This is the most common cause for wooden sills. Wood is porous; when the paint or sealant fails, water seeps in. Over time, the wood swells with moisture and then dries out, creating a cycle of expansion and contraction that eventually splits the wood. If the wood feels soft or spongy, you aren't just dealing with a crack—you’re dealing with rot.
If you have vinyl windows, ensure the small drainage holes (weep holes) are clear of debris. If they clog, water can pool on the sill and cause damage. Final Thoughts cracked window sill
To keep your window sills in top shape, follow these maintenance steps: This is the most common cause for wooden sills