What Is the Problem With Lead Roofing?

Cost and theft

Lead is a dense, refined metal, and it is priced accordingly. A lead flashing costs more per metre than a plastic, aluminium or felt alternative, and the labour is skilled, because every piece is cut and bossed by hand on site. That is the first reason people look at substitutes. Our guide to lead alternatives for roofs compares them honestly.

The same value makes lead a target. Sheet lead is stolen from church roofs, schools, empty buildings and sometimes from houses, leaving the roof open to the rain. Theft is mostly a risk where the roof is accessible and unobserved. Alarm systems, anti-theft marking and substitute materials on exposed roofs are the usual answers, and our page on lead theft from roofs goes through them.

Movement and cracking

The biggest technical problem is thermal movement. Lead expands and contracts noticeably as it warms in the sun and cools overnight. If a sheet is fixed rigidly or laid in too long a run, the movement has nowhere to go and the metal fatigues, then splits along a crease. Most cracked lead is the result of detailing, not the material itself.

The remedy is to limit the length of each sheet, use rolls, drips and clips that let the metal slide, and choose a thickness that suits the span. Read more in our answer on why lead cracks. Sealant used as the only fixing is the other common fault: it hardens, shrinks and lets water behind.

Weight, staining and handling

Lead is heavy. A roll of sheet takes two people to carry up a ladder, and a large flat roof of lead places real load on the structure, which has to be allowed for in design. That weight also makes it harder to work in cold weather, since the sheet stiffens a little and needs more careful dressing.

Staining is a cosmetic issue. New lead releases a white carbonate powder in the first months, which can streak brick and stone below. Patination oil and good drip details limit it; the white powder on new lead page explains how.

The health question

Lead is toxic if swallowed or breathed in as dust or fume, and that is the honest centre of the concern. For a roof that is simply sitting there, the risk to a household is low: the metal is stable, the surface oxidises into a hard grey layer and very little comes off it. The risk is mainly to the people working with it, which is why a roofer washes hands before eating and does not burn, grind or sand lead without proper control. Our short answer on whether lead flashing is dangerous deals with rainwater collection and the home itself.

Lead and the building itself

A less obvious problem is what lead does to the structure beneath it. It is heavy, so a large lead-covered area, such as a flat roof or a deep bay, adds a real load that the joists have to carry. Lead also reacts with some timbers and with fresh lime or cement, which can eat at the underside of a sheet that lies in direct contact with damp mortar. A separating layer of underlay or building paper stops that, and any old lead lifted for inspection often shows whether one was fitted.

Run-off is another point. Lead carbonate washes off in rainwater, so water from a large lead roof should not be collected for drinking or watering vegetables. This is a practical point for owners of period houses with lead gutters, and it is separate from the low risk of a quiet roof sitting over your head.

Telling a lead problem from a roofing problem

Many leaks blamed on lead turn out to be something else. A damp patch beside a chimney may be the lead flashing, but it may also be the pointing, a cracked pot or a missing cowl. A stain under a valley can come from the lead or from the tiles that overlap it. Before anyone strips good lead, the cause needs finding, and that is what a survey with photographs from the roof is for.

So is it worth using?

For flashings, valleys and gutters on a roof that someone will maintain, lead is still the best-performing choice and it can be recycled at the end of its life. Where theft is a real risk, or the budget is tight, an alternative may suit better. We will tell you which applies on your roof, with photographs, before you decide. Our approach to lead in general is set out under how we approach lead on Basingstoke roofs.