Field notes · 2026-09-22

Daylight under a closed garage door, and what the shape of it tells you

Stand inside the garage on a bright afternoon with the door down and the lights off. If a line of daylight runs along the bottom of the door, that line is information, not only a draft. Most people look at it and think about rubber, because rubber is the part that touches the floor and the part a hardware store sells.

Rubber is one of three answers. The other two are the floor and the door itself, and in the Red River Valley the floor is the one that moves. A slab and a driveway apron that sat flat in September lift when the frost drives in and settle again at thaw, while the bottom edge of the door stays a straight steel line that follows none of it.

So read the outline of the light before buying anything. An even strip, a wedge, a gap in the middle, two gaps at the ends. Each points somewhere different, and two of them are not seal problems at all.

Why can I see daylight under my closed garage door?

Three things put light under a closed door. The bottom seal is worn, torn, or the wrong profile for the retainer it sits in. The floor under the door is not flat, so the seal touches in some places and floats in others. Or the door’s bottom edge is bent, or hangs out of level because one cable has slipped. From the driveway all three look identical.

Look once, properly, and you will not have to guess. Give your eyes a minute to adjust, then crouch and sight along the bottom from one jamb to the other, reading the light edge-on instead of staring into it. Note where it starts, where it is widest, and whether it reaches both ends.

Daylight around the seal in January is one of the six triggers listed on the garage door replacement page. That is the short version. The long version is which of the three you have, because one is a fifteen-minute part, one is a service call, and one is concrete.

What does the shape of the gap tell you?

The outline names the cause. Light that runs even across the full width is the seal itself. A wedge that grows toward one jamb is the floor or the door hanging low on that side. Light in the middle only is a bowed bottom section. Light at the two ends with the middle sealed is curled rubber and whatever packed in behind it.

• An even strip is seal wear, or a seal too small for its retainer. Rubber takes a compression set over a few winters and stops springing back to its shaped height. A replacement bead or T-end that does not fill the channel it slides into does the same from day one.
• A wedge widening toward one jamb is the important one. Either the floor under that side moved, or the door hangs low on that end because a cable unwound a turn on its drum. The cable version is a service call rather than a hardware-store trip, and it wants looking at soon, because uneven tension loads the bottom brackets crooked.
• Light in the middle with both ends sealed means the bottom section is bowed upward across its span, usually after an impact or a jack left where the door came down on it. No seal spans a curve. That section has to come back flat or come off.
• Gaps at the two ends only are curled seal ends, often with ice or sand packed behind the last inch of rubber. The corners take the meltwater and the plow spray, so they harden and lift first.

Why does the concrete decide this in the Red River Valley?

Garages here sit on valley clay that holds water, and water that freezes in it lifts whatever is on top. Frost drives down through a Grand Forks winter, the slab and the apron in front of it rise, and they never rise evenly. At thaw the ground lets go and things settle, sometimes back to where they started and sometimes not quite.

The hinge in all of this is the joint where the garage slab meets the driveway apron, which is exactly the line the door closes on. Two pours, different thicknesses, different fill, moving independently, so the apron can sit proud of the slab by late winter and flush again in June. A gap that was not there in September is real. People second-guess it because the door did not change, and the door is all they looked at.

Drainage is the other half. If the apron falls back toward the door rather than away from it, meltwater from a warm afternoon runs to the low line under the door and refreezes after dark. Ice at that line works like a jack, on the same joint, every freeze cycle.

What fills a low spot in the floor, and what does not?

A bottom seal spans small differences. A taller bulb or double-bead profile has more travel in it than a flat U-shape, and matching the seal to the door’s retainer matters as much as size. A threshold seal, which is a rubber ramp bonded to the concrete at the door line, raises the floor to meet the door instead. Neither is a floor repair.

That distinction decides whether new rubber is worth buying. A seal makes up for the height it can compress through and no more. A threshold adds a lip the door closes against, which handles a shallow dip and keeps driven snow out at the line, but it cannot answer a corner that lifts past what rubber can compress, or a slab sitting low well inside the opening. Where the shape read as a wedge, new rubber often changes nothing. Bottom and perimeter weather seal in cold-rated rubber that stays flexible below zero runs $90 to $200 on the published price ranges.

One caution on thresholds in this climate. They hold water at the door line by design, and rubber bonded to the slab by refrozen meltwater is a separate problem with its own handling, covered in what cold does to a garage door.

Why did the door start reversing or bouncing after the first cold snap?

A heaved slab means the door meets the floor early. The opener runs the travel it was taught, feels resistance before it expects to, and reads that resistance as something under the door. So it bounces back up, or stops short with light still showing. Travel and force limits set against a July floor are wrong against a January floor, and nothing about the opener failed.

Almost everyone calls this a sensor fault, and most of the time they are right. The photo-eye version of a reversing door, and what to do about it, is answered on the garage door opener repair page. Separate the two before anyone buys parts. A door that reverses right at the floor, on cold mornings, in an opening whose gap changed this fall, is pointing down.

The wrong move is turning the down-force up until the door stays shut. That buries a floor that moved under a setting, and every cycle drives the bottom section into concrete it no longer matches. The limits do need resetting, but after someone has looked at the floor, not instead of it.

What does blowing snow through a half-inch gap actually do?

Fine prairie snow does not need much of an opening. A half-inch gap on a windward door passes enough of it in one blow to build a drift several feet inside the door. The slab is warmer than the air, so the drift melts, the water runs back toward the cold line under the door, and after dark it freezes there.

That ridge is what costs money. It holds the seal flat or rolls it under, so the rubber stops sealing where it is pinned and tears at the retainer when the door lifts against the ice. The gap is wider next week and the drift is bigger. Salt slush arrives on the same wind and sits against the bottom seam and the bottom brackets, which is where cables fray and where steel rusts from the inside face out.

Give that two or three winters and the conversation moves up the price list. A bottom section replaced runs $250 to $800 depending on whether your model’s panel is still made. Where the seam has rusted through on a builder-grade uninsulated door, there is no section worth buying and it becomes a new insulated door, $1,100 to $1,900 installed for a single and $1,600 to $3,200 for a double.

The window before the ground locks up

Most of the fixes here want temperature and a dry slab, which is why this read is worth doing in October and not in the dark in January. Threshold work needs the concrete dry and above the adhesive’s rated temperature, and anything that touches the concrete itself, grinding a high spot or patching the apron, needs curing weather. Once the ground locks up, that half of the answer waits for the thaw whatever anyone quotes you.

What stays doable in the cold is narrower. A worn seal can be changed on a mild day if the retainer is sound and the door line can be cleared and dried. A door hanging low on a cable gets fixed in any weather, because that one is hardware and it will not improve on its own. So is a bent bottom section, once snow is coming through the curve.

If you are already reading this in January with light under the door, record it tonight. Photograph the gap from inside with the lights off, note which end is widest, measure the widest point with a tape. Keep the drift knocked down and do not force a door frozen at the line. Come thaw, that photo is what tells a crew whether they are pricing rubber, a bottom section, or a floor, and whether the wedge you saw in February was still there in May.