Repairing (or Replacing) Rotted Windowsills

Both good weather and bad can damage windowsills over the years.

The sill--the flat shelflike base of the window frame--is prone to damage from rain, snow and ice, as well as the sun’s ultraviolet rays. Damp-dry cycles turn minor flaws in the paint into cracks, through which fungus invades the wood.

Warning signs of rot are mildew stains and peeling or checkered paint. To learn if rot is present, tap the area with a hammer or the handle of a screwdriver. If it feels spongy or sounds hollow, probe with an ice pick or awl to determine how deep the rot goes. The pointed tool will readily penetrate rotted wood until it hits sound wood.

If the rot is superficial, scrape or sand down to sound wood. If the damage is deeper but not extensive, epoxy resin and exterior-grade epoxy patching compound, available at hardware or boating supply stores, can be used to fill the wood. If the rot is widespread, it may be necessary to replace the sill.

Filling Rotted Wood

* Scrape away the soft rotted wood. Let the sill dry out. This may take as much as a month. Tape plastic over the sill if the weather looks bad.

* When the wood is dry, drill a honeycomb pattern of one-quarter-inch holes into (but not through) the affected wood. Put a piece of tape on the drill bit as a depth gauge. Drill the holes at an angle to make a more secure base for the epoxy treatments.

* Wearing rubber gloves, mix the two parts of the epoxy resin in a plastic bottle and slowly inject the mixture into the holes. Over the course of several days, the resin will seep into the pores in the wood, replacing the decayed wood with plastic that is as workable as wood. Add more resin as the first dose sinks in. Follow label directions for drying time.

* Again following label directions, mix the two parts of the epoxy patching compound. Apply it with a putty knife. Fill slightly over to allow for sanding. Filler hardens quickly, so work rapidly and don’t mix more than you need.

Replacing a Sill

If the damage is extensive, you can sheath the sill with metal or fiberglass. But the underlying rot will remain and gradually become more extensive. The ideal solution is to replace the sill. Here’s how:

* Raise the sash. From inside the house, remove the piece of wood on top of the sill (known as the inside sill or the stool) and the decorative wood below the sill (the apron).

* To remove the old sill, saw it in thirds, avoiding damage to the siding. Pry out the pieces. If they won’t come out, split the sill into smaller pieces with a chisel. But try to keep the old sill as intact as possible for use as a template. Check that the area beneath the sill is not affected by the rot. Remove any nails.

* Lumberyards sell windowsill stock in various thicknesses. Using the old sill as a guide, cut a new sill. Round the interior edges slightly with sandpaper so that it installs more easily.

* Seal the new sill with wood preservative and let it dry before sliding it into position.

* Nail the new sill to the trim with galvanized finishing nails. Sink the nailheads and fill the holes with wood putty. Fill any gaps where the sill meets the trim wit paintable exterior caulk.

* Reinstall the inside sill and the apron.

* Whether you replace or repair a sill, sand and prime it, then finish with two coats of good-quality exterior-trim paint to ensure its long life.