Wood window sills and frames are common in many homes. Window sills are the horizontal piece that sits across the bottom of the window and typically juts out into the room. The timber window frames are located on the outside of the house and do as the name suggests – they frame the window. Over the course of time and exposure to rain, snow and humidity unprotected timber will begin to rot. Wood rot is caused by a type of fungus that works its way into the timber and makes the wood spongy and soft. Soft spongy timber creates and entrance for water to run through and make its way into the home. If water or any type of moisture makes its way into the home it will damage the studs, floor joists along with floors and other supports. Addressing wood rot as soon as it is noticed is necessary to maintain the structural integrity of the home.

Better than fixing a wood rot affected window sill or window frame is preventing wood rot from occurring in the first place. Typically, window installers and building contractors paint window seals and frames with exterior grade latex or a penetrating timber sealer. Paints or sealers are not only applied so the house will look nice. The sealer or paint is applied to protect the wood from a variety of water issues including bowing, warping, twisting, cracking and rotting. Paints and sealers penetrate and coat the timber to form a barrier against damage. Keeping the frames and sills painted will in most cases prevent the fungus from entering the timber. Window frames and sills should be scraped, sanded and repainted every two years. If the climate in which you live has severe weather changes, painting or sealing the frames and sills yearly may be required.

Some window frames and sills are too rotted to repair and will require replacement. If the timber rot has penetrated through the sill or frame to the underlying materials, replacing the pieces is necessary. If the timber rot encompasses more than 40 percent of the face of the timber, replace the pieces. If the frames or sills have patches of rot – repair the sections. If the corners have been degraded by wood rot – fix them.

Checking Window Frames and Sills for Signs of Wood Rot

Examine the window sill thoroughly from the tops, sides and underneath. Sometimes the wood rot is very obvious. Obvious wood rot looks as if the wood has been chewed.

Feel the wood. Press on the wood. If the wood feels soft or gives in to your grip, it is rotted. Soft, spongy wood is a major sign.

Lightly jab a pick into the sills and frames to determine if the wood is rotted. If the pick easily enters the wood – you have rot.

Preparing Window Sills and Frames for Repair

Put on safety glasses and work gloves.

Scrape old paint off the wood surface with a paint scraper so that you can see what you are working with. Scrape the old paint from the entire frame in preparation for repainting later. Sand away any remaining paint with 80-grit sandpaper wrapped around a sanding block.

Use a framing or mortise chisel to carve out the sections of wood. Strike the end of the chisel with a mallet if the digging gets difficult. Continue to gouge out the sections of wood rot with the chisel and mallet until no spongy, soft or springy wood remains. You will be exposing the good salvageable wood.

Brush away the debris, bits of wood and dust with a small hand held whisk broom. If you have a shop vacuum, vacuum the wood surfaces to remove all dust and debris.

Repairing Window Frames and Sills

Equip a power drill with a ¼ inch bit. Drill holes into the wood every 1 to 2 inches into the wood about ¼ inch deep.

Make repairs to one section of the frame at a time or one whole sill at a time.

Dip a paintbrush into an epoxy liquid consolidant, which soaks into the wood and binds the fibers together in preparation for repair, and brush on the epoxy until the wood no longer absorbs the consolidant. If you are repairing framing or sills near an edge that is nailed or screwed into a subframe, drive in screws next to the original screws to make the repair stronger.

Let the consolidant dry until it is tacky to the touch.

On a disposable plastic plate, scrap piece of wood or other disposable holder, mix equal amounts of two part epoxy repair compound with the enclosed mixing stick. If you do not have a stick use a popsicle stick.

Dip a putty knife into lacquer thinner and scoop up the epoxy repair putty.

Press the repair putty deeply into the damaged wood surface to roughly rebuild the sill or frame. Build the repair putty up so it sits about 1/8 to ¼ inch higher and or wider than the frame or sill.

Let the epoxy patch dry until it is hard and solid. Drying times will vary depending on how thick the epoxy application is. Thinner epoxy will take much less time to dry than thicker patches.

Wrap 80-grit sandpaper around a sanding block and sand the hardened epoxy surface to smooth it out and make it level with the surrounding wood. Sand sides and edges to create a straight edge.

Wipe the frame and sill with a tack rag to remove bits of sanding dust.

Wrap 180-grit sandpaper around the sanding block and sand the surfaces a second time to make them smooth.

Wipe the area with a tack rag again to remove all sanding dust.

Complete repairs to all frames and sills.

Pour epoxy primer into a paint bucket. Dip a paintbrush into the primer and coat the window sill and frame. Be sure to coat the underside and small edges to completely seal out water.

Let the primer dry for three to four hours.

Pour epoxy paint into paint bucket. Dip a paintbrush into the paint and apply a coat to the window sill and frame. Let the paint dry and apply a second coat.