Garage Door Weather Seal Replacement: Diagnose the Gap First
A worn garage door weather seal announces itself as daylight under the door, a wet stripe on the floor after rain, leaves piling in the corners, or chew marks where a rodent found a way in. The right fix depends on which seal failed: the bottom astragal, the side and top stop molding, or both. Below is how to read the gap the way a technician would — and why the bottom seal is one of the few garage door parts a careful owner can replace safely, as long as it is sized right.
Request ServiceStart With the Daylight Test
Close the door on a bright day, turn off the garage lights, and look. Where the light leaks tells you which seal failed. A thin line of daylight across the full width of the door points to a flattened bottom seal. Light at one bottom corner only usually means the slab has settled or the door sits out of level — a new seal alone will not close that gap. Light along the vertical edges or above the door means the side or top stop molding has cracked, curled, or pulled away.
Gaps leave other evidence too: a dried water line on the floor, a drift of leaves in one corner, or gnaw marks on the seal ends. A mouse can pass through a gap about the width of a pencil, so chewed seal corners deserve attention.
A technician reads the same clues in a set order: floor line and slab first, then the seal profile and its condition, then the retainer, then door level and balance, and finally the stop molding contact along the sides and top.
Bottom Seals: T-Style Astragal vs. Nail-On
Most steel doors carry an aluminum retainer along the bottom edge. The seal itself is a vinyl or rubber astragal — a U-shaped or bulb-shaped strip with two T-shaped beads that slide into the retainer's channels. Those T-beads come in sizes; 1/4-inch and 5/16-inch are the common ones, and they do not interchange. A bulb profile bridges bigger floor dips than a flat U profile, which matters on older slabs.
Wood doors often skip the retainer and use a nail-on seal instead: an angled rubber flap fastened along the bottom edge with galvanized roofing nails. Matching a replacement means matching three things — the attachment style, the T-bead size or nail-on width, and the door width. Buy the seal at least as long as the door is wide and trim it after fitting.
Side and Top Seals: Vinyl Stop Molding
The gaps along the sides and top of the door are closed by stop molding nailed to the jamb. Modern stop molding is rigid vinyl with a soft co-extruded flap along its edge. The flap should just brush the face of the closed door — enough to block drafts and wind-driven rain without dragging.
Stop molding fails quietly. The flap cracks in sunlight, curls away from the door, or gets painted over until it turns stiff and stops sealing. A replacement should be nailed up with the door closed, so the flap sets against the door face at the right depth.
The Seal Protects the Door, Not Just the Garage
The bottom seal is also the door's own protection. On a wood door, the bottom rail wicks up standing water and rots from the inside out. On a steel door, rust starts at the folded seam of the bottom section and climbs. A split seal can even make things worse by letting the aluminum retainer trap water directly against the door edge.
That is why a flattened seal is worth replacing early: the seal is a minor part, while a rotted or rusted-through bottom section becomes a panel repair. If rainwater runs toward the door, pair the new seal with a glued-down garage floor threshold, which raises the sealing line above the puddle.
Sizing It Right — and Two Honest Cautions
Bottom seal replacement is one of the few garage door jobs a careful owner can do safely. Pull a few inches of the old seal out of the retainer, measure the T-bead on the cut end, measure the door width, and buy the matching profile with a little extra length. Clean the retainer channels first; a spray of diluted dish soap lets the new seal slide instead of bind.
- Never unbolt the bottom corner brackets. The lifting cables anchor there under spring tension, and loosening them can release that force violently. Seal work never requires touching them.
- A gap that survives a new seal is not a seal problem. A corner gap, a door that will not settle flat, or a bent retainer points to level, balance, or hardware trouble. Describe what you see and submit a service request through our contact form, and a technician can assess the door itself.
Frequently Asked Questions
How do I find the T-bead size of my old bottom seal?
Pull one end of the old seal a few inches out of the retainer and look at the cut end. You will see two round T-beads that ride in the retainer channels. Measure one — 1/4 inch and 5/16 inch are the common sizes, and the wrong one will either jam or fall out. Bring the scrap end to the store, or photograph it next to a ruler before ordering.
Why won't the new seal slide into the retainer?
Three usual causes: the T-bead size is wrong, the channels are packed with grit or corrosion, or the dry vinyl is binding. Clean the channels, spray them with diluted dish soap, and feed both beads together from one end. If the retainer lips are crushed or bent, the retainer itself needs replacing — and because it sits beside the bottom corner brackets where the lift cables anchor under tension, that swap is a job to request a technician for.
I installed a new bottom seal and water still gets in. What now?
A seal closes gaps; it cannot outwork drainage. If the driveway slopes toward the door or the slab has settled, water rides under even a fresh bulb seal. A glued-down floor threshold raises the sealing line, and a taller bulb profile bridges small dips. If daylight shows at one corner only, the door may be sitting out of level — describe that in a service request so a technician can check the door's level and balance, not just the seal.