Bay Roofs In Stretford, Urmston And Flixton

Key takeaways

• A bay roof is a complete roof in miniature, with a covering, a fall, a junction and drainage, all in a couple of square metres.
• It cannot be seen from the ground or from inside, which is why it goes decades without inspection.
• Square, splayed and curved bays each present a different degree of difficulty at the covering.
• The junction with the main wall causes more problems than the covering itself.
• An upstairs window on your own house is usually the best free view of it.

Why a small roof causes disproportionate trouble

A bay roof has an unusually high proportion of edge and junction to actual surface. It meets the main wall along its back edge, it has an outer edge at the front, it has two side edges, and it may have a small parapet or a raised trim. In a few square metres there are as many demanding details as in a much larger area of ordinary slope. Roofs fail at junctions, so the density of junctions is what predicts trouble, and by that measure a bay roof is one of the worst places on a typical house.

The shapes, and how much difficulty each brings

A square bay projects straight out with right angled corners, which is the simplest arrangement to cover and to flash. A splayed or canted bay has angled sides, which introduces additional corners where the covering must be formed around a change of direction. A curved or bow bay is the most demanding of all, since the covering has to follow a curve and any sheet material has to be worked to it. The more complex the shape, the more skill the original work required and the more likely it is that a later repair simplified something it should not have.

The back edge, where the water goes in

Along the rear of a bay roof, where it meets the main wall of the house, there should be a flashing let into a mortar joint and dressed down over the covering. It is the same detail as anywhere else and the same failure applies: where the mortar has weathered and the flashing has relaxed, or where somebody has sealed the joint instead of cutting it in, water runs behind and into the wall head. Because that wall head is directly above the front room, the resulting damp appears on the wall above the bay inside, which is one of the most commonly reported problems in housing of this type.

Coverings, and what is likely up there

Original bay roofs on housing of this period were commonly lead, sometimes zinc, sometimes bitumen felt depending on the specification and budget. Lead is excellent when laid in appropriately sized panels with allowance for movement, and fails early when laid in one large sheet, because thermal movement work hardens it until it splits. Felt has a much shorter life and perishes visibly, going brittle and crazed. Many bays have since been recovered in a modern membrane or fibreglass, which perform well provided the detail at the back edge and the outer trim was done properly.

Drainage, or the lack of it

A good many bay roofs have no gutter at all, on the assumption that the fall to the front will simply shed water over the edge. It holds up as long as the surface still drains forward and nothing has built up along the front. Once the covering deflects slightly, or debris builds along the front lip, water sits instead of leaving. On a surface this small, standing water is disproportionately significant, because there is nowhere for it to go and any weakness in the covering is under continuous exposure. Where a bay does have a small gutter, it is usually short, easily blocked and rarely cleared.

The free view most people never use

The best inspection point for a bay roof costs nothing: the upstairs window directly above it, in your own house. From there you can usually see the whole surface, and you are looking for standing water or a tide mark showing where it sits, moss or debris particularly along the back edge, a covering that looks crazed, split or blistered, a flashing that has lifted away from the brickwork, and any patch of sealant, which indicates a previous repair at that point. Two minutes at a window once a year would prevent a large proportion of the damp that housing of this type suffers.

What the damp inside looks like

Bay related damp typically appears at the top of the wall above the window rather than at the sill, and it often affects the ceiling of the front room near the window as well. That pattern distinguishes it from a window problem, which starts at the sill or the sides of the frame and works upward or outward. It is usually worse after prolonged rain rather than after cold weather, which distinguishes it from condensation. Getting that distinction right matters, because the three problems have entirely different remedies and the wrong one costs money without changing anything.

Repair or renew

If the covering is sound and the flashing has failed, renewing the flashing properly is the job and it is modest. If the covering is old felt that has crazed and split, or lead that has cracked along its fold lines, renewing the covering is generally better value than patching, because the area is small and the cost is dominated by access and preparation rather than material. Whatever is done, the specific question worth asking is how the back edge will be formed, because that detail decides whether the work lasts. While the covering is off is also the only realistic chance to see the timber beneath it.

When the bay is being reroofed anyway

Because a bay roof is small, the cost of work on it is dominated by getting somebody there and preparing the surface rather than by materials. That has a useful implication: the incremental cost of doing the job thoroughly rather than minimally is modest. If the covering is coming off, it is worth renewing the flashing at the back edge properly rather than reusing what is there, checking the timber deck beneath while it is exposed, forming a proper fall if the existing one is inadequate, and adding drainage at the front if there is none. Each of those individually would be hard to justify as a separate visit. Together, while access and preparation are already paid for, they turn a repair that lasts a few years into one that outlives most of the rest of the roof. Given how much damage a failed bay roof does to the wall and the room behind it, that is among the better returns available on any small piece of building work.

When the whole street has the same problem

Because these houses were built as a set with identical bays, the bay roofs on a street reach the end of their lives at roughly the same time. That produces a recognisable pattern: a period during which several neighbours discover damp above the front window within a few years of each other. It is worth talking to neighbours about it, both because their experience tells you what was found and what it cost, and because there may be scope to have the work done together. On a job where access and setting up dominate the price, two or three adjoining bays done in one visit is meaningfully cheaper than three separate visits.

Frequently Asked Questions

Why is the wall above my bay window damp?

Nearly always the bay roof, and nearly always at its back edge, where the metal should be buried in the brickwork and frequently is not. The damp appears above the window because that is where the wall head sits.

How can I see my bay roof?

Usually from the upstairs window directly above it in your own house, which costs nothing and shows the whole surface. Look for standing water or a tide mark, debris along the back edge, a crazed or split covering, a flashing that has lifted, and any patches of sealant indicating previous repairs.

Is it the bay roof or the window?

Look at where the damp starts. Bay roof problems show at the top of the wall above the window and often affect the front room ceiling too. Window problems start at the sill or the sides of the frame. Roof related damp also follows rain, whereas condensation follows cold weather.

Does a bay roof need a gutter?

Many were built without one, relying on the fall to shed water over the front edge, and that works only while the fall is adequate and the edge stays clear. Standing water, a stained wall beneath, or drips landing on the sill are all reasons to add drainage while somebody is already working on it.

What is my bay roof likely to be made of?

On housing of this period, most commonly lead, sometimes zinc or bitumen felt, and on many houses a later replacement in a modern membrane or fibreglass. Lead lasts well in correctly sized panels and splits early when laid in one large sheet, because it cannot accommodate thermal movement.

How often should a bay roof be checked?

Once a year is ample, and it can be done from an upstairs window in a couple of minutes. Because these roofs are invisible from the ground and from inside, they routinely go decades without anyone looking, which is precisely why they cause so much of the damp in this kind of housing.