Sealing a Threshold Gap the Right Way
Last lesson we measured and cut a door sweep. This one's about what's underneath it, the actual gap at the threshold, and getting it closed up right instead of just slapping the sweep on and hoping.
Here's the thing about a threshold gap. Sometimes a sweep alone fixes it. Sometimes the gap is bigger than that, or the door itself is dragging or sitting wrong, and if you just clamp a sweep over a real problem you're covering it up, not fixing it. So before you install anything, get down on the floor and actually look.
First, figure out what you're dealing with
Shine a flashlight along the bottom of the door from outside while someone stands inside with the lights off. Or just get low and look for daylight. You're checking for a few different things:
- A gap that's roughly even along the whole bottom. That's normal, that's what the sweep is for.
- A gap that's bigger on one side than the other. That usually means the door's not hung level, or the hinges have dropped a little. A sweep can still work but you might be shimming the hinge side of the threshold to even things out first.
- The door dragging on the threshold instead of gapping. That's a different lesson entirely, that's a planing job, and I want to talk about that separately because people mess it up.
Most of what you'll run into is the first kind. Even gap, just needs sealing. Good, that's the easy one.
The planing trap
I want to tell you about the first door I ever tried to fix, because it's relevant here and it's not my finest hour.
This was early on at the apartment. Door was dragging on the threshold, catching every time. My answer was to take the door down and run a plane along the bottom edge. Seemed obvious. Wood's too big, shave some off.
I took off way more than I needed. It felt fine right then, door swung free, I was pretty pleased with myself. Then summer came. Utah's dry most of the year but we still get humidity swings, and the wood swelled back out a little the way wood does. Now instead of a tight fit I had a gap you could see daylight through, all winter, every winter after, because you can't put wood back once you've shaved it off.
So here's my rule now, and it applies to threshold gaps too, not just planing: take off less than you think you need. Check it, close the door, see how it feels. Take a little more if you need to. You can always go back for more material. You can't undo it once it's gone.
If your gap is from a dragging door and not just a normal seasonal gap, plane in small passes and test between each one. Pencil a line where you think the high spot is, take one pass, check again. It's slower than just going at it, but slower is right here.
Sealing the gap itself
Once you know the gap is normal and even, here's the order I do things in:
- Clean the threshold. Vacuum out the grit, the hair, whatever's built up. You want a clean surface for anything to seal against.
- Check the threshold itself for damage. Cracked, warped, or rotted thresholds need to be dealt with before you seal anything on top of them. A sweep on a broken threshold is a band-aid.
- Install the door sweep to the bottom edge of the door, following the actual sweep you cut last lesson. Screw it on, don't just tack it, it takes real friction from opening and closing thousands of times.
- If there's still a gap between the sweep and the threshold when the door's closed, that's when I'll add a threshold seal, the rubber gasket kind that sits in a track on the sill itself. Not every door needs this. Most just need the sweep. But on an older place, or a door that's settled some over the years, you sometimes need both working together.
Test it with the flashlight trick again once you're done. If you're still getting daylight, something's off, either the sweep's not making contact along the whole width or the gap's bigger than a sweep can handle and you're back to looking at the door alignment.
A caution here
If you're taking the door off the hinges to work on the bottom edge, get a second person or use a wedge to hold it steady. Doors are heavier than people expect and a door falling on a hand is a bad afternoon. Also, if you're using a hand plane, it's sharp enough to take a chunk out of you same as it takes a chunk out of the door. Keep your fingers behind the blade, always cut away from yourself.
I don't mess with thresholds that have real rot or structural damage in a class setting, that's a repair or replace situation and probably means water's been getting in for a while. If you've got that, it's worth a longer look than we've got time for here.
Before next time
Go check your own doors with the flashlight trick this week, even the ones you think are fine. I still find one every October I'd missed the year before.