Leaking Bay Window Roof: Finding the Cause

The roof covering

Felt on a flat bay cracks and holds water as it ages. Lead on older bays can split. Tiles on pitched bays slip like any others.

The flashing

Where the bay roof meets the house wall, a strip of lead seals the join. If it lifts, water runs down behind it and shows at the back of the bay.

The window itself

Damp at the sides of the frame or on the sill is more likely failed seals or pointing than the roof.

How to tell

• Stain on the bay ceiling: suspect the roof
• Damp where the bay joins the wall: suspect the flashing
• Damp at the frame edges: suspect the window

The repair

A sound roof with one split can be patched. A worn-out covering is stripped, any rotten boards are replaced and a new covering is laid with fresh lead at the wall.

Why it matters

Water from a bay roof runs into the wall and window head below, so leaving it adds plastering and joinery to the bill.