A bay window or a porch is a small roof with a big job. It sits right over a front door or a principal room, it is seen from the street, and it is often the part of the house that has been patched most. On a Victorian or Edwardian terrace the covering is usually lead, laid over a timber frame that was built to carry it. Because these roofs are small, they are easy to neglect: you cannot see the top of them from the ground, nothing happens until a damp patch shows on the ceiling inside, and by then the water has often been getting in for some time. This guide describes how a lead bay or porch roof is put together, the faults that recur, and how to decide between mending and renewing. It also covers matching the lead to the house, what applies on a listed or conservation-area building in Bath, and the typical costs involved.
How a bay or porch roof is built
Most Victorian bay windows are canted, meaning the front is flat and the sides are angled back to the wall. The roof over them is a shallow shape that follows the plan: a flat or gently sloping top, with the angles at the corners running back to the house. A porch roof is simpler, often a single slope or a small hip, but it follows the same principles.
Underneath the lead there is a timber structure. Small joists, called firrings when they are tapered to give a fall, span from the front wall to the house. The joists carry a deck of boards or plywood, and the deck carries a separating layer and then the lead. The front of the roof usually finishes in a gutter or a timber fascia, and the back is tucked under a flashing at the house wall.
The lead itself is laid in a handful of pieces. The main sheet covers the flat or sloping top. At each angle, where two faces meet, the lead is dressed over a shaped timber roll, so the corner is a rounded rib, not a sharp fold. A hip roll is the same detail where two sloping faces meet at an outward corner, a feature of pitched bay roofs. The front edge is turned down into a gutter or over a drip. At the back, the lead turns up the house wall as an upstand, with a cover flashing wedged into a joint above it.
Because the structure is small, the whole roof can often be replaced in a short visit. Because it is small, a single mistake has a big effect. A fall that is too shallow, a roll left out or a flashing wedged badly can leave the roof leaking within a season or two. Larger lead roofs follow the same principles, and the thinking behind rolls, drips and bay layout is explained in the lead flat roof guide.
Why bay and porch roofs leak
The causes are fewer than you might expect, and they usually come down to water that cannot get away or lead that has been asked to do too much.
Ponding and poor falls
A flat bay roof has to slope towards its front edge or outlet. Manufacturers quote BS 6229 for a minimum finished fall of 1:80 and advise building to 1:40, leaving room for tolerances and later movement. A roof that has dropped at one corner, because a joist has rotted or the wall has settled, holds water in a pool. Standing water finds every weakness, and it makes the lead and the timber beneath it last less long.
Splits at the angles
The angles of a bay are where the lead has to turn and where the metal is under most stress. Cracks open along the roll or at the corner of a fold. Sometimes it is the lead itself, which has moved with temperature and is tired. Sometimes it is a poor dress, where the lead was folded sharply instead of rounded. A split at an angle is often the first leak a homeowner sees.
The wall abutment
At the house wall, the upstand and flashing keep the rain out. If the flashing has lifted, the pointing has gone, or the lead behind it is cracked, water runs down the wall and in at the junction. A damp patch on the ceiling just inside the wall is the typical sign. Our guide to lead flashings and soakers covers how that joint works.
Rot below
The timber deck and joists are the part you cannot see. Years of slow leaks cause wet rot, and the deck softens until it will no longer hold the lead flat. The lead then sags into the soft patch and cracks, which makes the leak worse. By the time the ceiling is stained, the structure may need more than a lead repair.
Repairing a lead bay or porch roof
A repair is sensible when the structure is sound and the faults are local. That might mean a split along one roll, a lifted flashing, or a corner where the lead has cracked. The work is done in stages. We first clear the roof of debris and moss and look for pooling. We then check the deck by pressing it and looking from beneath, if there is access, for dark staining or soft timber. Only then is a repair chosen.
A small split can be repaired in the lead itself, by removing the damaged piece and letting in a patch, joined by lead welding or a soldered or dressed joint, depending on the position. A lifted flashing can be re-wedged and re-pointed. A cracked roll can sometimes be rebuilt by lifting the lead on either side and re-dressing it. These repairs are quick and cost relatively little, and they keep the lead you already have, which matters on a roof where the existing metal is old and sound. If you want to know more about patching, our page on repairing split lead covers it, and our page on lead welding explains the technique for joining lead.
