02/10/2026
A door with two leaves 🍃, one active, one inactive.
Ever heard of a "constant latching flush bolt"?
It's very common to use a pair door with one leaf active and one inactive, especially to storage or utility rooms.
🏃♂️ The active leaf is used most often, providing human access through the opening.
🚫 The inactive leaf is usually left closed, and only opened when a large object needs to come through.
❓️ Could you just use a pair door with both leaves active?
Yes, of course, but then you'll be adding cost and complexity without much added benefit.
For any pair door with both leaves active and latching, each leaf needs to latch into something:
either a mullion, or the head of the frame and/or the floor (top and/or bottom bolts).
For a pair door with single leaf active, the active leaf can simply latch into the inactive leaf.
Fewer parts!
And what holds the inactive leaf in place?
Flush bolts. 🛠
There are three main categories:
🟢 Manual flush bolt
🟢 Automatic flush bolt
🟢 Constant latching flush bolt
All three of these are noted in 2024 IBC Section 1010.2.4 (Locks and Latches).
All types of flush bolts are installed in the inactive door leaf, with the operable part in the edge of the door facing towards the active leaf. The device is installed flush into the door leaf without any protrusions (except the button, in the case of an automatic flush bolt).
ℹ️ Interestingly, constant latching bolts were not identified in versions of the IBC earlier than 2024, although they have been widely manufactured and used in buildings for quite some time.
Here is a brief rundown on the three different types of flush bolts:
Manual flush bolt
- As the name implies, it is operated manually 💪, both for latching and unlatching.
- This is the simplest type, and is appropriate for many doors.
- Because its latching function is 100% manual, this flush bolt is prohibited on fire rated doors 🚫
Automatic and Constant latching flush bolts both have an automatic latching feature. Both types can be used on fire rated doors.
🔥✅️
The main difference between the two is that an automatic flush bolt immediately unlatches whenever the active leaf is opened, whereas a constant latching flush bolt must be unlatched manually.
Both of these types have appropriate applications.
A constant latching flush bolt is good for doors where it would be undesirable for the inactive leaf to become unlatched whenever the active leaf is opened (for example, exterior or high security doors).
An automatic flush bolt is good for doors where the inactive leaf will be used more often, or where it is inconvenient to manually unlatch (for example, doors that large/heavy carts will pass through often).
For any fire rated pair door with single leaf active where the active leaf latches into the inactive leaf, both leaves must each have a closer, and a device called a "coordinator". The coordinator ensures that the inactive leaf closes first, so that the active leaf can latch into it.
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