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When it flattens out or splits, you get fine grit on every shelf, hot air sliding in under the door all afternoon, and water across the floor the first time a summer storm blows through. Here is how the part works, how to tell yours is done, and why a gap at one end is usually not the seal at all.
How a garage door bottom seal actually works
Two parts do the job. The retainer is an aluminum channel with two narrow grooves running its length, screwed or riveted along the bottom edge of the door. The seal itself, called an astragal in the trade, is a rubber or vinyl strip with a matching bead along each edge, and those beads slide into the grooves. The middle of the strip hangs below the door as a loop or a bulb. That loop is the working part: it squashes when the door lands and spreads sideways to fill the small dips in your concrete. Seals are sold as T style or bead style. That name describes the edge beads, not the loop, so the new strip has to match your retainer.
Why the desert finishes a seal off early
Rubber and vinyl do their job through flexibility, and heat takes that away. An attached Las Vegas garage can push past 120 degrees in summer. The slab under the door gets hotter still in direct sun, and the strip sits squashed against that hot concrete for hours every night. Add years of UV on the outside face and the material takes a permanent set. It stops springing back and holds the flattened shape you can see when the door is up. Fine desert dust does the rest, packing into the retainer grooves and into the loop until the seal cannot fold the way it was designed to.
How to tell your garage door bottom seal has failed
Close the door, turn the lights off, and look along the floor line from inside. A healthy seal shows no daylight at all across the full width. Now put the door up and look at the strip. A good one hangs open and springs back when you pinch it. A finished one is creased flat, cracked along the fold, hard instead of rubbery, or pulled out of one groove so it droops or sits twisted. Dust tracks fanning inward from the floor line are the other reliable sign, because they show you where the air is getting in.
A gap at one end is usually the door or the slab, not the seal
This is the part people get wrong most often. Even daylight across the whole width points at the seal. A wedge of daylight, tight at one end and wide at the other, means the bottom edge of the door is no longer parallel to the floor. A new seal will not fix that. It will only wear through on the tight side. On the door side the cause is normally unequal cable winding on the drums, a bent bottom bracket, or a worn roller letting one corner hang low. On the concrete side it is a slab that has settled, cracked or lifted along one edge.
Threshold seals for a slab that is not flat
When the concrete is the problem, the fix goes on the floor instead of the door. A threshold seal is a rubber ramp bonded to the slab across the opening, so the door closes onto that instead of onto bare concrete. It suits a dished or cracked slab, a driveway that pitches toward the garage, and wind driven rain in monsoon season, because it gives the seal a flat raised line to land on. Check the drainage first, though. A threshold keeps water out, but if your floor slopes inward it will also keep water in. It belongs on a floor that drains toward the door.
What you can safely do yourself
Replacing the strip on a door with a sound retainer is a genuine homeowner job. Unplug the opener first so nobody runs the door while your hands are at the bottom edge. With the door closed, pull the old astragal out from one end. Clean the packed dust out of both grooves, cut the new strip slightly long, and feed the beads in using soapy water or silicone spray as a lubricant. What you never touch is the bottom bracket at each corner: the lift cable anchors there, under spring tension even with the door down.
When to call someone
Call rather than carry on if the gap is wedge shaped, if the door does not sit square in the opening, or if a cable looks loose or frayed. A bent or corroded retainer is the same answer, because replacing it means working on the bottom rail where the cables anchor. Those are load and alignment jobs. The springs and cables behind them are not DIY work: a torsion spring holds several hundred pounds of stored energy with the door closed. Swapping a seal on a straight door with a good retainer is a Saturday morning job, and anything past that line is worth a call to (855) 866-4555.