Weatherstripping

How to Stop Water Coming Under a Door

Water coming under an exterior door is a seal-and-drainage problem, and it is worth fixing fast — that same water soaks the sill and jamb and is a leading cause of frame rot.

Find where it is getting in

Water under a door comes from one of three places: the gap under the slab where the sweep should seal, the joint between the threshold and the floor, or water pooling at the door because it has nowhere to drain. On a wind-driven Ottawa rain or a spring melt, a small gap is enough. Watch during a storm to see where the water actually appears — a flashlight at floor level from inside, or a few paper towels laid along the threshold, will quickly show you whether the water is coming in under the slab or seeping at the threshold-to-floor joint. Pinpointing the entry path first saves you from sealing the wrong thing.

A worn or wrong door sweep

The sweep is the most common culprit. If it is torn, flattened, or set too high, water runs straight under. A sweep meant for weather should have a fin or blade that meets the threshold along its whole length. If you see daylight under the door, water gets in there too — the signs you need a new sweep are the same signs you will get water. Replacing it is quick, and choosing a seal that survives the cold matters too — see the best weatherstripping for Ottawa winters. It is part of our weatherstripping service.

A failed threshold or sill seal

The threshold itself has a seal where it meets the door and where it sits on the subfloor. Caulk fails, the threshold loosens, or an adjustable threshold has dropped too low to meet the sweep. Resealing or adjusting the threshold closes the path. If the sill has been wet a long time, check the wood — persistent water leads to frame and sill rot, which is a bigger repair.

Drainage and the approach

Sometimes the door is fine and the problem is water pooling against it — a step that slopes the wrong way, a clogged drain, or snow piled against the door. Even a perfect seal struggles against standing water. Improving drainage and keeping the threshold clear takes the load off the seal.

Adjusting an adjustable threshold

Many newer Ottawa exterior doors sit on an adjustable threshold — an aluminum cap with a row of screws that raise or lower it to meet the sweep. Over time the cap settles or someone backs the screws off to stop the door scraping, and the gap to the sweep opens up. Turning those screws clockwise raises the cap a fraction at a time; do it a quarter turn at a time, close the door, and check that it seals without binding. You want the sweep just kissing the cap along its whole length. If raising the threshold makes the door hard to open, you have gone too far — back it off slightly. This small adjustment fixes a surprising number of water and draft complaints without replacing anything.

The Ottawa freeze-thaw angle

Our climate makes water under a door worse than it would be in a milder place. In late winter and spring the snowbank against the step melts in the afternoon sun and refreezes overnight, driving meltwater at the threshold day after day. Ice can build up under the door and at the sill, holding the sweep up off the threshold so it no longer seals, and the same freeze-thaw cycle that lifts the sweep also opens hairline cracks in caulk and pries at a loosening threshold. This is why a door that stayed dry for years can suddenly leak in March. Clearing snow and ice away from the threshold through the winter takes a lot of the load off the seal — and is part of how you keep a door from freezing in the first place.

DIY versus calling a pro

Replacing a sweep, raising an adjustable threshold, or re-caulking a threshold joint are all reasonable homeowner jobs and the right first steps. Where it pays to call is when the threshold is loose or rotted underneath, when the sill or the bottom of the jamb has gone soft, or when you have sealed every obvious gap and water still gets in. Those point to a failed sill pan or hidden water damage in the frame, which need opening up to fix properly. If the wood at the base of the frame is dark, spongy, or crumbling, that is rotting frame repair, not a seal job.

Stop it before it rots the frame

Water under a door is not just a wet floor — it is the start of sill and jamb rot, and once the frame goes soft you are into frame repair. A worn sweep and tired threshold seal are cheap to fix now. We find the path the water is taking and seal it, flat-rate, across Ottawa and the Valley — and where the threshold itself has failed, we handle threshold and sill repair in the same visit.