Bay window roof problems and repairs

A bay window roof is the small roof over a bay, tucked under the main eaves. Because it is low, shallow and easy to forget, it is one of the commonest places for a leak to start. Most problems come from the lead flashing where the bay roof meets the wall, cracked or slipped tiles, or a tired felt covering on a flat bay roof. Caught early, each one is a small repair.

Why bay window roofs leak

A bay roof sits under the main gutter, so it takes the overflow whenever that gutter blocks. It also has a shallow pitch, so water moves slowly and finds every gap. In addition, many bay roofs are as old as the house, with original timber, lead and mortar that have had decades of weather.

The usual causes are:

• Lead flashing that has cracked, split or slipped out of the joint in the wall, which is the most frequent source of bay leaks.
• Cracked or slipped tiles and slates, where even one gap lets water run behind the covering.
• Felt on a flat bay roof that has blistered, cracked or split in the sun.
• Crumbling mortar on small ridges and hips.
• Blocked or overflowing gutters above, pouring water onto the bay.
• Rotten timbers underneath, once a leak has gone unnoticed for a while.

Warning signs your bay window roof needs repair

Look inside first, because you’ll often see damage there before you can spot it from the garden:

• damp patches or brown staining on the ceiling of the bay;
• bubbling paint or loose plaster at the top of the bay window;
• water dripping round the window head in heavy rain;
• a cold, musty corner in the bay.

Outside, look for missing or slipped tiles, lead that has lifted from the wall, cracked felt, or a sagging line to the roof.

Bay window roof repairs

The repair depends on what has failed:

• Replacing tiles or slates. Quick, as long as matching tiles are available.
• Re-dressing or renewing the lead flashing. The lead is cut back into the mortar joint, dressed down over the roof and repointed. On an old bay, new lead is often the better job.
• Re-covering a flat bay roof. Old felt can be replaced with new felt, glass-reinforced plastic (GRP) or EPDM rubber, which last longer.
• Timber repairs. Rotten rafters, boarding or fascias are cut out and replaced before the new covering goes on.
• Clearing or replacing the gutter above, so the bay stops taking the overflow.
• A new bay roof, when the structure is too far gone to patch.

Most bay roofs can be reached from a ladder or a small tower, though a roofer may still want scaffolding for a full re-cover.

Replacing the bay window at the same time

If the bay window itself is failing, fix the roof while the frame is out. The window fitter has to prop the bay roof during the work anyway, and any leak above will damage the new frame if you leave it. See bay windows for what a new uPVC bay costs, and why the corner posts need to be rated to carry the roof.

Bay window roof materials

Match the covering to the house: natural slate or clay tiles on period homes, concrete tiles where the main roof has them, and lead, GRP or EPDM on a flat bay. In a conservation area, the council may expect like-for-like materials on the front of the house; see planning permission for windows.

Getting it fixed

Ask the roofer what has failed and why, what they will replace, and how they will finish the lead. A roofer registered with a trade body such as the National Federation of Roofing Contractors is a good start. If you are getting the windows done too, get quotes for the bay window and ask whether the installer can arrange the roof.