angle bar

An angle bar is a vertical structural member that runs along the inside corner where two perpendicular window faces meet in a bay window, polygon window, or bow window. It sits at the junction between the two panes of glass or window frames and carries both the weight and the lateral stress of the assembly. On the exterior, it appears as a vertical division; on the interior, it forms a trim piece where the two window planes meet at a right angle or acute angle.

The angle bar differs from a standard mullion, which divides panes on a single flat wall plane. Because a bay window projects outward, its corner angle bar must handle shear and racking forces that a straight mullion never encounters. The bar must be rigid enough to prevent the two window frames from racking independently of each other, which would break the glass or seals. In timber framing, angle bars are typically made from solid wood stock, often 1.5 to 2 inches thick, selected for strength and dimensional stability. Metal angle bars in steel or aluminum serve the same function in commercial or modern residential work.

The angle bar's position makes it critical to the window's weathertightness. Water running down the exterior can collect at the corner, so angle bars are typically sloped or crowned at the top to shed water away. The junction also creates a node of potential air leakage if seals fail. The bar itself absorbs deflection: when wind pressure pushes the bay outward, the angle bar must flex slightly without breaking the glass or cracking its own joints with the adjacent frames. Poor installation, undersized material, or wood movement due to seasonal humidity change can cause the corner to rack, leading to gap formation and infiltration.

In period carpentry and restoration work, angle bars in timber are often tenoned or mortised into the frame head at the top and secured at the bottom. Modern work may use mechanical fasteners or adhesives. The challenge lies in accommodating wood movement perpendicular to the grain while maintaining a rigid corner. Angle bars in hardwoods like oak or maple resist seasonal movement better than softwoods, though all wood expands and contracts across the grain.

The term angle bar is primarily British and Commonwealth carpentry usage. North American builders more often call it a corner post, bay post, or simply the corner mullion. The principle is the same across traditions: a single vertical member that resolves the structural and weathering complexity at the point where a bay window's geometry changes direction.