Garage Door Mullion

A garage door mullion is an upright post set in the middle of a wide opening so that two or more doors can share it. Each door runs in its own track or guide, and the mullion carries the inside edge of the doors on each side of it. On a house the word usually means the strip between two windows. On a commercial building it means the center post between overhead or rolling steel doors, and it answers a practical problem: one very wide door is heavy, costly and slow, while two doors side by side with a post between them are easier to build and run.

Types of garage door mullions

• Fixed mullions stay in place. Cornell Cookson, a rolling door maker, describes its fixed guide mullions as the choice for wide openings where several doors sit next to each other, using little guide space.
• Removable mullions unbolt or unlatch from the floor and header so the full opening can be used now and then, such as moving large equipment through a shop. Cornell Cookson offers removable guide mullions for very wide openings that need full access only occasionally.
• Sliding mullions hang from a track under the header and slide to one side. Hanover Door's sliding mullion rides on six rollers, stays about half an inch off the floor, disconnects with deadbolts at the top and bottom, and parks against one jamb, taking up about 10 inches of it. Either door can still open on its own while the post is in place.
• Corner mullions hold the guides of two rolling doors at a right angle, so the doors can wrap a corner of a building.

What a mullion means for the doors

A mullion turns one opening into two. Each door is measured to its own clear width, from the jamb to the mullion, so the opening size on the order is not the full width of the wall. On rolling steel doors, the guides for both curtains mount to the mullion, so the post has to be anchored well enough to take wind load on both doors. Some sliding mullions carry a limit switch at the top, as Hanover's does, to keep the motor from running while the post is not locked in place. If a mullion is moved, check that both doors still line up in their guides before you run them.

Mullion or one wide door?

One door across the whole opening gives clear access every time but needs a heavier curtain or sections, stronger springs or a bigger operator, and a stronger header. A mullion with two doors lets each door be lighter, lets one side stay closed in cold weather while the other runs, and keeps a bay working if one door fails. A removable or sliding mullion gives both: two doors for daily use and the full width when it is needed.

Mullion or center post on a house?

Many homes have two single garage doors with a strip of wall between them. That strip is a framed wall section, often called a pier, with its own jambs on each side. It holds up the header and the house above, so it is part of the building, not part of the door. A true mullion is a post in one opening with a single header across the top, and that is why it can be unbolted or slid aside. If you want to turn two single doors into one double door, the pier comes out and the header has to be rebuilt to carry the load, which is framing work, not door work.

Signs a mullion needs attention

• A door rubs or binds on the side next to the mullion, or its curtain or rollers ride out of line in the guide.
• The post moves or rattles when a door runs or the wind blows, which points to loose anchors at the floor or header.
• A removable or sliding post will not lock back in, or its latches no longer line up with the floor pocket.
• The post was hit by a forklift or truck and is no longer plumb.

Stop running both doors if the post is bent or loose, because each door relies on it for straight travel.

What is the strip around the garage door called?

The boards on each side of the opening are the jambs, and the board across the top is the header. The strip fastened to them that the closed door seals against is the stop molding, which usually carries the perimeter seal.

What is a mullion on a sliding door?

On a sliding patio door or a window, the mullion is the vertical bar where two panels or frames meet. It is the same idea at a smaller size: a post that divides an opening and holds the edges of the panels on each side.

Commercial door service in Lakewood and Denver

G Brothers works on rolling steel, sectional, high-cycle and fire-rated doors for warehouses, storefronts and loading docks across the Denver metro, on every brand and operator type. For a door that binds against its mullion or guides, see rolling steel door repair or commercial garage door repair. Planning a new double opening? Start with commercial garage door installation.