What is a horn on a window frame?
A horn — in Dutch oor, literally “ear” — is a laterally projecting part. In building it usually refers to the horns of a frame: the parts of the head or the sill that project a little beyond the jambs and are built into the masonry. They give the frame a hold and an anchorage in the wall.
The principle is simple: because the head or sill runs on into the masonry, the wall grips the frame. Without horns the frame is supported only at the reveal and by its fixings.
Exposed and concealed horns
- Exposed horns project and remain visible in the masonry. You see them mainly on historic façades and on timber frames in a traditional elevation.
- Concealed horns are entirely hidden by the masonry. This is the usual arrangement in modern housing.
Other meanings of the Dutch “oor”
The word recurs in several places, and always means the same thing: a small projecting part.
- Leadwork — the projecting parts of lead flashings that are dressed into the bed joint are called ears. On a pitched roof they are also known as dog ears.
- A construction detail — the transition from a square to a moulded section, for example on the ribs and wall plates of timber vaults.
- Fortification — a projection at the shoulder angle of a bastion, also called an orillon, which gave cover to the flank.
- Paving — lying “on one ear” means that a road drains to one side only rather than to both.
Points to note
- Bed the horns fully in mortar; a half-filled joint around a horn is a path for water.
- Timber horns must be treated or painted on all faces before they are built in — this is a classic place for rot to start.
- When replacing a frame in existing masonry the horns have to be cut free; allow time for that.
Related terms
- Window frame
- Sill
- Head
- Wood rot
- Dog ears in leadwork
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