Astragal

Diagram of an astragalus as part of a Ionic order column

An astragal is a moulding profile composed of a half-round surface surrounded by two flat planes (fillets). An astragal is sometimes referred to as a miniature torus. It can be an architectural element used at the top or base of a column, but is also employed as a framing device on furniture and woodwork.

The word "astragal" comes from the Greek and Latin for "vertebra," and the original astragals used in architecture were made in a beaded design, thereby resembling a vertebra.

On doors

An astragal is commonly used to seal between a pair of doors. The astragal closes the clearance gap. The vertical member (molding) attaches to a stile on one of a pair of doors (either sliding or swinging) against which the other door strikes, or closes. Exterior astragals are kerfed for weatherstripping. Also flush head and foot bolt hardware is commonly mortised into the astragal to hold the inactive door in place, when both doors operate, at the top and bottom.

Also known as “meeting stile seals,” the term can refer to the raised half-round overlap where pairs of doors meet, such as is the case with French doors. An astragal is designed to be applied to one or both doors of a pair at their meeting edges (meeting stiles). The astragal closes the clearance gap for the purpose of either providing a weather seal, ensuring privacy, preventing sound from leaking in or out of a room, minimizing the passage of light between the doors, or retarding the passage of smoke or flame during a fire.

Doors are typically the weakest link in any partition that is designed to block sound. This is often due to poor sealing around the perimeter of the door. Astragals, perimeter gasketing, drop seals and door sweeps can all be used to prevent sound from leaking through cracks around the door perimeter.

On windows

In cabinet making, an astragal can mean a bar separating panes of glass, either vertically or horizontally. This use is also common with window manufacturers.

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