Normally, lead flashings should not exceed 1.5m in length, with laps of not less than 100mm. The flashing should be tucked into a mortar joint 25mm deep and at least 75mm above the tiling level, and the lead should be wedged into place. After that, the joint should be pointed with mortar or sealed with suitable exterior grade sealant. Following these guidelines can help ensure that your lead flashing lasts for as long as possible, without any issues or leaks.