Typical UK costs for small lead repairs are given by MyJobQuote's chimney leadwork guide (22 September 2026): a patch repair at £150 to £250. That figure is for chimney leadwork, so treat it as a rough marker for what small lead patches cost, not as a price for a bay. A moderate repair of a roof, according to MyBuilder's roof repair cost guide (2026), runs £500 to £1,000. Your fixed quote follows the survey.
A repair stops making sense when the same roof is being patched again and again, or the deck is soft. If a small repair would leave you with a roof whose timbers are decaying beneath it, the better advice is to renew, and you will hear that straight after the survey.
Renewing the roof: what a full job involves
A full renewal strips the lead from the bay or porch, takes the deck up, inspects and repairs the timbers, and builds the roof again. The steps are straightforward in outline.
- Lift the old lead and set it aside to see how it was laid and what code it was.
- Take up the deck and check the joists, wall plate and any bearers for rot. Cut out and replace anything soft.
- Re-form the falls with new firrings so that the roof drains properly. This is the moment to correct a fall that has been wrong for years.
- Lay a new deck and a separating layer, then fix new rolls.
- Lay the lead, dressing it into the angles and turning it down into the gutter or over the drip.
- Fit the upstand and cover flashing at the house wall and point it in the right mortar.
Each part is given a code, and a small bay or porch often takes one of the middle ones, picked to suit the length of its runs. How to choose is covered in our lead codes guide. The cost is dominated by the area of lead, the amount of timber repair and the scaffold. Typical UK ranges from MyBuilder's flat roof cost guide (2026) put lead at £150 to £200 or more per square metre. Over a small area that is a modest total, but the timber work and access can add to it.
Where the roof is not listed and not on a conservation-area frontage that calls for lead, there are other coverings. MyBuilder's guide gives EPDM at £90 to £130 and GRP at £95 to £150 per square metre as typical UK ranges. They are cheaper than lead, and they are a fair choice on a rear porch no one sees. On a front bay in a street of lead-roofed bays, the match matters, and we would advise lead. We set out the options in the survey and let you decide.
Matching lead, listed buildings and the Bath context
On a period house the roof is part of the front elevation, and bay and porch roofs are designed as part of the architecture. The profile of the rolls, the shape of the gutter and the colour of the weathered lead all contribute to the look of the house, and a modern replacement that ignores them will stand out.
Bath adds its own layer, and heritage status shapes what a front bay can be given. A listed house needs the council's approval before its lead changes in material or looks, and doing it unapproved is an offence. An unlisted house sitting inside the conservation area, which reaches over about half of Bath, can usually be re-leaded like for like without drama, though a different material deserves an early word with the planners. World Heritage status covers every street. A few streets also carry Article 4 directions, so consult the B&NES policy map
The practical advice is to keep the lead where it is sound and to renew like for like where it is not. The same weight of lead, the same roll profile and the same gutter detail keep the character of the house. On a listed building, a change to the detail, even a small one, needs agreeing with the conservation team in advance. At the survey we explain the consent route, and where the Building Regulations apply to the work we deal with that side.
Lead is also a target for thieves, and a bay roof that is easy to reach from a wall or a gate is more exposed than a pitched roof high above. If your lead has been stolen, our page on lead theft from a roof explains what to do first. The usual advice is to make the opening watertight and record the damage, and an alternative covering is sometimes sensible on a roof that is repeatedly stolen from, although not on a listed building.
Looking after a lead bay or porch roof
Lead on a bay or porch wants little care, though not none. Because it is small and often out of sight, it is easy to forget until the damp appears. A short routine keeps small faults from becoming big ones.
- Clear leaves, moss and silt from the roof surface and the gutter at least once a year, and after a storm.
- Check that the outlet and downpipe run freely, since a blocked outlet turns the roof into a shallow pond.
- Look for staining or flaking paint on the ceiling or wall below, which can be the first sign of a slow leak.
- Keep ladders, planters and fixings off the lead, and do not walk on it.
- Have the roof looked at after any work on the neighbouring wall or window, because scaffolding and building work are common causes of cracked lead.
When you do need help, the survey is where we start. We go up, look at the lead and the flashing, check the deck from above and below where we can, and keep pictures of your own roof to show you what we found. Afterwards you receive an itemised fixed price in writing, and our 10-year written guarantee covers the workmanship. A roof letting water into a room is made safe first, because we prioritise leaks and storm damage. Should a repair be all it needs, a repair is what you are quoted.
The rest of our leadwork is described under lead roofing.