Water Coming In Under Your Garage Door? Causes, Fixes & How to Keep Your London Garage Dry

Introduction

You walk into the garage the morning after a heavy London thunderstorm and there it is: a thin sheet of water spread across the floor, a dark tide line along the bottom of the door, and maybe a soaked cardboard box that was sitting a little too close. It is one of the most common calls we get in the spring melt and again through the summer storm season, and it is one of the most misunderstood. Homeowners often assume the door itself is broken. Usually it is not. Water under a garage door is almost always a combination problem: the seal at the bottom of the door and the ground it sits on are working together, or failing together, to decide whether the rain runs off or runs in.

This guide walks through why water gets under a closed garage door in the first place, how to tell an actual leak apart from condensation (which looks identical on the floor but has a completely different fix), the causes ranked from easiest to hardest to solve, the repairs a homeowner can safely handle, the ones that need a professional or even a concrete contractor, and realistic 2026 costs in the London area. Every external link here has been checked to point at a live page on a recognized industry body. Water is patient and cement is porous, so the sooner you find the real cause, the less you will spend fixing what the water damaged.

First, Rule Out Condensation

Before you spend a dollar sealing anything, make sure you are actually dealing with water coming in from outside. A surprising number of "leaks" are condensation forming on the inside of a cold door or pooling on a cold slab, especially in late fall and early spring when a warm, damp garage meets a cold panel. Condensation and a real leak leave nearly identical puddles, but they call for opposite fixes: a leak needs better sealing and drainage, while condensation needs better insulation, ventilation, and humidity control.

Three quick tests separate them. First, timing: a leak appears during or right after rain or snowmelt, while condensation shows up on cold, humid mornings regardless of rain. Second, location: a leak pools at the door line and works inward, while condensation forms evenly across the slab or drips from the door and ceiling. Third, the paper-towel test: tape a dry paper towel to the inside face of the bottom panel overnight in dry weather. If it is damp in the morning with no rain, you have condensation, not a leak. Our full garage door condensation guide covers that problem in detail. If the water only ever arrives with the weather, read on.

Why Water Gets In: The Real Causes

Water under a closed door is rarely one thing. It is usually the seal and the ground failing to cooperate. Here are the causes, ordered roughly from the cheapest and most common to the most involved.

1. A worn, hardened, or missing bottom seal

The single most common cause. The flexible strip along the bottom edge of your door (the astragal, or bottom weatherstrip) is the first and most important line of defence against water. It is a length of rubber or vinyl held in an aluminum retainer, and it is designed to squish flat against the floor and form a continuous barrier when the door closes. It is also a wear part. Sun, ozone, road salt, and thousands of open-close cycles turn it brittle over five to ten years. Once it hardens, it stops conforming to the floor, cracks at the corners, or tears away from the retainer entirely, and every low spot in your driveway becomes an entry point.

The good news is that this is the easiest and cheapest fix on the list, and it is genuinely a homeowner-friendly job on most doors. Our step-by-step bottom seal replacement guide walks through measuring the retainer, buying the correct T-end or bead-end profile, and sliding the new seal in. If your seal looks fine but water still gets past it, the problem is almost certainly below the door, not on it.

2. An uneven or worn concrete floor

A garage door seal is only as good as the surface it presses against. Older slabs settle, crack, spall, or wear a shallow dip right where the door meets the floor. A standard bottom seal can bridge a gap of maybe half an inch. Once a low spot is deeper than that, no ordinary seal can close it, and water pours through the channel every time it rains. This is the classic case where a homeowner replaces the bottom seal, sees no improvement, and gets frustrated. The seal was never the problem.

The fix for an uneven floor is a threshold seal: a low, wedge-shaped rubber ramp glued to the floor directly under the closed door. It rises to meet the door and creates a dam that both the door and the concrete press against. It is the right answer for a wavy or dished slab and for driveways that slope gently toward the garage. Our garage door threshold seal guide explains how to choose the right height and install it, and our companion article on the gap under a garage door covers how to diagnose whether the gap is in the seal, the floor, the door, or the opener limits.

