What Is Lead Flashing?

06/04/2022

Lead flashing is an important feature of any roofing system because it prevents water from entering the structure through a joint.

Prior to the availability of lead roof flashing, a variety of measures for preventing water intrusion were used, including angling shingles away from the joint, situating chimneys at the ridge, and constructing steps into the side of chimneys to deflect water. With the development of produced lead flashing codes, it became possible to employ it to reduce water penetration at connections like chimneys, vent pipes, roof-to-wall walls, windows and doors.

The use of prefabricated flashing has greatly improved building durability and minimised indoor mould issues by preventing water penetration at barriers such chimneys, vent pipes, walls that abut roofs, window and door openings, etc. Preventing leaks near roof windows or skylights is also crucial. Furthermore, flashing is crucial to guarantee the roof’s integrity before installing solar panels. (Keep a look out for our upcoming article on how to best fit solar panels without damaging your roof.)

Lead roof flashing, which has its own set of rules and codes, can be exposed or hidden and is commonly made of sheet metal such as copper, lead or aluminium. Lead flashing should have expansion joints for long lines to prevent metal sheets from deforming owing to expansion and contraction, and it should not stain or be stained by neighbouring materials, and it shouldn’t react chemically with them.

TOP TIP: Some roofers don’t know this, but copper nails should be used for lead; aluminium can sometimes react to lead and water, and can corrode the lead. If you have an unexpected leak from your lead flashing and can’t work out why; aluminium nails may have been used!

Chimney Flashing

Along the edges of the chimney stack, step flashing is commonly used. At the place where the chimney joins the roof, chimney flashing prevents water from entering the building through the roof.

Flashing is sometimes put onto a chimney in two parallel parts; the cover/ counter/ cap flashing and the base flashing. The counter-flashing is built into the chimney and overlaps a replaceable piece of base flashing. The name “apron” is still sometimes used to describe the flashing below a chimney. Lead strips were occasionally used for flashing edges. A small gable-like assembly known as a cricket with cricket flashing, or a rear flashing or back pan flashing on narrow chimneys without a cricket, may be present on the uphill side of a chimney. Flashing can be inserted into a reglet, which is a groove found in chimneys and walls.

Step Flashing

When a low roof reaches a wall, a roof meets a chimney, or a conservatory roof meets a house wall, step flashing allows the roof to abut the brickwork. The term ‘step’ refers to a pattern in the lead. This pattern is made by a sequence of cuts that allow the lead to penetrate into the courses between the bricks, resembling the steps of a staircase. The purpose of going through the trouble of making the stairs is to keep the bricks from being cut and so damaged, looks neat and is a nice detail of quality and thoughtful roofing.

Apron Flashing

Because of its versatility, apron flashing (also known as cover flashing) is the most frequently-seen types of flashing used in roofing applications. On the front and backs of chimneys, abutting brickwork on flat roofs, and where a roof meets the brickwork of a home or outbuilding, this sort of flashing can be installed. It’s also installed around bay windows, on slate and tile roofs where they meet a wall or a porch, and on conservatory roofs.

Which lead codes are used for lead flashing?

The most common type of lead roof flashing is Code 4 milled lead. This is recommended because it offers a good balance of malleability and resistance to fatigue splits. These fatigue splits can be induced by expansion and contraction. Lead soakers — the unseen waterproof parts that sit beneath tiles or slates – are made of Code 3 lead, which is thinner and lighter.

The length of a lead sheet for lead flashing should not exceed 1.5 metres. If the lead is reduced any farther, expansion and contraction may cause the lead work to split. When installing flashings, make sure they overlap by at least 100mm. This is an excellent general rule of thumb for cutting lead flashing, but there are some circumstances where you might want to increase the overlap, such as prevailing high winds, driving weather, or very high flashings where the overlap could be prone to water creep.

You can read more about lead and how to work safely with lead in this blog post.

And, for further explanation about lead codes for flashing, you can read this article.

How to install and repair lead flashing

We always recommend getting experienced roofers to install or repair lead flashing. However, it can be useful to be informed about what will happen to your roof, and and the processes that we will go through to repair the flashing.

When cutting and bending lead work for a sloped roof that abuts a wall, three measures are required:

1. The Chase Depth is the amount of lead that will protrude through the space between brickwork, usually a few millimetres short of bottoming out for ease of installation.
2. The distance the flashing ‘drops’ down the wall to meet the roof is known as the upstand or ‘Drop.’
3. The Cover, also known as the ‘overlap,’ is the quantity of lead that will cover the tiles or slates beneath to make them watertight.The height of the upstand or drop is typically 150mm, or two courses of brickwork, though it might be less if the roof is being installed under a window or for aesthetic reasons. You can reduce this measurement to 75mm, but make sure there are no other elements that might make it unsuitable, such as splashing from above, driving weather, or snow drifts.The cover or overlap onto the roof is typically measured at 150mm, however if you are considering increasing or decreasing the measurement, keep in mind the pitch of the roof, exposure to the elements and the look of the roof and the building.

Making a lead bung or ‘chock’ is the most popular method of securing lead flashing. Roll a 25mm (or less) strip of lead into an oval shape just wider than the mortar chase you’re mending, then drive it beneath the surface of bricks to make a wedge. Use hall clips or nylon flashing clips as an option. If you’re installing a long run of flashings, these can be faster and easier to use, but they’re also more expensive than the classic lead bung approach.

There are two methods for pointing lead roof flashing: classic mortar pointing or silicone mastic pointing. If you want to use mortar, make sure it extends all the way to the back of the chase to prevent flashing leaks. If you’re going to use silicone mastic, check sure it’s safe to use with lead. Some mastic can react badly with lead over time.

Finally, lead roof flashing with patination oil is a viable option. This will dull the new milled lead’s initial gleam and protect it from the white stains left by flowing water.

If you have noticed a leak in your roof, this may be down to the lead flashing needing to be repaired. Give us a call on 01288 381256 if you need experienced roofers, based in Cornwall.