The short answer
A lead roof corrodes underneath when moisture gets trapped against its underside and cannot dry out. Warm, damp air from the rooms below reaches the cold lead, condenses on it and sits there, and the metal slowly breaks down from beneath, where nobody can see. The remedy is to stop that air reaching the lead and to let the deck breathe, following BS 5250 on moisture in buildings.
Lead can look perfect from above while it rots from below. The usual cause is damp air with nowhere to go.
How condensation attacks lead from beneath
Lead is tough in the open air, where rain runs off it and wind dries it. The underside is different. It is a closed, dark space, and anything wet that lands there stays wet.
Where does the moisture come from?
Rooms below make a lot of it. Kitchens, showers, wet laundry and sleeping bodies all release moisture as vapour, and the warmer the air, the more of it can be carried. That warm air finds its way up through gaps in the ceiling, round pipes, past light fittings and through the deck joints. When it meets the cold underside of a lead sheet, the vapour turns back into water. That is condensation.
Winter is the worst time, because the lead is at its coldest just when the house is at its warmest and dampest. Left alone, the droplets sit on the metal and on the timber deck, a cycle that repeats through every cold spell. Lead reacts with moisture and with some of the substances the damp draws out of the timber below it, and the underside slowly turns to a crust and loses thickness. From above the sheet looks flat and sound until a soft spot or a hole appears.
Why ventilation is the cure
You cannot stop warm air rising entirely, so the answer is a pair of measures: keep moist air out of the roof, and give any that gets in a way out.
What does BS 5250 say?
BS 5250 is the British Standard on handling moisture in buildings, and BSI lists the 2021 edition as current, though under review. It advises on design without being law, and it sets out how to design roofs so that vapour is controlled and allowed to escape. A roof that follows it has a vapour barrier or sealed ceiling on the warm side and a clear path for air movement on the cold side.
Two builds are common. In a warm roof the insulation lies over the deck; in a cold roof a ventilated void separates ceiling and deck. The second type relies on air flowing through that void. If insulation has been stuffed in to fill the gap, or the vents at the eaves are blocked, the air stops moving and the condensation stays.
Spotting trouble before the lead gives way
Because the damage starts below, the early signs are often inside the house, or along the roof edges, where the underside can be seen. Ask whether the void above the ceiling has been boarded or insulated since the roof was built, because extra insulation pushed into an old void is a frequent trigger.
What does an owner notice?
Damp stains on a ceiling under a lead roof that do not match a rain event are the first clue. A musty smell in the room, flaking paint around a bay or dormer, and timber that feels soft near the eaves all point at moisture. From the outside, look for lead that has gone dull and chalky along its edges, or dents and soft patches where the deck underneath has moved.
Do not assume a stain means a leak from above. The same mark can come from condensation, a faulty flashing or a failed joint, and the cause decides the cure. Our answer on the signs of failed flashing helps you tell the two apart. Our roof survey looks at both sides, checks the ventilation and shows you photos of what it finds.
Putting it right
A corroded sheet cannot be saved. Once the underside has thinned, patching the top does not restore it, and new lead laid over a damp deck will go the same way.
What does a proper repair involve?
The old lead comes up and the deck is checked. Wet or rotten boards are cut out and replaced. Then comes the part that decides how long the new lead lasts: the build-up beneath the lead is corrected. That may mean a vapour control layer on the warm side, clear eaves ventilation, an air path above the insulation, or a change to a warm roof design. A new roof of this kind needs the right thickness of lead too, and our page on lead codes lists what each weight is used for.
A job like this can touch Building Regulations when the deck or insulation changes, so we tell you what applies before work starts. For the wider service, see lead work. Boards soaked over many winters may also be decayed, so we check the joists and boards before settling what to replace. You see photographs of the deck and the underside of the lead before any figure is given.
More on lead and damp
Can I stop condensation by adding a vent myself?
A vent alone often is not enough, and a badly placed one can pull moist air into the void. The air path, the insulation and the ceiling seal work together. Have the build-up checked first, and ask your roofer how the void will be ventilated before any extra openings are cut.
Is underside corrosion covered by simply replacing the lead?
No. New lead over the same damp deck corrodes again. The fix lies in removing the moisture source, repairing the deck and correcting the ventilation. The lead goes back on last. That is why a survey of the build-up comes before any price is set.
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