Rotten window sills are one of the most common calls we get, and this job in Lower Merion Township in September 2025 is a textbook example.
What we found
The house is stucco with painted wood window trim. The September 22 photos show the bottom of the trim soft and split at the corners, where water runs off the sill and soaks into the end grain. A downspout runs right below, so the wall stays wet after every rain.
Lower Merion TwpSep 2025
Lower Merion TwpSep 2025Cutting it out
The sill and the bottom of the trim were cut out until we reached solid wood. With the old wood out, you can see how far the rot went under the window.
Lower Merion TwpSep 2025Rebuilding in PVC
The new sill and trim are cellular PVC. It does not rot, holds paint and can be shaped like wood. Behind it goes a layer of flashing so any water that gets past the sill is turned out over the stucco, not into the wall.
Lower Merion TwpSep 2025Why sills rot first
- Water sits on a flat or back-pitched sill instead of running off.
- End grain at the corners soaks up water like a straw.
- Caulk at the joint between sill and casing cracks within a few seasons and holds water in.
If you push a screwdriver into the corner of a sill and it sinks in, the rot is already there. The earlier it is fixed, the less of the window has to come out.
