How to Fix a Leaking Basement Window: Window Wells, Caulking, and Sill Flashing (2026)

Frequently Asked Questions

Why is my basement window leaking even though it is above the waterline?

Above-grade basement windows can still leak for several reasons: the window well has filled with soil, mulch, or debris and the drain at the bottom is clogged, allowing water to pool and press against the frame. The caulk sealing the window frame to the masonry has cracked or separated, letting water wick in behind the frame. The sill flashing — the metal or vinyl piece that sheds water away from the bottom of the window — is missing, bent, or corroded. Rain that falls directly into an uncovered window well amplifies all three problems. Fixing the well drain is almost always the first step because even the best caulk cannot hold against standing water.

How do I clear a clogged window well drain?

Start by removing all debris from the well: leaves, mulch, soil, and insects. Dig down to the drain opening at the bottom of the well (typically a 4-inch perforated pipe or gravel pit). Use a plumber's snake or garden hose to flush the drain line — run water and watch whether it exits at the foundation footing or daylight drain nearby. If the drain is completely blocked or was never installed, the fix is to remove the bottom 12 inches of gravel, lay new perforated drain pipe surrounded by drainage fabric, backfill with clean washed gravel (not pea gravel, which compacts), and recheck drainage. A functional well drain keeps water from ever building up against the frame.

What type of caulk should I use on a basement window frame?

Use a polyurethane caulk (such as Sikaflex or Tremco) or a silicone-polyurethane hybrid rated for below-grade masonry applications. Standard silicone caulk works on glass but does not bond well to concrete or CMU block, and cheap latex caulk degrades within one season below grade. Apply caulk to the joint between the window frame and the masonry on all four sides, pressing into the joint with a caulk tool. On the exterior, also caulk the top and sides — leave the weep holes at the sill bottom open. Do not caulk the interior perimeter alone; the leak originates from outside and must be sealed there.

What is sill flashing and how do I install it?

Sill flashing is a bent piece of metal (typically galvanized steel or aluminum) or pre-formed vinyl that sits at the base of the exterior window opening. It slopes outward so that water hitting the sill runs off and away from the frame rather than pooling at the joint between the frame and masonry. To install: remove the old sill trim or flashing if present. Cut aluminum flashing stock to length (available in rolls at hardware stores) and bend to a 10-degree slope using a sheet metal brake or a straight edge. Slide the back leg under the window frame, seal the top edge with polyurethane caulk, and secure with masonry screws. Lap the side sections over the sill piece so water flows continuously outward.

Do window well covers actually stop leaks?

Yes — a properly fitted window well cover eliminates the primary cause of well flooding, which is direct rainfall into the well. A polycarbonate dome cover lets in light while shedding rain off to the sides of the well. A corrugated metal cover is less transparent but more durable. Covers attach to the foundation wall with expansion anchors and to the window frame or sill with clips. They do not replace a working drain — if your drain is clogged, rainfall that hits the cover still runs into the well from the sides. Fix the drain first, then add the cover as a secondary defense. Covers also keep out rodents and debris, which cause drain clogs in the first place.

My window well fills with water every heavy rain even with a cover. What is wrong?

If the well floods despite a cover, the problem is almost always the drain. The drain pipe from the well bottom is either clogged, collapsed, or was never connected to a functional outflow. Check where the drain exits: it should daylight on the exterior of the foundation below grade or connect to a sump pit. Use a drain snake to clear any blockage. If the drain is connected to a sump: confirm the sump pump is working. A secondary cause is grading — if the ground around the well slopes toward the foundation, surface water channels into the well faster than the drain can handle. Regrade the soil or install a shallow French drain around the well perimeter to redirect surface flow away from the house.

Leaking basement window fix: (1) Clear and repack the window well drain with clean gravel so water exits rather than pools. (2) Remove old caulk from the window frame perimeter and apply fresh polyurethane or silicone caulk rated for below-grade use.

A basement window that leaks during rain is almost never a window problem — it is a drainage and sealing problem around the window. Fix the water path first and the window stays dry.

What You Need

• Polyurethane masonry caulk — bonds to concrete and masonry, stays flexible below grade
• Window well cover — polycarbonate dome — sized to your well diameter
• Aluminum flashing roll — for sill flashing fabrication
• Perforated drain pipe with sock — for relining the well drain
• Drainage fabric landscape cloth — to wrap the gravel pit and prevent soil intrusion
• Masonry screws — Tapcon 3/16 — for anchoring flashing and well cover brackets

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Step 1: Assess the Leak Source

Before touching caulk or flashing, spend five minutes finding where the water is actually entering.

During a rainstorm (or with a garden hose): Stand in the basement and watch the window frame. Mark where water first appears with painter’s tape.

• Water at the sill bottom: the well drain is pooling or sill flashing is missing
• Water along the sides of the frame: caulk failure or no caulk present
• Water weeping through the masonry below the window: the leak is higher up and tracking down — check the sill and frame perimeter above

Check the window well exterior: Dig into the gravel at the bottom. You should find a perforated pipe or a gravel pit that leads to a drain. If it is mud and compacted debris with no visible drain, the drain is either clogged or was never installed.

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Step 2: Clear and Rebuild the Window Well Drain

This step alone resolves the majority of basement window leaks.

