The short answer
Often, yes. On a cold roof, where insulation sits between or below the joists, the void under the deck must be ventilated so that moist air does not condense on the underside of the lead and the timber. A warm roof avoids that by putting the insulation above the deck with a vapour control layer beneath, which removes the need for a ventilated void. Either way, lead should sit on a separating layer, not directly on bare timber.
Why moisture matters to lead
Lead is durable from above, where rain and sun form a protective carbonate film on its surface. The underside is a different story. Moisture trapped between the lead and the deck can set up corrosion, and certain timbers give off acids that eat into the metal; oak and western red cedar are well known for it. Condensation can form when warm, damp air from the house rises through the ceiling and meets the cold back of the lead sheet. If it cannot escape, the deck stays damp, the lead corrodes unevenly and the timber can rot. You may notice the effect as dark patches, pitting or white deposits under old sheet, long before any water drips into the room.
Cold roof, warm roof
Flat and low-pitched lead roofs, such as those on bays, porches and small flats, are built in one of two ways. In a cold roof, the insulation sits at ceiling level, and the void above it stays cold. That void has to be ventilated, typically by openings at opposite edges so that air passes through, and the ceiling below needs a vapour control layer to hold back moisture from the house. The airflow carries off any damp that gets through.
In a warm roof, the insulation is laid above the deck and below the lead, with a vapour control layer under the insulation. The deck stays warm and dry, and no ventilated void is needed. Both can work, but each must be built correctly. Mixing the two ideas, such as stuffing insulation tightly against the deck in a cold roof with no airflow, is a classic cause of failure. Our page on whether a flat roof needs ventilation covers the principle for other coverings.
What goes between the deck and the lead
Lead should never sit directly on bare boards. A separating layer, often a geotextile or specialist underlay, goes on first, with the joints and fixings protected, to stop the metal being scuffed by the timber and to let the deck breathe a little. Lead on a timber deck is also laid in bays with rolls or welts to allow for expansion, and the sheets are limited in length for the same reason. On a roof with a poorly detailed junction, even a good build-up can fail. For the lengths, see the maximum length of a lead sheet.
What about lead on pitched roofs?
For flashings, valleys and soakers on a pitched roof, the question is mostly answered by the ordinary ventilation of the loft and the roof void. A pitched roof with blocked eaves and moisture-laden air is hard on the underlay and the timber, and lead sitting on that timber suffers too. Good loft ventilation helps everything, as our guide on condensation in the loft explains. The lead at a chimney back gutter, for example, still needs a dry bed, which is part of why a well-built gutter has a proper board and fall; see what a chimney back gutter is.
Signs the build-up is wrong
A few symptoms point to trouble under the lead. The ceiling below may show damp patches that appear in cold weather and not in rain. The lead may bubble, ripple or crack along the sheet more than movement alone would explain. Timber at the edge of the roof may feel soft, and a musty smell from the void is a giveaway. Droplets on the underside of a lifted corner of sheet tell you moisture is condensing there. If you see these signs, do not simply patch the surface. Cold-weather condensation on a flat roof needs the whole build-up looked at, including the ceiling, the insulation and the vapour control layer.
Regulations and what to ask
When a lead roof is replaced as part of other work, the insulation standards of the Building Regulations can apply, and the ventilation details are part of getting that right. Ask your roofer whether the roof will be warm or cold, where the vapour control layer goes, how the void is ventilated if there is one, and what separating layer is used. A clear answer to each is a good sign. Vagueness about the build-up is a reason to ask for a second opinion.
We survey the existing roof, lift a section of lead where needed to look at the deck, and photograph what we find. If the deck is wet or the timber is soft, we say so before pricing the job. You can return to the lead roofing hub, visit the homepage, or ring 01256 213956.
Every roof is different. A free survey with photos settles it for yours.
Get a free survey