How to Fix Window Flashing: Stop Leaks at the Sill, Head, and Sides

Window flashing is the unsung hero of the building envelope — when it works, you never think about it. When it fails, water enters the wall framing where you cannot see it, and by the time interior staining or mold makes the problem visible, the rot and damage can be extensive.

Window flashing is the unsung hero of the building envelope — when it works, you never think about it. When it fails, water enters the wall framing where you cannot see it, and by the time interior staining or mold makes the problem visible, the rot and damage can be extensive. Fixing window flashing early, when you first notice staining or can identify a failed sealant joint on the exterior, is dramatically cheaper than repairing the wall after years of hidden moisture intrusion.

This guide covers diagnosing flashing problems, making accessible repairs from the exterior, and replacing the sill pan flashing in cases that require removing the trim.

What You Need

• Self-adhesive flashing tape 6-inch — rubberized asphalt or butyl, for sealing flanges and laps
• Metal drip cap flashing — aluminum or galvanized, for window head
• Flashing sealant caulk — polyurethane or hybrid, for sealing flashing edges
• Exterior wood filler — for repairing minor rot damage in trim
• Pry bar trim puller — for removing exterior casing without damage
• Housewrap tape — for sealing housewrap-to-flashing transitions

Step 1: Diagnose Where the Water Is Getting In

Water appearing inside the house near a window can come from several different points. Knowing which joint has failed tells you what kind of repair is needed and how much access you will need.

Inspect the exterior from close range. Use a ladder to examine the window from outside. Look for:

• Caulk joint at the window head (top): A cracked, shrunken, or missing caulk bead between the top of the window frame and the casing or siding above it is the most common and most accessible failure point. Water running down the wall face enters this gap and is channeled behind the window frame.

• Caulk at the sides: The vertical joints between the window frame’s flanges and the siding or casing should be continuously sealed. Gaps at these joints allow water to track behind the window frame under wind-driven rain conditions.

• Sill condition: Water staining or peeling paint concentrated at the bottom of the window, or soft wood at the sill exterior, indicates the sill pan flashing has failed or was never installed correctly. This is the most serious failure.

• Missing or corroded head drip cap: The metal drip cap above the window head directs water out over the siding rather than down behind the casing. If the drip cap is missing, corroded through, or the siding was installed over it (so water behind the siding hits the exposed window head flange), water enters at the top.

Test with a hose. With a helper watching from inside, run a garden hose slowly over specific areas of the window exterior — start at the sill, then the sides, then the head. Have the helper call out when water appears inside. This pinpoints the entry location quickly.

Check above the window too. Water can enter the wall above the window, travel down behind the sheathing, and appear at or below the window interior. Inspect the roof-to-wall joint, any penetrations above the window (light fixtures, electrical, plumbing), and the siding condition for signs of water entry above the window opening.

Step 2: Accessible Repairs — Caulk and Exterior Tape

If the failure is at the head caulk joint or the side flange joints, you can make a durable repair from the exterior without removing trim.

Re-caulking the head and sides:

1. Remove all existing caulk from the failed joint using a utility knife and a caulk removal tool. Do not caulk over old caulk — the new caulk will only adhere to the old layer, and the old layer’s failure mode will eventually carry the new caulk with it.

2. Clean the joint surfaces with a stiff brush to remove any remaining caulk residue, dust, or loose paint. Wipe with a dry cloth.

3. Apply painter’s tape to both sides of the joint to produce a clean bead line.

4. Apply polyurethane or hybrid polymer flashing sealant (not standard silicone) into the joint. Polyurethane sealant adheres to more materials, accepts paint, and remains flexible over a wider temperature range than standard silicone.

5. Tool the bead immediately with a wet finger or a caulk tool, pressing it firmly into the joint and ensuring good contact on both sides. Remove the painter’s tape before the caulk skins over.

6. Allow full cure before rain exposure — typically 24–48 hours depending on the product.

Reinforcing with flashing tape:

For joints that have failed repeatedly or show signs of movement (gaps that open and close seasonally), flashing tape provides more durable protection than caulk alone.

