Why Lead Flashings Are Still Best for Roofs

Lead has been keeping British roofs watertight for centuries. It is dense, durable and extremely malleable, so it can be dressed neatly around chimney stacks, parapet walls, dormers and abutments. Unlike some modern alternatives, it does not become brittle with age. Instead, it weathers to a protective grey patina and can last 50 years or more when installed correctly. For UK homes, that combination of long life and workability is hard to beat.

That does not mean any piece of lead will do. Success depends on choosing the right code, forming the details properly and allowing the metal to move. Lead expands and contracts with temperature changes, so fixings must hold it securely while letting it slide.

Lead codes: choosing the right thickness

Lead sheet for roofing is graded by code, which determines thickness and weight. For most domestic flashings, Code 4 is the sensible minimum. Code 5 suits longer runs, exposed positions and areas subject to foot traffic. Code 3 is generally reserved for soakers, while Code 6 and above are used for valley gutters and heavy-duty cover flashings. Check the roll marking to BS EN 12588 before work begins.

• Code 3: soakers and small, sheltered details.
• Code 4: standard chimney aprons, side flashings and cover flashings.
• Code 5: exposed chimney flashings, parapet cover flashings and long lengths.
• Code 6 to 8: valleys, gutters and situations needing extra rigidity.

As a rule, horizontal laps should be at least 100mm and vertical laps at least 75mm, with more overlap in wind-driven rain zones. Always allow for thermal movement. Fixings should be stainless steel or copper clips, never tight nails through the face of the lead.

Chimney flashings that actually shed water

A chimney is the most common place for leaks, because it interrupts the roof plane and collects water from several directions. A proper lead installation usually has four parts: a front apron, side flashings, a back gutter and cover flashings chased into the stack. The apron should dress down over the roof tiles and turn up at least 75mm against the chimney. The back gutter needs a fall towards the roof and a generous upstand, as that is where water and debris gather.

Chases cut into the chimney should be at least 25mm deep and pointed with a flexible mortar or lead sealant. Avoid fresh cement-rich mortar touching new lead, as alkalis can stain it badly. Apply patination oil as soon as the lead is installed. On wide chimneys, use two-piece flashings rather than one long sheet, so movement does not tear the metal.

Parapet walls, soakers and step flashings

Parapet walls need a cover flashing that caps the top and turns down both faces. This is usually Code 5 or Code 6 lead, fixed with clips that allow movement. At the roof junction, step flashings and soakers work together: each slate or tile has a soaker beneath it, and the step flashing covers the joint. Soakers are short pieces of Code 3 lead, while the step flashing is often Code 4. The upstand should be at least 75mm.

Never let lead touch aluminium, galvanised steel or new oak, cedar or sweet chestnut. These can cause corrosion or staining. Use an isolating membrane where contact is unavoidable. Also keep lead away from copper, which can accelerate corrosion in damp conditions.

Lead and stove flues: what to avoid

Wood-burning and multi-fuel stoves run much hotter than a traditional open fire. Lead melts at around 327°C, so it must never be wrapped directly around a single-wall flue or any surface that reaches high temperatures. Where a twin-wall insulated flue passes through a roof, use a proprietary lead slate or flashing with a high-temperature collar, and maintain the clearances specified by the flue manufacturer.

For masonry chimneys serving a stove, lead flashings at roof level are usually fine, because the flue gases are contained within the stack. The critical details are at the top: the pot, the terminal and the crown. Make sure the flue is swept annually, the chimney is sound, and a carbon monoxide alarm is fitted in the room. Building Regulations Approved Document J covers installation requirements in England and Wales, with similar rules across the UK.

Maintenance that adds decades

Lead is low maintenance, but not maintenance-free. Check flashings twice a year and after severe storms. Look for lifted edges, cracked mortar, loose clips, white staining or small holes. Clear moss and debris from back gutters and parapet outlets, because standing water will eventually find a way in. Repoint crumbling mortar joints before water gets behind the lead.

• Apply patination oil to new lead if it has not already been done.
• Keep gutters and back gutters clear of leaves and grit.
• Trim overhanging branches that can scrape or pierce flashings.
• Have the chimney swept and inspected annually if you burn wood or solid fuel.
• Repair small tears promptly with a lead sealant or a competent roofer.

Get the details right and lead flashings will quietly do their job for decades. They are not the cheapest option, but measured over the life of a roof, they are often the best value. For British weather, that reliability is exactly what a home needs.