Why Is My Bay Window Damp?

Bay windows are lovely features on a lot of Victorian and Edwardian houses round here, and they’re one of the places I’m most often asked to look at for damp.

Why bays are vulnerable

A bay sticks out from the main wall, so more of it is exposed to the weather. It usually has its own small roof with flashings where it meets the house, and sometimes its own gutter. The walls under and between the windows can be thinner than the main wall, and frames, panels and sills are often timber. Lots of joints, lots of cold surfaces, lots of places for water to get in.

Common causes

• The bay roof. Split lead, failed felt, slipped slates and especially failed flashing where it meets the main wall. Damp shows around the window head and bay ceiling, worse after rain.
• Gutters. A blocked or leaking gutter above can pour water onto the bay roof and down the face of the bay.
• Window frames and sills. Failed sealant, cracked sills and sills without a proper drip let water into the wall below the window.
• Low-level damp. High ground, paths or render carried down at the front can bridge the DPC.
• Rotten timber. Wet rot in timber sills, frames and panels lets water in and gets worse if left.
• Cracks. Bays can move differently from the main house, and cracks at the junction let rain in. Significant or growing cracks need looking at by a suitable professional.
• Condensation. All that glass and thin wall makes the bay the coldest part of the room. Heavy curtains and a sofa pushed into the bay trap moist air, giving black mould on reveals and under the window board.

Working out which

Look outside first, ideally after rain: the bay roof and flashing, gutters, sills and ground level. Worse after rain means water getting in. Black mould that’s worse in cold weather means condensation. It can be both.

Fix the outside defects first, then deal with ventilation and air movement inside. Let the walls dry before replastering or redecorating.