A window at the bottom of a bucket
Every other opening in a house is above the ground it drains to. A basement window in a well is below it, inside a three-sided container that has one outlet, and that outlet is normally a vertical pipe under the gravel connecting to the footing drain. The whole arrangement is fine while the pipe works. When it does not, the well is a bucket and the window is the overflow point.
The volumes involved are small, and that is the surprise. A 42 by 24 inch well is 7 square feet of opening. Eighteen inches of depth to the sill is 10.5 cubic feet, or 78 gallons, and the gravel in the bottom takes back part of that. A garden hose delivers that much in a few minutes. So can a modest catchment during a real downpour, which is why the failure is often described as sudden: the well does not fill gradually over a storm, it fills in the ten or fifteen minutes when the rain is heaviest.
Intensity, not depth, is the input
For a dry well or a storage tank the number you need is the total depth of the storm, because you are storing the whole thing. For a window well it is the intensity, because the question is whether the rate coming in beats the rate going out for long enough to fill 78 gallons. A one inch storm delivered over eight hours never troubles a well. The same inch delivered in twenty minutes is running at three inches an hour and is a different event entirely.
The intensity figure has to come from rainfall frequency data for your own location and this page does not supply one. What is worth understanding is that short-duration intensities are much higher than the average rate of a storm, and a well fills on the peak rather than the average.
Where the water comes from
| Source | Typical share | What changes it |
|---|---|---|
| Rain falling directly into the opening | Small on its own | A sealed solid cover removes it entirely |
| Ground shedding toward the well | Often the largest steady source | Grade at the well; a settled hollow around the rim |
| Roof discharge from a nearby downspout | Can dwarf everything else | Extension length and direction |
| Water already in the backfill | Slow, unmodelled here | Site conditions, not something to calculate |
The roof line is the one to check first. A downspout serving 400 square feet of roof, releasing three feet from the wall next to a well, at three inches an hour, is delivering 12.5 gallons a minute all by itself. Against a 78 gallon well that is six minutes to the sill with nothing else contributing. The downspout discharge calculator works out what any given downspout is carrying, and moving it is a cheaper afternoon than anything else on the list.
Testing the drain, which is the only thing that matters
The calculation above is really a way of showing how little time you have if the outlet is not working. Testing whether it is working takes a bucket. Pour several gallons into the well on a dry day and watch. Water that disappears steadily indicates an outlet doing something; water that stands and slowly seeps into the gravel indicates a drain that is blocked, silted, crushed, or was never connected to anything in the first place.
Wells are also gradual accumulators of the material that blocks them: leaves, roof grit, soil washing off the surrounding grade, mulch. A well that drained perfectly for fifteen years can be full of silt in its gravel without looking any different from above, because the silt is below the stone. Clearing the gravel and replacing it is a modest job; finding out that the pipe below it is broken is not, and that is where it stops being a homeowner project.
Covers help with the leaves and the direct rain, and they introduce a constraint. Where a basement window serves as an emergency escape opening, what can be placed over it and over its well is regulated, and the requirements differ locally. That is a question for the building department rather than something to work out from a product description.
What a flooded well does next
Water that reaches the sill goes through the window, and basement windows are not designed to hold back a head of water. What follows is a fast entry at one point, which is why a window well shows up as one of the location answers on the basement water entry organizer and why it is one of the entry points that is easiest to confirm — you can stand outside during rain and watch it happen, which is not true of most of the others. If it has already happened, drying comes before repair, and the water damage drying calculator covers what that takes.
Before digging anywhere on the property, have the underground utilities located. In much of North America that is a free call-before-you-dig service and using it is a legal requirement, not a courtesy. Separately: excavating alongside a foundation can undermine it. Soil next to a footing is carrying load, a trench cut beside or below the footing can let that soil move, and the failure does not always show up on the day. This page gives no excavation procedure and none should be inferred from it. Work close to a foundation, below the footing line, or in soil that will not hold a face belongs with a contractor or an engineer who is standing on the site.
Where roof water and site water are allowed to go is set locally and it varies enormously. Discharging onto a neighbouring property, into a street gutter, into a storm sewer, into a sanitary sewer, or into a watercourse may be required, permitted, permitted only with approval, or prohibited outright depending on where you are, and some jurisdictions also set minimum setbacks and grading requirements at the building. None of that is stated on this page as fact, because none of it can be. Your local building department or stormwater authority is the source, and it is worth a phone call before anything is dug, because water arriving on someone else's land is one of the more common ways a drainage project turns into a dispute.
Questions people ask
Why is my window well filling with water?
Something is delivering water faster than the outlet takes it, and there are only three deliverers: rain landing in the opening, ground shedding toward the well, and roof discharge from a downspout near enough to reach it. The outlet side has only one usual answer, which is that the drain under the gravel is blocked, silted or was never connected. Testing that with a bucket of water on a dry day tells you more than any amount of watching during a storm, and it is the check to do first.
How do I know if the window well drain is blocked?
Pour several gallons in and watch what happens. Water that drops away steadily means something below is taking it. Water that stands, or soaks slowly into the gravel and then sits, means the outlet is not doing its job. Be aware that clean gravel will absorb a few gallons into its voids before anything reaches the pipe, so use enough water to get past that, and give it a few minutes rather than judging on the first thirty seconds.
Does a window well cover stop the flooding?
It stops the fraction that falls directly into the opening, which for a typical well is a real but modest share of the total, and only if it is sealed against the wall rather than sitting loosely on top. It does nothing about ground shedding toward the well or a downspout discharging beside it, which are usually the larger sources. It also brings a constraint with it: where the window serves as an emergency escape opening, what may cover it and its well is regulated locally and is worth confirming before buying anything.
How much water does it take to reach the window?
Less than people expect. Multiply the inside width by the projection to get the opening area in square feet, multiply by the height to the sill in feet, and multiply by 7.48 for gallons. A 42 by 24 inch well with 18 inches to the sill holds about 78 gallons before the gravel is subtracted, and gravel occupying the bottom six inches at a 35 percent void ratio takes about 17 of those gallons away. Sixty gallons is a few minutes of a heavy downpour concentrated by a modest catchment.
Can I add a drain to a well that does not have one?
Sometimes, and it depends entirely on what the outlet would connect to, which is not visible from the surface. A well drain normally ties into the footing drain, and whether one exists, where it runs and whether it is functioning are the governing questions. Excavating alongside a foundation to find out is work that can undermine the footing if it is done badly, and it belongs with a contractor rather than with a weekend. Before any of it, the cheaper interventions — moving roof discharge, regrading the ground around the rim, clearing and replacing the gravel — are worth exhausting.