Calculate lead flashing length, rolls, and weight by type and code
Lead flashing is one of the most critical weatherproofing details on any UK roof. It forms the watertight seal where the roof surface meets a wall, chimney, dormer, or other vertical surface, preventing rainwater from penetrating the building at these vulnerable junctions. Despite being one of the oldest roofing materials still in use, lead remains the gold standard for flashing because of its exceptional malleability, durability, and resistance to corrosion -- a properly installed lead flashing can last over 100 years. In the UK, lead for roofing is specified by code number, which indicates the thickness and weight per square metre. The Lead Sheet Association (LSA) publishes detailed guidance on which code to use for each application. Code 3 (1.32mm, 14.18 kg/m2) is the thinnest grade suitable for short flashings and soakers. Code 4 (1.80mm, 20.41 kg/m2) is the standard choice for most flashing applications, including step flashing, cover flashing, and apron flashing. Code 5 (2.24mm, 25.40 kg/m2) is used for valley gutters, parapet gutters, and any flashing with a long unsupported run where thermal movement is a concern. This calculator helps you determine the length of lead flashing needed, the number of standard rolls to purchase, and the total weight of lead required. It supports four common flashing types: step flashing (at the junction of a pitched roof and a wall), cover flashing (horizontal junctions), apron flashing (at the base of chimneys and dormers), and valley flashing (where two roof slopes meet in an internal angle). Understanding the total girth -- the combined width of the upstand (turned up the wall) and cover (over the tiles) -- is essential for selecting the correct roll width. Standard roll widths are 150mm, 240mm, 300mm, 390mm, 450mm, and 600mm, and the calculator automatically selects the nearest standard width that accommodates your specified girth.
To calculate lead flashing requirements: 1. Select the flashing type from the dropdown. Step flashing is used where a pitched roof meets a wall at an angle (for example, the side of a chimney or abutment wall). Cover flashing runs horizontally along a junction. Apron flashing sits at the base of a chimney or dormer where it meets the roof slope. Valley flashing lines the internal angle where two roof slopes meet. 2. Enter the run length in metres. For step flashing, measure the length of the wall abutment along the slope. For cover and apron flashing, measure the horizontal length of the junction. For valley flashing, measure from ridge to eaves along the valley line. 3. Select the lead code. Code 4 is the default and suitable for most applications. Choose Code 3 for short vertical flashings under 500mm. Choose Code 5 for valleys, parapets, and exposed locations. 4. Enter the upstand height in millimetres. This is how far the lead turns up against the wall or vertical surface. The Lead Sheet Association recommends a minimum of 75mm, with 150mm being standard practice. The upstand is typically tucked into a mortar joint (raked out to 25mm depth) and secured with lead wedges. 5. Enter the cover width in millimetres. This is how far the lead extends over the roof tiles or flat roof surface. A minimum of 75mm is required, with 150mm being standard. Greater cover provides better weather protection in exposed locations. 6. Review the results showing total flashing length, number of rolls needed, total lead weight, and the roll width selected. For step flashing, the length is calculated from individual pieces at 225mm intervals (matching standard UK brick courses).
The flashing calculation depends on the type of flashing: Total girth = upstand height (mm) + cover width (mm). For standard 150mm + 150mm: total girth = 300mm. The calculator selects the nearest standard roll width equal to or greater than the total girth. Standard widths are 150, 240, 300, 390, 450, and 600mm. For step flashing: each piece matches a brick course height of 225mm. Number of pieces = run length / 0.225, rounded up. Each piece has a total length equal to the girth. Total flashing length = pieces x girth / 1000 (metres). For a 5m run: pieces = ceil(5 / 0.225) = 23. Total length = 23 x 300 / 1000 = 6.9m. For cover, apron, and valley flashing: the flashing runs continuously along the junction. Total length = run length. Rolls needed = total length / 3 (standard 3m roll length), rounded up. Lead weight = flashing area (m2) x weight per m2 for the selected code. Flashing area = total length (m) x roll width (m). For 6.9m of 300mm-wide Code 4 lead: area = 6.9 x 0.3 = 2.07 m2. Weight = 2.07 x 20.41 = 42.25 kg. The weight calculation is important for structural considerations and for estimating the cost of lead, which is sold by weight. Current UK lead prices fluctuate based on the London Metal Exchange rate, but Code 4 lead typically costs between 8 and 12 pounds per kilogramme from builders' merchants.
When working with lead flashing, always follow the Lead Sheet Association's best practice guidelines. Lead should be patinated (coated with patination oil) immediately after installation to prevent white lead carbonate staining on adjacent surfaces. Step flashing should be installed with the brick courses as the wall is built, or chase-cut into the mortar joints on existing walls. Never use silicone sealant as a substitute for properly installed lead flashing -- it has a lifespan of only 5-10 years compared to lead's 100+ years. In conservation areas or on listed buildings, the local planning authority may require traditional lead flashing rather than modern alternatives. Always use lead fixings (wedges, clips) rather than steel nails, which can cause galvanic corrosion when in contact with lead.