Window Rot: Why Windows Rot, the Warning Signs, and When It Can Be Repaired

Wood windows rarely rot all at once. Rot starts in one or two places where water sits, spreads slowly, and is often hidden under paint until the wood is soft. Catch it early and a window can usually be repaired. Leave it, and it spreads into the sill, the frame and eventually the wall around it. Here is why windows rot, what to look for, and how to tell whether yours can be saved.

Why windows rot

Wood rot is a fungus, and it needs moisture to grow. Dry wood does not rot. So the question with any rotting window is where the water is coming from and why it is not drying out.

• Failed paint and glazing putty. Paint and putty are what keep water out of the wood. Once they crack and peel, rain soaks into the bare wood, most often at the bottom rail of the sash and on the sill.
• Condensation inside. In winter, moisture condenses on cold glass and runs down onto the bottom of the sash and the interior sill. Season after season, that is enough to start rot from the inside.
• Water that cannot drain. A sill that has settled flat or back toward the house, drainage paths blocked with caulk or paint, or a missing drip cap above the window all hold water where it does damage.
• Overflowing gutters and splash. Clogged gutters dump water down the face of the house and over the window head. Lawn sprinklers and roof runoff without gutters do the same to lower windows.
• Caulk in the wrong place. Caulking over joints that are meant to drain, or over cracks in wood that is already wet, traps moisture rather than keeping it out.
• Age. None of this is fast. Rot is usually the result of years of small maintenance gaps.

Warning signs of window rot

Outside, look for:

• Peeling or blistered paint on the sill and the bottom of the sash
• Cracked, missing or loose glazing putty around the glass
• Open joints where the sill meets the side jambs
• Wood that feels soft or spongy. Press it with a screwdriver: sound wood resists, rotted wood gives or crumbles.
• Dark staining or a gray, fibrous look to bare wood

Inside, look for:

• Water stains or discoloration on the sill, the trim or the wall below the window
• Peeling or bubbling paint and wallpaper near the window
• A musty smell, which means wet material somewhere
• Black spots of mold on the sill or the bottom of the sash
• A sash that has started to stick or no longer closes tightly

Interior signs often mean the problem has gone beyond the window itself, so it is worth having the wall around it checked too.

Can a rotted window be repaired?

Very often, yes. Rot is usually confined to the parts of a window that hold water: the sill, the bottom rail of the sash, and the lower ends of the frame. Those areas can be cut out and rebuilt with new wood, or consolidated and filled when the damage is shallow, then primed and painted. The rest of the window, including old-growth wood in older homes that is often more rot-resistant than new lumber, stays in place.

Replacement starts to make more sense when rot runs through most of the frame and sill, when the window has other problems such as failed insulated glass across many units, or when the cost of repairs approaches the cost of a good new window. A free inspection will tell you which side of that line you are on. See wood rot repair and window restoration.

How to keep windows from rotting

• Keep paint and glazing putty intact, and touch up the sills and bottom rails first, since they take the most water.
• Keep gutters clear, and make sure downspouts and sprinklers are not soaking the walls.
• Wipe up winter condensation, and run bath and kitchen fans to bring indoor humidity down.
• Do not caulk over drainage paths or seal wet wood.
• Check windows once a year, ideally in fall, with a screwdriver test on the sills.

Window Makeover has repaired and restored wood windows in the Indianapolis area since 1999. If you are seeing any of these signs, schedule a free inspection or call 317-770-4550.