Why is my bay window roof leaking?

A bay window sits proud of the house wall, and its roof has to deal with a lot in a small space. There is usually a flat or low-pitched top, a set of narrow sloping faces if the bay is canted (angled rather than square), and a junction where all of it meets the main wall. Each of those joints needs lead or a sealed membrane to turn water away.

The roof is also easy to forget. It sits below the main gutters, out of sight from the garden and above the line of vision from the street, so a slow failure may drip into the front room for a long while before anyone spots the ceiling mark. Bay roofs also take the full weight of any leaves, moss and debris blowing down from the main roof above, and small gutters on a bay clog quickly.

Lead is the traditional covering for a bay roof top and for the flashing where the bay meets the wall of the house. Here the flashing, a waterproof metal strip dressed over the joint, does the work. Code 4 or Code 5 lead, the lighter and heavier of the common grades, is typical for this work. MyJobQuote (22 September 2026) says well-installed lead flashing can last over 80 years, so a failure at a bay usually points to a fitting problem or damage, rather than simple age.

Look for a top edge that has lifted, a split along a fold, and pointing or sealant falling away from the brickwork. On Victorian and Edwardian terraces, the lead at the back of a bay is often old and has been patched with mastic more than once. Repeated sealant is a sign that the underlying metal has stopped doing its job. Our page on leaks where a roof meets a wall explains the wall detail that most bay flashings depend on.

Many bay roofs built in the twentieth century have a flat top covered in bitumen felt. Felt tears, blisters and shrinks away from the edges. Torch-on felt can last 10 to 20 years according to Less, and a poor job can fail in under 10, so a bay roof from a decade or more ago may simply be due for renewal.

A few warning signs: bubbles that move under your fingers, cracks across the surface, pooled water that does not drain, and edges that have curled. Because a bay top is so small, patching felt rarely pays. Recovering the whole top in a modern membrane or lead is usually the cleaner repair, and the saving over a patch shows up in not being called back after the next storm.

Where a bay has a pitched roof on each face, the covering is usually the same tile or slate as the main roof, laid with hips (the angled external corners) between the faces. Valleys and hips on a bay are short but sharp. A single slipped tile at a hip can let water run straight down into the wall plate.

Cracked slates, nail sickness (rusted nails letting slates slide) and cement bedding that has perished all show up here. Cutting tiles neatly round such a small area takes skill, and a rushed repair with sealant is one of the commonest reasons a bay roof leaks again. Our note on roof valley repair costs covers what drives the price where two slopes meet, and similar reasoning holds for a bay hip.

When a single flashing has lifted and the remaining covering is healthy, repairing it makes sense: refit the lead, rake and re-point the wall joint, replace the odd tile. If the felt is cracked all over, or the lead has several splits, recovering the whole bay roof makes more sense. Rotten timbers beneath the covering, which only show once it is opened, mean the carpentry comes first.

Access matters to the price. A ground-floor bay may need nothing more than a ladder and a low platform. An upper-floor bay may need scaffold, and MyBuilder's guide, updated in August 2026, gives £850 to £1,200 for a standard two-storey scaffold. Treat that as a national typical range; on a small job it can outweigh the repair itself. Our survey establishes whether a scaffold is needed at all.

Our survey ends with pictures of this particular bay and a price in writing for just the work the fault demands.

Stand back across the road or in the garden of a neighbour who is happy to let you look, and check for slipped tiles, moss growth, sagging at the front edge or gutter overflow staining below the bay. Inside, look at the ceiling of the room under the bay for rings or bubbling paint, and at the top corners of the window frame for damp.

If water is getting in now, put a container underneath and clear the room edges. Do not step onto a bay roof to investigate: they are thin, and many are not built to carry a person. A bay behaves much like a porch roof, and cold-weather checks are covered in our page on roof repairs in winter. Our general roof repairs service covers bays, and the finished work carries a 10-year workmanship guarantee in writing.