Repair rotten wood on a window frame or sill

Use this guide when you find flaking paint, soft spots or dark staining on wooden window sills or frames, especially on the bottom rail. You need a screwdriver or bradawl, chisel, wire brush, wood hardener, two-part wood filler, filling knives, sandpaper, exterior primer, undercoat and topcoat, and a dry spell of weather.

Step by step

1. Map how far the rot goes

   Press a screwdriver or bradawl into the wood along the sill and bottom of the frame: sound timber resists, while rotten wood feels soft and the tip sinks in easily. Mark the edges of every soft area with a pencil. If rot covers more than roughly a third of a sill, goes right through the frame, or the window no longer closes squarely, a joiner may need to splice in new timber or replace the section. Small patches under a hand's width are ideal for a DIY filler repair.