Roofing Flashings – Why Size Does Matter

Lead has been used as a roof flashing since roman times and is still an excellent choice today where a durable flashing material is required. Unlike some other metal flashings it is highly resistant to corrosion and has stood the test of time. It is extremely malleable, allowing it to be formed into complex shapes. A well installed lead roof flashing can last in excess of 70 years.

However, the material requires a great deal of skill to achieve correct results. In this article I would like to discuss some of the common mistakes made during installation. I have repaired hundreds of lead roofing flashings where the fault can be traced back to the initial installation. This is not to blame the roofer that installed the flashing. The fact is information on the thermal properties of lead has only become widely available in recent times.

Thermal movement

The biggest cause of failure in lead sheet flashings is caused by thermal movement. This is due to the Coefficient of thermal expansion. Simply stated the lead expands in volume as the temperature increases. In a roofing situation the temperature variations can be wide. The cycle of strong sunlight followed by cloud cover mean the lead is constantly moving. This constant expansion and contraction of the roofing material will eventually cause fractures or splits in the flashing.


Fortunately, there are a few measures we can take to minimise the damage caused by thermal movement. Size definitely matters. Care must be taken to ensure the correct grade of lead is used for each situation. Lead sheet comes in rolls graded by the width. Between 1.3 to 3.5mm are the most common. The smaller widths are used for roof soakers and the heavier codes used for parapet and box gutters. When you purchase your lead from your roofing supplier tell them what you want it for. They will advise you of the correct grade for your application. Don’t scrimp on materials. The thicker grades are more expensive but essential if you want to avoid roof leaks.


The length of the flashing size is of paramount importance. Roofing flashings that are cut too long will deteriorate faster. Expansion and contraction is greater in larger sheets. Thus the lead will split faster. As a rule of thumb you should keep all your cuts down to 1.5 meters in length, or 4.9 feet. In summary, ensure you have the correct grade lead for the area you wish to flash and keep all your cuts under 1.5 meters in length.