Why is the bay window the dampest part of my house?

Ask a surveyor where damp turns up in a Victorian or interwar house and the bay window is near the top of the list. Nothing about it is defective in principle; it simply assembles more weak details into one small area than any other part of the building, and each of them is a known leak point on its own.

What you'll need

• Binoculars
• Torch
• Access to lift a floorboard in the bay, if suspended (optional)

Step by step

1. Start with the bay roof.Most bays have a small lead, felt, asphalt or tiled roof at a shallow pitch. Look for ponding, splits at the folds in lead, blisters in felt, and the junction with the main wall above. That junction is the single commonest source.
2. Check the abutment and the flashing above the bay roof.The flashing where the bay roof meets the main wall must be dressed into a chase and pointed, with a cavity tray above it on a cavity wall. A cement fillet instead of a flashing is a guaranteed future leak.
3. Check the outlet.Bay roofs drain to a small gutter or an outlet that blocks with leaves and moss more readily than a main gutter because it is small and rarely looked at. A blocked bay roof holds water against the wall continuously.
4. Look at the sills and the mullions.A bay has more sill per square metre than any other window and often has joints in the sill run. Check every joint, every drip groove and every end.
5. Work out what the bay's walls are.Bays are frequently built thinner than the main walls, and in an interwar house the main walls may be cavity while the bay itself is solid brick. Thinner and solid means colder and more permeable, so it grows condensation and takes in rain more readily.
6. Look underneath.The bay's floor often projects on cantilevered joists beyond the main wall, with cold outside air under it and around three sides. That is why bay floors are cold and why condensation forms at the skirting there.
7. Check the ventilation of the bay's floor void.Airbricks serving that void are frequently buried by later paving or planting, which rots the cantilevered joist ends — a structurally significant problem hidden under a floor.
8. Separate the rain problem from the cold problem.Bays commonly have both. Fix the water first, because a wet wall is also a cold wall, and reassess the condensation afterwards.

Tips

• Insulating between the joists of a bay floor and blocking the draught at the skirting is one of the most noticeable comfort improvements available in an old house, but never at the cost of blocking the underfloor ventilation.
• Bay roofs are small, awkward and often patched. If yours has been repaired several times with different materials, a proper recovering is usually cheaper over ten years than another patch.
• Heavy curtains drawn across a bay cut the bay off from the room's warmth, which is why mould so often appears behind them.

Common mistakes

• Assuming the damp above a bay is coming from the main roof — It is nearly always the bay's own roof, its flashing or its gutter. The main roof is a long way above and its water does not usually arrive there.
• Blocking the airbricks under a bay when laying a new path or patio — The cantilevered joist ends over a bay are among the most vulnerable timbers in the house, and stopping the airflow under them starts wet rot in a place nobody inspects.

Questions people ask