If you’ve done all the above and the damp returns within a month, or you see a white, fluffy salt deposit (efflorescence) on the sill, your problem is structural. Likely causes:
To prevent damp under window sills from becoming a recurring issue:
We see this all the time: damp patches appearing directly under the window sill.
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Every spring, walk around your house with a caulk gun and a flashlight. Check under every window sill. If you see a hairline crack, seal it that day . Clean weep holes. Trim back any plants touching the sill.
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Don’t just paint over it! The problem will come back. 1️⃣ Check the exterior sealant for cracks. 2️⃣ Improve ventilation in the room. 3️⃣ If the plaster is soft, you may need a professional to treat the area. If you’ve done all the above and the
That damp patch is rarely rising damp from the ground. It’s almost always or condensation . Run this quick checklist:
🧱 Condensation: Cold air outside + warm air inside = water droplets forming on the sill. 🪟 Failed Sealant: That tiny crack in the exterior caulking is an open invitation for rainwater. 🕳️ Gaping: Over time, the gap between the window frame and the wall opens up.
Damp under the window sill is rarely a disaster. 80% of cases are fixed with a paperclip (weep holes), a tube of silicone (exterior seal), or a dehumidifier (condensation). Tackle it this weekend before it becomes next year’s mold problem. Check under every window sill
Ignoring damp under window sills can lead to a range of problems, including:
Swipe left to see how we fixed this one! ➡️