How to Waterproof a Below-Grade Window Well in 4 Steps

TL;DR: Waterproofing a below-grade window well requires four layers: a corrugated liner to hold back soil, 6-8 inches of gravel at the bottom for drainage, caulk or hydraulic cement at the foundation-to-liner joint, and a polycarbonate cover to keep rain out. A single heavy storm can send 200+ gallons through an unprotected window well; these steps redirect water before it reaches your foundation.

_Last reviewed: July 2026 · 6 min read_

A below-grade window well that isn't waterproofed becomes a collection basin during every rain. Water pools against the window, seeps through the frame, and ends up on your basement floor—or worse, behind drywall where you won't notice until mold appears. The fix involves four components working together: a liner to contain soil, gravel to drain water downward, sealant where the liner meets the foundation, and a cover to shed rain before it enters the well.

Okoniq Property Hub lets you log the installation date, track cover inspections after storms, and attach photos of the liner and grading so the next owner (or your future self) knows exactly what's in place.

Why does a below-grade window well collect water in the first place?

The window well sits lower than the surrounding grade, so gravity directs surface runoff and roof downspout overflow straight into it. Without a liner, soil erodes into the well and blocks any drainage path; without gravel, water has nowhere to go except against the foundation; without a cover, every inch of rain fills the well like a bucket. A 3×4-foot well can collect over 200 gallons in a 3-inch storm if it has no cover. The window frame itself is rarely designed to be fully submerged—most egress windows have weep holes that assume water will drain away, not stand against the glass for hours. Once water finds a path through the sill or frame, it follows the foundation wall downward and spreads along the basement floor or inside the stud cavity if the basement is finished. The basement waterproofing post covers interior drainage systems for when water has already entered; this guide stops it before it reaches the window.

What type of liner and gravel setup actually keeps water out?

A corrugated metal or composite plastic liner sits in the excavated well and holds back the surrounding soil—galvanized steel lasts 15-20 years in most climates, while composite (often recycled plastic) lasts 30+ years and won't rust. The liner must extend at least 4 inches above final grade and rest tight against the foundation. Dig the bottom of the well another 6-8 inches below the window sill, then fill that depth with ¾-inch crushed stone (not pea gravel, which packs too tightly). This gravel layer acts as a dry well: water percolates downward and disperses into the surrounding soil instead of pooling. If your soil is heavy clay, run a 4-inch perforated drainpipe along the bottom of the gravel bed and slope it to daylight or tie it into your basement dehumidifier sump system. Backfill the outside of the liner with clean fill—no topsoil or mulch, which absorbs water and collapses into the well. Slope the final grade away from the liner at ½ inch per foot for at least 6 feet; this sends surface water past the well instead of into it.

How do you seal the joint between the liner and the foundation wall?

The gap where the liner meets the foundation is the most common leak point. After the liner is in place and backfilled, caulk the seam with polyurethane or a paintable exterior-grade sealant rated for foundation movement—avoid pure silicone, which doesn't stick well to concrete or metal over time. For joints wider than ¼ inch or where the foundation is actively wet, use hydraulic cement instead: mix it to a thick putty, press it into the gap, and shape it smooth before it sets (you have about 5 minutes). Hydraulic cement expands slightly as it cures, locking into pores in the concrete. If water is already seeping through the joint, the cement will set underwater—silicone and polyurethane won't. Check this seal every spring; freeze-thaw cycles can crack it. The ceiling water stain diagnose article walks through similar leak-tracing logic when you're not sure where moisture is entering.

What kind of cover works best, and how do you secure it?

A polycarbonate dome or flat grated cover keeps 90% of rain out of the well while still allowing emergency egress—essential if the window is a code-required exit. Polycarbonate domes shed water to the sides and let in more natural light than metal grates; flat steel grates are stronger for areas with heavy foot traffic or snow load but can rust. Both styles must sit on a frame that's lag-bolted or masonry-anchored to the foundation wall or the liner flange, not just resting loose—wind will lift an unsecured cover during a storm. The cover should overhang the liner by 2-3 inches on all sides so water runs onto the ground, not into the well. If you live where snow drifts against the house, remove snow from the cover within 24 hours; 2 feet of wet snow weighs 40+ pounds per square foot and can buckle polycarbonate. Covers with hinges let you open the well for window cleaning or egress testing without removing the entire panel. Plan to replace polycarbonate every 10-12 years as UV exposure makes it brittle; metal grates last 20+ years if painted every 5 years. The chimney fireplace inspection checklist includes a similar annual cover-check step for chimney caps.

Should you add a window well ladder or egress step system?

If the window is a code-required egress (common in finished basements with bedrooms), you're required to have a means of exit—either a permanent ladder or stacked concrete blocks that form steps. A galvanized steel ladder bolted to the foundation inside the well costs $60-120 and meets most local codes; ladder rungs must be spaced no more than 18 inches apart and extend to within 6 inches of the window sill. Modular plastic step systems fit inside the well and double as planters; they're easier to climb than a vertical ladder but take up more space. Inspect ladder bolts and welds every year; rust at the mounting point can fail under body weight. If the well is deeper than 4 feet, consider a second handrail along the liner wall. Document the ladder installation date and load-test schedule in Okoniq—a failed egress ladder is a life-safety issue, not just a maintenance nuisance.

FAQ

How often should I clean out a window well after waterproofing it?

Twice a year—once in spring after snowmelt and once in fall after leaves drop. Pull out accumulated leaves, dirt, and debris that can clog the gravel layer or block weep holes. If you have trees overhead, check the well after every major storm.

Can I plant anything inside a waterproofed window well?

Only if you use a raised planter box that sits on top of the gravel, not directly in it. Planting in the gravel destroys the drainage layer as roots and soil fill the voids. Small succulents in a metal or plastic container work; avoid anything with deep roots or high water needs.

Do I need a sump pump inside the window well itself?

Not if the gravel drains properly into the surrounding soil. A sump is only needed if you have solid clay, a high water table, or if the well sits below the level where gravity drainage can exit. In that case, install a small pedestal sump in a bucket at the lowest point of the gravel bed and pipe it to your main basement sump or to daylight.

What's the best way to retrofit a window well that was never lined?

Excavate the soil around the window to expose the foundation wall, install the liner against it, backfill the outside, then add gravel and a cover as described. If the window frame is already damaged from past leaks, replace it before you waterproof—sealing around a rotted sill just traps moisture inside the wall.

How do I know if the window well is draining correctly after a storm?

Check the well within 2 hours of the rain stopping. If water is still pooled at the bottom, either the gravel is clogged, the soil around the liner is saturated (common in clay), or you need a drain line. Dig down to the gravel layer; if it's full of silt, replace it. If the gravel is clean but water won't drain, add a 4-inch perforated pipe sloped to daylight or to your sump system.

This is educational information, not structural or waterproofing engineering advice. If your foundation shows active cracks, horizontal displacement, or water enters through the wall itself (not just the window), consult a licensed structural engineer or foundation contractor before attempting any DIY waterproofing.