3. Driveway grading that slopes toward the garage

This is the big one, and the one nobody wants to hear. If your driveway slopes down toward the garage rather than away from it, every heavy rain sends a sheet of water straight at the door. No seal is designed to hold back a moving flow of water under pressure; seals are built to block splash and standing damp, not to act as a dam against a stream. On homes where the garage sits below the level of the street or yard, this is often the true root cause, and the bottom seal is just the part that gets blamed.

You cannot fix grading with a rubber strip. The real solutions live outside the door: a trench drain (a slotted channel drain set across the driveway apron just outside the door, which catches the water before it reaches the threshold), regrading or resurfacing the apron so it pitches away from the garage, or improving the yard drainage that feeds the driveway. These are concrete and landscaping jobs, not garage door jobs, but a good door company will tell you honestly when the door is not the problem so you spend your money in the right place.

4. Wind-driven rain and poor side and top seals

Not all water comes under the door. During a strong storm, wind can drive rain sideways through gaps around the perimeter: the jamb seals down each side and the header seal across the top. If your water intrusion happens only during windy storms and shows up along the sides rather than in a puddle across the floor, suspect the perimeter weatherstripping rather than the bottom seal. These seals age the same way the bottom one does. Replacing the perimeter stop moulding with fresh vinyl-backed weatherstrip is a straightforward job covered in our broader garage door weather seal guide.

5. Snowmelt, ice, and the spring thaw

London winters add a seasonal twist. Snow piled against the door melts in a January thaw or a March warm spell and the meltwater behaves exactly like rain, except it often arrives when the slab is still cold and the seal is stiff and less flexible. Worse, water that seeps under the door and refreezes overnight can freeze the door to the floor, which is a separate headache covered in our frozen garage door guide. If your only water problem is during the thaw, the fixes are the same (seal and threshold), but a fresh, still-flexible seal matters even more because cold rubber seals poorly.

How to Diagnose It Yourself in Ten Minutes

You can narrow down the cause without any tools. Do this the next time it rains, or simulate it with a garden hose.

1. Watch during actual rain. With the door closed, look from inside with a flashlight along the bottom edge. Is water coming in evenly across the whole width, or only at one or two spots? Even entry across the width points to grading or a hardened full-length seal. Entry at one or two spots points to low areas in the floor or torn seal corners.
2. Check the seal by eye and by hand. Open the door and inspect the bottom weatherstrip. Squeeze it. Fresh rubber is soft and springs back; a failed seal is hard, cracked, flattened permanently, or torn at the ends. Look for daylight along the closed door from inside during the day.
3. Test the floor with a level or a straight board. Lay a straight edge across the floor where the door lands. Rock it. A visible gap under the straight edge is a low spot the seal cannot bridge on its own.
4. Check which way the driveway slopes. Stand at the door and look down the driveway. Pour a bucket of water on the apron just outside the door. If it runs toward the garage, grading is a factor. If it runs away, grading is fine.
5. Note whether wind matters. If water only appears in wind-driven storms and shows up at the sides, focus on the perimeter seals, not the bottom.

Between those five checks, most homeowners can tell within one rainfall whether they are looking at a ten-dollar seal, a fifty-dollar threshold, or a conversation with a concrete contractor.

The Fixes, From Easiest to Hardest

Replace the bottom seal (DIY-friendly)

If the seal is hard, cracked, or torn, start here. It is inexpensive, it is the most common cause, and on the vast majority of residential doors it is a job a careful homeowner can do in under an hour with no special tools. Measure your door width, identify whether your retainer takes a T-end or bead-end seal, buy a little extra length, and slide the new seal through with a bit of dish soap as lubricant. Full instructions are in our bottom seal replacement guide. A fresh seal alone solves a large share of water problems on doors with a reasonably flat floor.

Add a threshold seal (DIY-friendly)

If the floor is uneven or the driveway slopes gently toward the door, a threshold seal is the fix. It glues to the concrete and forms a raised dam that both the door and the floor seal against. Choose a height matched to your worst gap, clean and dry the slab, dry-fit it, then bond it with the adhesive it comes with. One trade-off to know: a threshold creates a small lip you drive over and sweep against, and it can trap water inside if you wash the floor, so it is best where the primary threat is water coming in from outside. Our threshold seal guide covers the height selection and install.

