Short answer: The rubber seal on the bottom of your garage door and the weatherstripping along the sides and top help keep out drafts, water, dust and pests. Replace them when they are cracked, flattened, torn or letting daylight through. Many bottom seals slide into a channel on the door and can be swapped by a homeowner. Never loosen the bottom brackets while you work, because they hold cables under high tension.

The seals around a garage door are easy to forget until the garage gets cold, a puddle shows up after rain, or leaves start collecting in the corner. They wear out slowly, and replacing them is one of the more manageable garage door jobs, with a few safety limits.
An easy check: on a bright day, close the door, turn off the garage lights and look for light around the edges.
If there is a gap on only one side, or the door sits crooked, a new seal will not fix it. An uneven door can mean a cable, spring or balance problem. That needs a professional. Stop using the door if it looks lopsided and read our spring warning signs.
Sometimes the door simply does not close all the way. That can be an opener setting or a sensor issue. Our post on why a garage door won't close covers the common causes.
Bottom seals come in different shapes to fit different retainers, so match what you have. Measure the width of the door and take a photo of the end of the old seal, or bring a piece of it with you when you shop.
If the retainer channel is bent, rusted through or pulling away from the door, the job gets bigger. That is a good time to have a professional take a look.
This trim is usually nailed or screwed to the door frame. When you replace it, set it so it touches the door lightly. If it presses too hard, the door can drag and the opener has to work harder. Work from outside with the door closed, and keep the opener unplugged.
A new seal can change where the door stops. Plug the opener back in, run the door a few times, and make sure it closes fully and reverses when it meets an obstruction. Follow your opener manual for the safety reverse test. Our garage door safety basics explain why that feature matters.
A yearly tune-up checks the door's balance, rollers, springs and opener, the parts that keep it closing evenly. See what a tune-up checks.
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Look for daylight under the closed door, drafts, water or debris coming in, and rubber that is cracked, flattened or torn.
Often, yes, if it slides into a channel. Raise the door partway with the opener, then unplug it, work from in front of the door rather than under it, and never loosen the bottom brackets, which hold cables under high tension.
A floor that is not level can cause a small gap. A door that sits crooked can mean a cable, spring or balance problem. Stop using it and have a professional check it.
It helps with rain blowing under the door. If water runs toward the garage from the driveway, a seal alone may not be enough, and a threshold seal or drainage fix may be needed.