerospace. He has accumulated over 20 years experience in manufacturing management and has progressed from supervision to executive management.