One of the difficulties with finding out how much lead you need to order for your flashing is taking into account the “upstand”. When a bit of flashing encounters a wall, it needs to go slightly up (called the “drop”) before entering the brickwork (the “chase”) to make sure that the whole join is watertight. But how much upstand does lead flashing need?
Lead flashing upstands can be divided into two types. One is cover flashing. This is when the flashing meets a wall at a horizontal angle. The upstand rises straight up in a horizontal line before being chased into the brickwork. The other is step flashing, which occurs when flashing meets a wall at a diagonal angle. Step flashing makes a step pattern on the wall, rising up until it meets a gap in the brickwork, then following that until the upstand has nearly shrunk completely, then it rises again.
Minimum Upstand
The usual minimum recommended upstand for flashing is somewhere around 150mm. This allows it to rise up two levels of brick. Since upstand will need to be chased into the bricks, it’s always recommended to calculate your upstand in terms of how many layers of brick it will have to rise – 75mm, 150mm, 225mm etc.
75mm upstand might be required if the flashing encounters something where it cannot rise above (such as a window) and might also be optional if your house is in a particularly dry area. However, in the UK, it is highly recommended to use 150mm of upstand wherever possible.
Minimum Chase
The chase is where the upstand turns to enter the brickwork. In order to create a waterproof join, the chase needs to enter quite far. In standard conditions, it should enter at least 25mm into the brickwork. This gap will need to be carved out of the brick mortar – ideally with an angle grinder. However, sometimes 25mm risks causing damage. This might be because the mortar and bricks are too obviously aged or because there is a lintel or a similar feature that could get undermined.
In these cases the chase should only be 20mm. This will create as waterproof a join as possible while recognising the dangers of cutting too deeply and compromising something else.
By combining your upstand and chase, you can better estimate the precise level of flashing that you need.
