Water Coming In Under the Garage Door

The main event, and the reason the usual advice underperforms here. A bottom seal squashed against concrete resists gravity well and pressure poorly. When a microburst spreads out at ground level, air stacks up against the face of the door and looks for the only opening in that wall — the line where the door meets the slab — and drags water through with it.

The seal doesn't have to be bad for this to happen. It has to be less than perfect, which after a couple of Arizona summers it always is. Rubber that has gone stiff touches the high spots and bridges everything else, and every one of those bridged gaps becomes an inlet under pressure.

What helps is a seal with more squash in it and a flat surface for it to close against. What doesn't help is putting something absorbent on the inside, because that's downstream of the pressure rather than in front of it.

How you can tell: It only happens in wind-driven rain. The wet area fans in from the whole width. A quiet all-day rain produces nothing.

If water arrives during quiet rain with no wind involved, the seal isn't the story — supply is. A driveway is supposed to fall away from the opening. Plenty around the valley fall toward it by a small amount, either because the original grade was marginal or because the ground has moved since.

An apron with a slight backward fall turns every rain event into a sustained delivery of water at the door line. Instead of a brief pulse during a squall, you get a steady supply for as long as it rains, and a seal that would shrug off a passing storm gets a long, patient test that it eventually fails.

The same thing happens when a driveway is resurfaced or an overlay is added without accounting for the door. Raise the concrete just outside the opening by a quarter inch and you've built a shallow reservoir that empties inward.

How you can tell: Water arrives in ordinary rain, not just in wind. A hose run on the apron pools near the door or runs toward it rather than away.

An easy one to miss because the cause is ten feet above the symptom. Flat and low-slope roofs in this valley discharge through scuppers and canales, and pitched roofs discharge through gutters and downspouts. Where any of those dumps within a few feet of the garage opening, a storm delivers a concentrated column of water onto the apron in exactly the place you least want it.

The volume is what makes this different from ordinary rain landing on the driveway. A roof plane collects over a large area and delivers it through one small outlet, so the apron near the door receives many times what falls on it directly. In a heavy cell that's enough to overwhelm any seal, regardless of condition.

The other version is a gutter that has failed rather than one that's badly placed — a blocked outlet, a joint that has come apart, or a section that has sagged and now overflows at the low point, which is often right above the door.

How you can tell: A concentrated wet patch at the same spot after every storm, and a scupper, downspout or roof edge directly above it. The silt deposit is heaviest at that one point.

Under pressure, water behaves like anything else being pushed — it finds the weakest point rather than distributing itself politely. On a garage door the weakest point is almost always one of the two bottom corners.

The reason is structural. The bottom seal belongs to the door and ends at the door's edge. The vertical jamb seal belongs to the frame and often stops short of the slab. Between them is a small triangular void that no single part is responsible for closing, and a driven storm aims straight at it. Once water is through that void it runs inward along the slab and spreads, which is why a corner entry point can produce a wet area that looks like it came from the whole width.

There's a second route to the same corner, and it's the one people miss because it doesn't come under the door at all. DASMA describes it in TDS #1503: on some door designs, rain sitting on the face collects in the joints between sections while the door is closed, and that trapped water “often runs or wicks its way to the ends of the door, bypassing the perimeter door seal and entering the interior space.” DASMA is blunt that this “can result in a substantial amount of interior water.” If your bottom seal is genuinely soft and contacting, both corners read dark from inside, and you still get water at the ends after a storm that hit the face of the door hard, this is the likely mechanism — and the remedies are seals between the sections and an enhanced perimeter seal from the manufacturer, not more rubber at the floor.

The silt evidence settles which one you have. Sediment concentrates where the flow was fastest, and that's the inlet — water that wicked out of a section joint arrives higher up and leaves a run mark on the inside face of the door rather than a sediment fan on the slab.

How you can tell: The heaviest sediment is at one corner. The wet area is a streak that starts at an end of the door rather than a fan from the middle.

Rubber can follow a certain amount of contour and no more. Half an inch of deflection is a generous figure for a bulb seal. Where the concrete under the door dishes, slopes, or has cracked with a lip on one side, the seal is spanning a gap and water walks straight through underneath it in any storm at all.

This is the specific situation a threshold is built for, and it's the option most homeowners have never been offered. It's a heavy vinyl or rubber bead bonded to the slab where the door lands, and it does two jobs a seal can't: it raises the closing surface to a consistent height, and because it's bonded down, it also acts as a low dam against water running across the apron.

Worth being honest about the trade-offs. You'll feel it driving over. If the garage floor slopes inward it can hold water and grit on the inside face. And it has to be positioned to match where the door actually lands, which is not always where people assume.

A bonded threshold is the retrofit version of something DASMA specifies properly at the concrete. TDS #197 — the sheet covering water under the bottom of a door — describes two details. The first is a slightly inclined lip or sill that the door rests on, drawn at a 3° to 5° slope. The second, which DASMA says “improves the door seal and greatly aids in disallowing water infiltration,” is a rectangular notch cut into the lip of the slab, below slab grade, at least ½ inch deep and sized to take up to a 2-inch-wide track and up to a 2-inch-thick section so the door can still fully close. Those are pour-day details, so on an existing garage they're a saw-cut conversation with a concrete contractor rather than a door job. Knowing they exist is still useful: if you're building or replacing a driveway, this is the moment to get it right, and it's worth more than any seal you can buy afterward. One caveat — TDS #197's scope explicitly excludes the jambs and header and excludes driven or pressurized water, so a perfect sill still doesn't answer a monsoon hitting the face of the door.

How you can tell: The rubber is soft and in good condition, the door is level, and water enters at a specific low area every time. A straightedge on the concrete shows daylight under it.

Worth ruling out before anyone spends money on rubber. Water enters garages by several routes, and the floor slopes toward the door, so a leak at the back of the garage produces a wet patch at the front. Everybody then inspects the door.

The usual alternatives are the base of the walls, where stucco weep screed sits close to grade and landscaping has been built up against it; expansion joints and control joints in the slab; cracks in the floor that admit water when the ground outside is saturated; and the service door, which typically has an older and worse seal than the main door and gets none of the attention.

The distinguishing evidence is direction of travel. Water that came under the main door leaves silt at the door line and spreads inward. Water from elsewhere leaves its heaviest deposit somewhere else entirely, and the door line is just where it ended up.

How you can tell: The floor is wet but the silt line and the wet perimeter don't start at the door. Damp along a wall base, or a wet line following a crack, points elsewhere.

When water comes in at the same end every single time and never at the other, look at how the door is hanging before you look at anything else.

A door hangs level because both lift cables carry it equally. When one cable slips, spools unevenly on its drum, or comes off, that corner drops and the opposite corner lifts. The high corner now has a permanent opening that no seal will close, because the gap tapers along the door's length and rubber can only compress by a fixed amount.

The reason this belongs on a page about water is that we get called for a wet garage, find a door that hasn't been level for months, and have a considerably more urgent conversation than the one the customer booked. An unevenly hung door works its rollers sideways in the track, and that is the state immediately before a door comes off its track.

How you can tell: Water only ever at one end. The gap under the door visibly tapers from one corner to the other, and one bottom corner sits closer to the slab than its opposite.