Cut a piece of self-adhesive flashing tape to span the failed joint with at least 2 inches of overlap on each side. Peel the backing and press the tape firmly into place, starting at the center and working outward to avoid bubbles. Use a roller or a plastic J-roller to press the tape fully to the substrate — especially at the edges and corners.

At the window head, the flashing tape goes under the siding edge, not on top of it. If you cannot slide the tape under the siding, the most complete repair requires removing the lower course of siding above the window (often just one board) to lap the flashing tape correctly.

Step 3: Repair a Failed Head Drip Cap

If the metal drip cap above the window is missing, corroded, or was installed under the window’s nailing flange rather than over it, this is a significant water entry point.

Removing the old drip cap: Use a flat bar to carefully lift the lower edge of the siding above the window. The drip cap slides out from behind the siding — do not pry the siding far out, just enough to free the cap. Remove all old caulk from the window head.

Installing new drip cap:

1. Apply a bead of flashing sealant to the top flange of the window frame where the drip cap will rest.

2. Slide the new aluminum drip cap behind the lower course of siding and over the window head flange. The cap’s vertical leg goes against the sheathing; the horizontal leg laps over the window head.

3. Seal the joint between the bottom of the drip cap and the window head with flashing sealant. Do not caulk the top of the drip cap to the siding — this joint must remain open to allow any water behind the siding to drain out.

4. Apply a layer of self-adhesive flashing tape over the drip cap’s vertical leg and onto the sheathing above it, if the sheathing is accessible. This seals the joint between the cap and the wall.

Step 4: Replace Sill Pan Flashing

A failed sill pan is the most common cause of serious water damage at windows — and the one that requires the most access to repair correctly. If the sill pan has failed, a caulk repair on the exterior will only delay the damage.

This repair requires removing the exterior window casing (the decorative trim frame around the window exterior) and often the lowest course of siding directly below the window. The window itself typically stays in place.

Remove the exterior casing. Use a trim puller or a stiff putty knife to carefully pry the casing away from the wall. Score the paint line between the casing and the siding before prying to avoid tearing the siding face. Remove all fasteners.

Inspect what you find. With the casing removed, you can see the window’s nailing flange against the sheathing, any existing flashing membrane, and the condition of the sheathing. Look for:

• Black staining or soft sheathing (rot) — soft areas need to be cut out and patched before re-flashing
• Old flashing tape that has lost adhesion — remove it and clean the surface
• No flashing at all (common in homes built before the 1990s) — the repair is an installation, not just a repair

Install the sill pan:

1. Cut a piece of self-adhesive flashing tape wide enough to span the window opening plus 6 inches on each side (or use a purpose-made sill pan product). The standard width for most residential windows is 9-inch tape.

2. Fold end dams at each corner. These are vertical folds that rise up the rough opening sides and prevent water from draining off the sill pan corners into the wall cavity. Crease the corners sharply and seal any overlapping areas with additional flashing tape.

3. Peel and stick the sill pan into the rough opening sill, pressing firmly. The sill pan should slope slightly outward — if the rough opening sill is perfectly level, add a thin shim at the back edge to create a slight outward pitch.

4. Lap the side flashing pieces over the ends of the sill pan by at least 2 inches.

5. The head flashing goes last, lapping over the top of the side pieces.

6. Reinstall the window casing using corrosion-resistant fasteners. Caulk the joint between the casing and the siding on all sides with polyurethane sealant. Do not caulk the joint between the casing bottom and the sill — leave a small drainage gap.

Preventing Future Flashing Failures

Flashing fails early when dissimilar materials are in contact (some sealants degrade certain membranes), when caulk is applied to dirty or wet surfaces, or when the lap order is reversed. The cardinal rule of flashing: water runs downhill, so every lap must go over the piece below it. Any lap that goes under the piece below it is a funnel.

Inspect your window exterior caulk lines every two years — especially the head joint — and touch up any cracks before they open far enough to let water in. A tube of polyurethane caulk and 20 minutes of touch-up work every other year prevents years of hidden water damage.

Related Reading

• How to Fix a Leaking Window
• How to Caulk Around Windows
• How to Fix a Damaged Window Frame