How to tell if a window sill is rotted

I'm Sam, a framer and handyman. For about 12 years I've been building houses from the frame up around Land O' Lakes and Pasco County, and fixing what goes wrong in them. In Florida, windows take a beating from rain and humidity. Rot often starts at the sill — the flat piece at the bottom of the window — or at the interior stool (the ledge inside) and the bottom of the side jambs. Paint can hide it for a long time.

If you catch it early, you can often stop the water and plan a solid repair. If you ignore it, the rot can work into the trim and, on a bad one, into the framing around the opening.

Look before you poke

Start with your eyes, from inside and outside.

• Peeling or bubbling paint on the sill, the stool, or the lower jambs.
• Dark staining or a gray, fuzzy look to the wood.
• A swollen, wavy, or uneven interior stool. That usually means water has been sitting in the wood.
• Gaps in the caulk where the trim meets the window or the wall.
• Soft-looking wood at the outside corners of the sill, where water sits.
• A window that suddenly sticks, rattles, or doesn't sit square.

On vinyl windows, also look at the weepholes along the bottom of the frame. If they're clogged, water can sit in the frame and work into the wood behind it.

Probe gently

Solid wood pushes back. Soft wood doesn't.

Take an awl or a flat screwdriver and press — don't stab — into the bottom of the outside sill, the corners, and the interior stool. If the tip sinks in, that wood is gone, even if the paint still looks decent. Check under the lip of the sill and at the ends, where water runs off, or doesn't.

Don't go chopping chunks out of the wall just to "see how far it goes," especially if the wall feels spongy. You're inspecting, not demolishing.

Find the water path

Rot is a water problem first and a wood problem second. If you only paint over soft wood, the rot keeps going.

Common causes I see around here:

• Failed or missing caulk at the joints
• A sill that tips back toward the house instead of shedding water out
• Clogged weepholes on vinyl windows
• Missing or short flashing above the window
• Sprinklers hitting the trim all season
• A clogged gutter dumping water down the wall

A temporary stop — clearing weeps, recaulking sound joints, and keeping water out — can buy you time while you plan the repair. That is not the same as a fix. The damaged wood still has to be cut back to solid material and replaced, and the water path has to be corrected.

What you can handle, and when to call a pro

A small area of soft exterior trim, with solid wood behind it and a clear leak you can stop, is a reasonable weekend repair for a careful homeowner. Cut back to sound wood, prime all sides of the new piece, seal the joints, and make sure water can drain away.

Call someone who does this work if:

• The sill plate, jack studs, or sheathing feel soft
• The window has dropped, pulled away from the wall, or won't operate
• Soft wood runs past the trim into the wall
• You can see daylight or feel air moving around the frame
• You aren't comfortable on a ladder or working around the opening

Those are signs the framing may be involved. That's past a weekend patch, and it's not a place to guess. Don't chip at structural wood just to map the damage if the wall feels spongy. Get a pro to look at it.

This is general information, not a substitute for a licensed pro. If the wall feels soft, stop and have it checked.

I don't have a standalone window-rot guide in the shop yet. If damp Florida air and moisture in the house are part of what you're dealing with, the Florida Humid-Home Mold & Mildew Fix-It Guide — $5 walks through what to check in a humid house. You can also see all the guides and tools.