Remove all well contents down to the drain:

1. Lift out any existing gravel by hand or with a trowel. Set it aside — you will not reuse it if it is mixed with soil.
2. Locate the drain opening at the base of the well. It is typically a 4-inch pipe opening or a gravel column leading to a perforated pipe below grade.
3. Use a plumber’s hand snake or flush with a garden hose to clear the drain line. Watch for water to exit at the foundation exterior or at a connected sump pit.

Rebuild the drain bed:

1. Line the bottom and sides of the well pit with drainage fabric (landscape cloth), lapping it up the sides 12 inches.
2. Place 6 inches of clean, washed crushed stone (3/4-inch) in the bottom.
3. Lay a 4-inch perforated pipe with a filter sock across the bottom, angled to drain toward the existing drain outlet or sump connection.
4. Cover with another 6 inches of crushed stone.
5. Fold the drainage fabric over the top of the stone and trim flush with the well liner edge.

Fill the remaining depth with washed gravel or decorative stone — not topsoil, mulch, or soil — to within 2 inches of the window sill.

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Step 3: Regrout and Recaulk the Window Frame

With the well drained, address the frame sealing.

Exterior frame perimeter:

1. Use a utility knife or oscillating tool to cut out all existing caulk from the joint between the window frame and the masonry surround — all four sides.
2. Clean the joint with a stiff brush and remove loose masonry dust and old caulk fragments.
3. If the joint is wider than 3/8 inch, install foam backer rod first to give the caulk proper depth.
4. Apply polyurethane masonry caulk in a continuous bead, pressing it into the joint with a caulk tool. Tool smooth.
5. Leave the weep holes at the sill bottom open. Do not caulk the bottom edge closed — trapped water behind the frame has no escape.

Interior frame perimeter (secondary seal):

If the interior drywall or block shows staining, apply interior hydraulic cement to any visible cracks in the masonry block around the window before caulking the interior perimeter with standard paintable caulk.

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Step 4: Install or Replace Sill Flashing

Sill flashing ensures that any water that reaches the sill runs outward instead of pooling at the frame-to-masonry joint.

Fabricate aluminum flashing:

1. Measure the width of the window opening from inside edge to inside edge of the masonry reveal.
2. Cut aluminum flashing stock to length with tin snips.
3. Bend the flashing to create a back leg (2 inches, vertical) and a sloped sill (3 inches, angled 10–15 degrees down and outward) and a drip edge (1/2 inch, bent down at the front).
4. If you have a sheet metal brake, use it. Otherwise, clamp the aluminum between two boards and bend over a straight edge.

Install:

1. Apply a bead of polyurethane caulk to the back of the back leg.
2. Slide the back leg under the window frame or behind the exterior casing.
3. Drive masonry screws through the back leg into the masonry — drill pilot holes with a masonry bit first to avoid cracking.
4. Caulk the top edge of the back leg where it meets the frame. Do not caulk the drip edge at the front.

Pre-formed vinyl sill flashing (sold in window supply stores) is an acceptable alternative if cutting and bending aluminum is outside your comfort zone.

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Step 5: Install a Window Well Cover

A window well cover is the last line of defense. It prevents rain from falling directly into the well and reduces debris accumulation that clogs drains.

Sizing: Measure the inside diameter of the window well liner and the width of the window. Buy a cover that extends at least 2 inches beyond the liner on each side.

Installation — polycarbonate dome cover:

1. Hold the cover against the foundation wall above the well. Mark the anchor locations on the masonry.
2. Drill 3/16-inch pilot holes with a masonry drill bit.
3. Drive Tapcon masonry screws through the back flange of the cover bracket into the wall.
4. The front of the cover rests on the window sill or clips to the sill edge.
5. Confirm the cover sheds water away from the foundation — it should slope outward slightly when installed.

Covers snap off for emergency egress (required by code in sleeping areas). Confirm the cover releases from inside before considering the job done.

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Step 6: Final Check

After completing all repairs:

1. Run a garden hose into the window well at full pressure for 5 minutes. Watch the well — water should drain within 2 minutes and not pool against the frame.
2. Check the interior window frame for any new moisture. If the well drains properly and the frame is sealed, the window should stay dry.
3. After the next rain, inspect again. A small amount of condensation on the interior glass is normal. Running water or wet masonry indicates a remaining gap in the caulk or a drain that is still partially blocked.

If water still enters despite fixing all three issues (drain, caulk, flashing), the problem may be hydrostatic pressure from a high water table — a condition that requires interior waterproofing membrane or a sump system rather than surface repairs.

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When to Call a Professional

DIY repairs handle the large majority of basement window leaks. Call a waterproofing contractor if:

• The well floods even with a working drain and the ground around the house slopes toward the foundation on multiple sides — grading work or a perimeter French drain is needed
• You see horizontal cracking in the foundation wall near the window — this indicates structural movement, not a waterproofing issue
• Water enters below the window through the block or poured concrete — this is hydrostatic pressure requiring interior drainage board, dimple mat, or an interior drain tile system

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Related Reading

• How to Fix a Wet Basement — full basement waterproofing options beyond window repairs
• How to Install a Sump Pump — managing groundwater when surface drainage is not enough
• How to Seal Basement Walls — interior and exterior masonry sealing techniques