Replace the perimeter (side and top) weatherstripping

For wind-driven rain entering at the edges, replace the stop moulding weatherstrip around the door frame. This is a moderate DIY job: pry off the old stop, cut new vinyl-backed stop to length, and nail it on so the flexible flap just kisses the closed door. Done right, it seals out both water and drafts. See our weather seal guide for the full walkthrough.

Fix the drainage or grading (professional)

When the driveway slopes toward the garage, the durable fix is a trench drain across the apron, regrading the apron to pitch away from the door, or resurfacing. These are concrete and drainage jobs handled by a paving or landscaping contractor, not a door company. They cost the most, but on a garage that floods every storm because it sits low, they are the only permanent answer. A rubber seal on a flooding driveway is a bandage, and it will fail again.

Interim measures while you plan

If a big storm is forecast and you have not fixed the root cause yet, temporary defences buy time: a reusable fabric flood barrier or water-filled dam laid across the door line, sandbags on the apron, or a rolled towel and a wet-dry vacuum on standby. These are stopgaps, not solutions, but they can save a finished floor or stored belongings during one bad night.

Why Getting the Water Out Matters

It is tempting to shrug off a little water on a concrete floor. The trouble is what the water touches on its way in and what it leaves behind. Standing water and chronic damp at the base of the door accelerate rust on the bottom panel, the bottom-corner brackets, the track, and the fasteners, which is exactly the corrosion our rust prevention guide is built to help you avoid. Road salt carried in on tires makes it worse, because salty water is far more corrosive than clean rain. Beyond the door hardware, a wet garage raises humidity, feeds mildew on drywall and stored items, and in an attached garage can push moisture toward the house. Fixing the water early is cheaper than replacing a rusted-through bottom panel or a seized roller later.

Realistic 2026 Costs in the London Area

Prices on this page are before HST.

Water intrusion is one of the cheaper garage door problems to solve when it is a seal or threshold issue, and one of the more expensive when it is grading. Realistic 2026 numbers for southern Ontario:

• Bottom seal (astragal), parts only for a single door: $20 to $45 for the rubber; a full retainer plus seal runs more.
• Bottom seal replaced on a service call: typically $120 to $220 all-in for a standard door, folded into a tune-up if you book one.
• Threshold seal kit, parts only: $45 to $110 depending on door width and profile height.
• Threshold seal supplied and installed: typically $180 to $300 for a single door, more for a double.
• Perimeter (side and top) weatherstrip replaced professionally: typically $150 to $300 depending on door size and moulding condition.
• Trench drain or apron regrading (concrete or landscaping contractor): commonly $1,500 to $5,000 or more, depending on length, access, and how much slab has to move. This is a specialty trade, not a door repair.

Two honest notes. First, every door company has a minimum service-call fee, so a ten-dollar seal in a service call is mostly the truck, the technician's time, and a proper close-and-balance check afterward. Second, if the true cause is grading, spending on seals first is money poorly placed. A reputable company will tell you when the door is not the problem, and will not sell you a threshold to fix a drainage failure.

How to Keep It From Coming Back

Water intrusion is easy to prevent once the root cause is solved. Build these into your routine.

1. Inspect the bottom seal twice a year. Spring and fall, squeeze the seal and look for daylight along the closed door. Replace it at the first sign of hardening or cracking, before the next storm season, not after the flood. This slots neatly into our fall garage door maintenance checklist.
2. Keep the door line clear. Leaves, dirt, and ice packed in the channel where the door meets the floor hold water against the seal and stop it from sealing flat. Sweep it out. In winter, clear snow away from the base rather than letting it pile against the door.
3. Watch the driveway. If you notice water beginning to pool toward the garage after a few seasons of settling, address the grading before it becomes a chronic flood.
4. Store valuables off the floor. Shelving and pallets keep boxes and tools out of the splash zone, so an occasional wet floor never becomes a loss.
5. Keep up with general maintenance. A well-maintained, well-balanced door closes squarely and seals evenly. Our garage door maintenance guide covers the routine, and the DASMA homeowner maintenance guide is a worthwhile annual read with a printable checklist.

