Direct answer
The most effective way to stop water under a garage door is to combine a new, properly sized bottom seal on the door with a glued-down rubber threshold on the slab, then tighten up the side/top weatherstripping and address drainage outside. This pairing creates a compressible seal at the door and a raised water dam on the floor, even if the concrete is slightly out of level.
Why water gets in
- Worn or wrong-size bottom seal: The rubber bulb or T-seal hardens, cracks, or no longer meets the floor.
- Uneven slab: Concrete settles or heaves, leaving gaps at corners or in the center.
- Wind-driven rain: Blows past the sides/top where weatherstripping is loose or missing.
- Drainage issues: Downspouts, grading, or driveway slope push water toward the door.
Target specs:
- Threshold height: 1/2–3/4 in
- Bottom-seal compression when door is closed: 1/4–3/8 in
- Driveway/ apron slope away from door: 1/8 in per foot (minimum)
Tools and materials
- Tape measure, straightedge/level, chalk line
- Utility knife, tin snips or hacksaw
- Pliers (locking pliers helpful), flat screwdriver
- Cordless drill/driver (for new retainers)
- Caulk gun
- Degreaser and stiff brush; shop vac
- Rags and mineral spirits or acetone (final wipe)
- PPE: safety glasses, gloves, knee pads, ventilation
Materials (choose as needed):
- Garage door bottom seal matching your retainer: T-style, P-bulb, or U-seal (EPDM or vinyl). Rodent-resistant options with embedded fiber/steel are available.
- Aluminum bottom retainer (if yours is bent/rotten or wrong profile)
- Rubber garage door threshold kit (adhesive included or use polyurethane construction adhesive)
- Side/top weatherstripping (vinyl flap on wood/aluminum trim) or brush seals for metal roll-up doors
- Polyurethane concrete sealant for small floor cracks
- Optional: concrete patch/self-leveler, or channel drain if water volume is high
Estimated cost: bottom seal $15–$40; new retainer $20–$60; threshold kit $50–$120; side/top seals $20–$60; adhesives/sealants $10–$25. Time: 1–2 hours for seal + threshold, plus adhesive cure time.
Step-by-step: Stop the water
1) Diagnose the contact line
- Close the door on a strip of chalk or use a flashlight at dusk. Mark light gaps.
- Check floor for high/low spots with a 4–6 ft level or straightedge.
2) Replace the bottom seal
- Identify the profile: Look at the existing seal’s cross-section and the retainer channel. Most are double-T tracks; some are P-bulb push-ins.
- Remove old seal: Open the door, unplug the opener, and clamp the tracks with locking pliers so the door can’t move. Slide out the old seal; use pliers if stuck.
- Prep: Clean the retainer channel; deburr sharp edges.
- Install new seal: Lubricate with a little soapy water or silicone spray. Feed the T-edges into the channel and pull evenly from both sides. Cut to length and crimp the retainer ends with pliers so the seal can’t creep out.
- If the retainer is damaged or the wrong type, replace it with an aluminum retainer screwed to the bottom rail, then slide in the new seal.
Tip: For uneven floors, choose a larger bulb/U-seal or an adjustable double-fin style to fill dips.
3) Add a rubber threshold on the floor
- Dry-fit the threshold against the closed door so the seal compresses ~1/4–3/8 in. Mark position with a chalk line.
- Surface prep: Degrease and scrub. Rinse and dry thoroughly. Do a final wipe with mineral spirits or acetone.
- Adhesive: Run continuous 3/8–1/2 in beads of polyurethane adhesive within the marked area, especially at the front and back edges.
- Set the threshold: Lay it in place, then close the door to press it down evenly. Weight it with a few bricks.
- Cure: Keep the door closed and avoid driving over it for 12–24 hours (check adhesive label).
Orientation: The raised rib typically faces outward to form a dam; follow the kit’s arrows. Trim ends cleanly with a utility knife.
4) Seal sides and top
- Replace or adjust the vinyl flap weatherstripping on the jambs and header. It should lightly press against the door without binding. For steel roll-up doors, use brush seals.
5) Manage outside water
- Extend downspouts 6–10 ft away. Clear gutters.
- Seal hairline slab cracks with polyurethane sealant.
- If water still pools, consider a channel (trench) drain across the driveway in front of the door, tied to proper drainage. This is more involved but very effective for heavy runoff.
Safety considerations
- Unplug the opener and clamp tracks so the door doesn’t move while you work.
- Never loosen torsion spring hardware.
- Ventilate when using solvents/adhesives; wear gloves and eye protection.
- Mind trip hazards: choose a threshold height you can safely step/drive over.
Tips for best results
- Match the seal profile to your retainer. If unsure, bring a 1 in sample to the store or check the door model.
- In cold climates, EPDM seals stay flexible longer than basic vinyl.
- If a corner still leaks, add a door-bottom corner plug or notch the seal to fold upward at the ends.
- Rebalance/test the door after changing seal thickness. The opener’s downforce may need a slight tweak per the manual.
- For minor unevenness, a thin polyurethane self-leveling bead under the threshold can fill dips.
Common mistakes
- Using silicone caulk to glue a threshold—it won’t hold. Use polyurethane construction adhesive.
- Installing the threshold too far inside the garage, leaving a water track under the door.
- Picking the wrong seal profile (T vs. P/bulb) or too-small bulb that won’t reach the floor.
- Skipping surface prep; dust/oil will cause bond failure.
- Creating a threshold so tall the door won’t close or the car scrapes it.
When to call a pro
- The slab is out of level by more than about 3/4 in, or water still intrudes after seal/threshold upgrades. You may need grinding, a concrete “speed bump” water diverter, or a trench drain.
- The door bottom rail is damaged, sagging, or the door is warped.
- Spring/balance adjustments are needed beyond opener settings.
- You want a permanent drainage solution (cutting concrete, tying into storm drainage).
With a new bottom seal plus a well-bonded threshold and tuned side/top weatherstripping, most garages become watertight in a single afternoon. Add simple drainage fixes outside, and you’ll handle even wind-driven rain reliably.