Daylight Under Your Closed Garage Door? The Floor Moved, Not the Door

You close the garage door, step back, and see a line of daylight running under it — maybe a sliver on one side, maybe a gap you could slide a magazine through. The instinct is to assume the door is broken or hung crooked. Usually it isn't. Nine times out of ten the door is fine and the concrete underneath it has moved.

Understanding that difference matters, because it changes the fix entirely — and a like-for-like seal swap, the obvious move, often doesn't solve it.

Why the floor is the real culprit

A garage slab isn't a permanent, perfectly flat surface. It's a slab of concrete sitting on soil, and soil moves. Over years, seasonal moisture swings, tree roots, freeze-thaw cycles, and the settling that happens under any slab can leave the floor low in the middle, high on one edge, or gently sloped toward the driveway. The door was cut and sealed to meet the floor as it was on installation day. When the floor drops or tilts, a gap opens — even though the door hasn't changed at all.

You can usually tell it's a floor problem rather than a door problem by the shape of the gap. A settled slab typically leaves a gap that's wider in the middle or biased to one side, following the dip in the concrete. A door that's genuinely off — off its track, or with a broken bottom bracket — tends to sag as a whole and usually comes with other symptoms: binding, noise, or a door that won't sit square in the opening.

Why a straight seal swap often fails

Here's where a lot of DIY fixes go wrong. The standard reaction is to pull the old bottom weatherstrip and pop in an identical replacement. If the floor has settled, that new seal meets the same low spot the old one did, and the gap comes right back. You've spent an afternoon and the daylight is still there.

The real fix is to match the seal to the floor as it is now:

• A deeper or larger-profile bottom seal (the U-shaped or bulb-style astragal that slides into the retainer) can bridge a bigger gap than the thin strip that came standard. Going up a size is often all it takes.
• A threshold seal — a rubber ramp bonded to the floor itself — meets the door from below and seals against an uneven slab, water intrusion, and drafts. It's the go-to when the slope is significant or water is running in.
• Sometimes the retainer (the aluminum channel the seal slides into) is bent or the bottom bracket has shifted, and that needs correcting first.

The point is that sealing a settled slab is a matching problem, not a replacement problem. You're choosing a profile that fits the floor's real shape, not swapping in whatever was there before.

Why the gap is worth closing

It's tempting to live with a little daylight, but the gap does real work against you over a season:

• Energy and comfort. An attached or semi-conditioned garage bleeds heat and lets cold air pour in under the door all winter.
• Water and debris. A low spot channels rain, leaves, and driveway grit straight into the garage. Over time, standing water at the threshold rots stored items and works on the door's bottom section.
• Pests. A half-inch gap is an open invitation to mice, insects, and everything else looking for a warm, dry place as the weather turns.
• The door itself. A bottom section sitting in water or grit corrodes and delaminates from the inside out — turning a seal problem into a panel problem.

When to call it in

If the gap is small and even and the slab looks flat, a larger bottom seal is a reasonable DIY. Call a technician when the gap is uneven or one-sided, when water is getting in, when the door binds or sits crooked in the opening, or when you've already swapped the seal and the daylight came back. A pro will read whether it's the floor, the seal, the retainer, or the door's alignment, and fit the profile that actually matches your slab — often in a single visit.

Daylight under the door is the floor telling on itself. Seal it to the floor you have, not the floor you had.

Up & Down Garage Doors diagnoses and fixes bottom gaps, seals, thresholds, and alignment for homeowners and property managers — matching the fix to the floor, not just swapping parts. If daylight is showing under yours, call (475) 267–4105 or reach us online.

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