When to Call a Professional

Replacing a bottom seal or gluing down a threshold is well within reach for most homeowners. Call a professional when the water keeps coming after you have replaced the seal (which usually means a floor or grading issue that needs diagnosis), when the door does not close squarely so one side seals and the other does not, when you see rust or damage on the bottom panel or brackets that needs attention, or when the true cause is drainage and you need the right trade. A door technician can also confirm whether the door's travel limits are set so it closes fully onto the seal, since an opener that stops a hair early leaves a gap no seal can fill. The DASMA garage door safety tips are clear that any work involving the springs, cables, or bottom brackets belongs to a trained technician, and the bottom-corner hardware sits right in the splash zone. If you want to vet a company first, the International Door Association maintains a consumer directory of qualified professionals.

Frequently Asked Questions

Is a little water under the garage door normal?

A trace of dampness during an extreme storm is common and not alarming. A recurring puddle or a sheet of water across the floor after ordinary rain is not normal and points to a failed seal, an uneven floor, or a grading problem. If it happens with every rain, it is worth diagnosing.

Will a new bottom seal alone stop the water?

Often, yes, if the floor under the door is reasonably flat and the driveway does not slope toward the garage. If you replace the seal and water still gets in, the cause is almost certainly below the door: a low spot in the concrete (fixed with a threshold seal) or grading that sends water at the door (fixed by a drain or regrading).

What is the difference between a bottom seal and a threshold seal?

The bottom seal attaches to the door and moves with it. The threshold seal glues to the floor and stays put, forming a raised ramp the door closes against. A bottom seal handles a flat floor; a threshold handles an uneven floor or a gently sloping driveway. On tough cases, using both together gives the best result.

How do I know if it is a leak or condensation?

Timing and pattern. A leak appears with rain or snowmelt and pools at the door line. Condensation appears on cold, humid mornings regardless of weather and forms evenly on the slab or drips from cold surfaces. Tape a dry paper towel to the inside of the bottom panel overnight in dry weather; if it is damp with no rain, it is condensation. Our condensation guide covers the fix.

Can water under the door damage the garage door itself?

Yes. Chronic damp and salty meltwater rust the bottom panel, the bottom-corner brackets, the track, and the fasteners, and raise humidity that feeds mildew. That is the corrosion our rust prevention guide exists to prevent. Fixing the water early is far cheaper than replacing rusted hardware or a corroded bottom panel.

My driveway slopes toward the garage. What can I do?

That is a grading problem, and no rubber seal will fully solve it because seals are not built to hold back moving water under pressure. The durable fixes are a trench drain across the apron, regrading or resurfacing the apron to pitch away from the door, or improving yard drainage. A threshold seal can help as a secondary defence, but the drainage is the real fix, and it is a concrete or landscaping job.

Does this get worse in winter?

It can. Snow piled against the door melts and behaves like rain, cold stiffens the seal so it conforms less well, and water that seeps in and refreezes can freeze the door to the floor. A fresh, flexible seal matters even more in cold weather. Our frozen garage door guide covers the freezing side of the problem.

Conclusion

Water under a garage door looks like a door failure, but it is really a conversation between three things: the seal on the door, the concrete under it, and the way the ground outside sends water toward or away from the opening. Start by ruling out condensation, then work from cheap to expensive: check and replace the bottom seal, add a threshold if the floor is uneven, refresh the perimeter seals if wind-driven rain is getting in at the sides, and only then look at drainage if the water keeps coming. Most London homes are solved at the seal-and-threshold stage for well under a couple hundred dollars. The ones that flood every storm because the driveway slopes the wrong way need a drain, and the honest move is to spend that money where it actually works.

If you have replaced the seal and the water still finds its way in, or you would rather have a London technician diagnose it in one visit, run the balance and close-limit checks, and quote the right fix, our team is happy to help. Call 519-619-7901, reach us through our contact page, or browse doors and openers on our price estimator. AA Advantage Doors has kept London-area doors sealed, balanced, and running since 2009; our BBB profile has the service history.