Double Doors, Flush Bolts, and Door Seals
The bronze interlock/hook can be set back far enough that the 2-1/4" doors allow for a centered extension bolt in the passive door to bypass the hook and lock into the bronze. The exterior will show the lower part - sometimes a full width bronze threshold, sometimes a 1-1/4" wide x 3/8" tall bronze interlock. There is no way to avoid this.
With adjustable commercial sills, a vinyl bulb is used. The aluminum track that holds the bulb is recessed into a plow into the bottom of the door and mostly concealed.
If you are using Endura or similar sills for paired units, there is no good solution beyond interior surface bolts. The Europeans have better solutions (sills), but the materials are hard to get here and often demand significant changes to the jambs and rabbets that my traditional market will not accept.
From contributor N:
I once saw a door seal that fit up into a dado in the bottom of the door and dropped down when the door was closed. It worked by some type of push button that installed on the hinge edge of the door and when the door was closed, the button would push against the jamb and the sweep would drop down, then when the door was opened, the sweep would retract back up into the door. I don't know what it was called or if it would help you.
From contributor D:
Automatic door bottoms that mortise into the door bottom are available, but will not solve the problem of the retaining bolt on paired doors. In fact, it will make the use of such a bolt near impossible, and certainly problematic. A surface mount type will avoid the problem but look unwieldy. Other drawbacks are that the automatic door bottoms will freeze up in cold weather exposures and not retract when opened.
From contributor J:
We do this a couple different ways on US style butt hinge doors. For a 2 1/4" door the flush bolt can be positioned off center. Then use either an automatic bottom or adjustable bottom offset the other way. Pemco automatic bottoms are too large for this and do not work that well. Resource Conservation sells one that is a lot smaller in section and works better. I like their adjustable bottom better because of the simplicity. I don't think there is enough room on a 1 3/4 door to do this.
Another method that may work on 1 3/4 is to cut a small rabbet in the bottom outside face of the door and use a European tilt turn type gasket to butt up against the threshold. This would work without offsetting the bolt but is visible from the outside. On a 2 1/4 door the rabbit can be deeper and not noticeable. These gaskets are available from a few sources in the US.
We quit using all metal thresholds because of bad thermal conductivity. You can actually get ice growing inside in cold weather. We have had no problems with frost or freezing the gaskets with thermally broken or wood thresholds.
Modern, more insulated construction, insulated glass and the fact that homes are cooled and heated more changes the way doors and windows need to be constructed.
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