A French-door mullion — also known by its trade name *astragal* in North America and as a *deurnaald* in Dutch — is a vertical, fixed or removable centre post that sits between the two leaves of a double door. It is held in the frame, meets each door with a rebate and provides the stop against which the active leaf closes.
Function
The mullion handles several jobs in one element:
- Closing stop — it gives the active (first-opening) leaf a face to close against
- Lock housing — the active leaf’s lock engages into the mullion
- Draught seal — closes the gap between the two leaves
- Sound seal — completes the acoustic line between rooms
- Fire and smoke seal — essential in fire-rated double doors
Fixed or removable mullion
| Type | Properties | Used in |
|---|---|---|
| Fixed mullion | Built into the frame | Standard double doors, small entrance doors |
| Removable mullion | Demountable or attached to the inactive leaf | Wide openings for furniture moves, hospital trolley doors, fire-brigade access |
The removable mullion is locked to the passive leaf by an espagnolette or other shoot bolt. In day-to-day use it functions like a fixed mullion, but it can be lifted out when the full opening is needed.
Construction
A traditional timber French-door mullion consists of:
- A vertical post — usually hardwood, with a rebate on both sides
- Two stop profiles — for the active and passive leaves
- Lock case and keepers — for the main lock and any flush bolts
- A nose bead or overlap — in fire-rated versions, providing extra smoke sealing
In modern frames the mullion is sometimes replaced by a scissor mechanism or a double-leaf shoot-bolt arrangement, so the full opening is clear when both leaves are open.
Where they are found
French-door mullions appear in:
- Grand doors in monumental buildings — churches, town halls, mansions
- Double internal doors — between reception rooms, often “*en suite*”
- Fire-rated double doors — on escape routes
- Garage and industrial doors — in two leaves for wide openings
- Garden or terrace double doors — common in older homes
Maintenance
The mullion joint has to be airtight for it to work properly. Pay attention to:
- Replacing perished draught seals in good time
- Re-adjusting strike plates as the doors settle
- Inspecting timber for warping each year
- Replacing fire-stop strips only with equivalently certified profiles
Related terms
- Double door
- Door frame
- Jamb
- Hardware
- Espagnolette
- Fire resistance
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