What is Roof Flashing?: Ashbrook Roofing Supplies LTD
A Roof Flashing seals joints between different sections of your roof. The roof flashing will be used around a Velux window, the chimney, along valleys, around the dormer window and around any vent pipes. Roof flashings may also be required to seal the gap between roofs of one property and the one next door.
Without this protective roof flashing to cover the gaps between the sections of your roof, rain, snow, wind, pollution (even wildlife) will freely enter your building. If it wasn’t there, the building would quickly fall into dis-repair.
Not only does flashing protect the building against falling rain, roof flashings need to contract and expand along with the rest of the building as the temperature changes throughout the day. Considering that it is normal for the different parts of a building to be made of different materials (brick for the chimney and slates on the roof), flashings also have to cope with these mixed materials contracting or expanding at different speeds. Flashing need to be malleable so that it can be tightly formed over different materials of differing profiles – without splitting or tearing during the process.
As if all this wasn’t enough, they need to last as long as the very building it protects. In most circumstances you will want it to last for 50-100 years.
Despite modern alternatives, lead flashings are the one material that has stood the test of time above all others. It’s impressive to think that lead flashing has been around for centuries. And certainly, in the UK it’s very likely to be around for many more.
As can be seen in our previous blog understanding roof flashing, lead has been used for millennia due to its capacity to provide a high degree of durability, coupled with malleability, strength and ease of fitting.
The jargon that surrounds it can confuse even the most experienced. Here’s our quick guide to understanding lead flashing:
Lead is Manufactured to specific industry-wide thicknesses or lead codes. The higher the code, the thicker your lead sheet will be. (The number next to the code actually describes the weight per square foot).
Code 3 Lead, Code 4 Lead and Code 5 Lead are the most commonly used and found on many roofs. Code 6, 7 and 8 are typically used for high specification, ornate or flat roofs.
The Different Codes of Lead & Their General Usage:
Lead flashing is typically manufactured in rolls of either 3 meters or 6 meters in length. Treasure your roofing supplies merchants who will cut these rolls at different lengths for you!
Different types of roofing work call for different widths (from 100mm to 1.6m!) The most common widths of lead sheet used are 150mm, 240mm, 300mm and 600mm.
Fixing Lead Flashing:
Lead is cut using lead snips, folded into place, or beaten using lead bossing stick and lead beaters.
The lead is fixed into place using Hall Clips or Fixings Clips, and/or Lead Sealant / Lead Mastic. Nails are sometimes used (but carefully to enable the sheet to expand and contract without buckling).
After installation, the lead must be protected with Lead Patination Oil. Without this, the lead will quickly react to rain or moisture in the air and go white. This process is called carbonation and will cause staining of both the lead and surrounding brickwork or tiles.
Other Uses For Roofing Lead:
Lead is also used to make lead slates (used where the boiler pipe sticks out of your roof), lead roses & lead dots (both used for fixing lead to a roof, but also used as a form of decoration).
Lead can be cleaned using Lead Cleaner and Lead Restorer. Patination oil should be applied afterwards. Never use a pressure washer or abrasives such as wire wool or wire brushes.
Lead Alternatives exist. Their primary advantages are that they offer no value to thieves (so reduce the likelihood of theft). In addition, they are lightweight, easy to install and are non-toxic. This makes alternatives to lead flashing very popular, especially to those who are harvesting their rainwater for secondary purposes like watering their vegetable patch or diverting the rain into fish ponds.
Lead codes determine the weight (and hence the thickness) of lead sheet. The number in the code simply equates to the weight in lbs, per square foot of lead.
The importance of using the correct lead code:
When installed correctly, the lead will often outlast the life of the building. Where lead is installed incorrectly, especially where a thinner than recommended code has been used, it will deteriorate rapidly.
This is because lead is susceptible to three major influences.
Installation of Flashing
On anything but the simplest of flashings, the lead sheet must be repeatedly folded and worked into place. If you attempt to do this using a thinner code of lead sheet than that recommended, the lead is very likely to split, leaving the building exposed to water penetration.
A thicker code of lead aids the roofer or builder in two ways. It allows the lead to be folded without splitting. A thicker sheet of lead (the thicker the sheet the higher the code), also enables the lead to be beaten around corners without a dramatic thinning of lead (remember that lead should not be thinned by more than 25% of its thickness).
Expansion & Contraction:
On even relatively mild days, the lead will expand in the heat of the day, and contract during the cool of night. For small areas of lead, this generally does not cause a problem. Where lead has been used to cover large areas – such as over a dormer window, or on a flat roof, unless thicker codes of lead are used, it will start to buckle, twist and split.
Lead suppliers have recognised this, and generally, recommend different thicknesses of lead for different purposes. A summary of the best practice is listed below.
Choosing the correct lead code:
We all know that a lead flashing acts as a seal over joints between different parts (or different components) of a building.
In its most simple format, a lead flashing may be used to seal the join between the roof of a house extension, and the main wall of a building.
Perhaps fewer of us understand how the different components of a lead flashing work.
Managing variances of heat is a key challenge to the lead roofer (getting the lead up onto the roof is one of the others!).
Like all metals, heat causes lead to expand, whilst the lowering of temperature causes it to contract. As lead has a low melting point, this expansion and contraction can take place even in the relatively cool climate of the UK.
Before continuing, it’s worth noting that lead flashing has successfully been used for centuries to protect buildings against the rain. If the right lead flashing code is specified, and it is then correctly installed by a skilled roofer, lead flashing will normally outlast the lifespan of any building.
On very small areas of lead, normal expansion and contraction will have minimal effect. However, as the areas of lead become larger, this expansion and contraction has a more significant impact. At a certain point, if the wrong lead flashing code has been used, it will split allowing rain or snow to enter into the fabric of the building.
Table of lead codes
Code Nominal thickness lbs per sq foot Kg/m2 Recommended max sheet size Typical use
Code 3 lead 1.32mm 3 14.98
Code 4 lead 1.80mm 4 20.41
Code 5 lead 2.24mm 5 25.40
Code 6 lead 2.65mm 6 30.07
Code 7 lead 3.15mm 7 35.72
Code 8 lead 3.55mm 8 40.26
There are three main types of lead flashing. Understanding the differences between them, can help you ensure the right one is specified and used on your projects.
Milled Lead (Rolled Lead)
Manufactured under a fully automated, computer controlled process, this produces lead of the highest quality and consistency.
During production, a large lead ingot is repeatedly passed between two hydraulic rollers until a sheet of lead of an exact thickness and consistency is formed. This production method ensures that each and every roll of lead will be of a similar thickness along the entire length of the roll.
Because of the consistency from roll to roll, this is the preferred roof flashing material for many roofers, architects and contractors. Additionally, buildings being produced to high specification, may request the use of lead manufactured to BS EN 12588. Milled Lead is the only lead flashing that is produced to this standard.
Most experienced roofers prefer using milled or rolled lead. There is common agreement that it is easier to work, less liable to split and less likely to contain the brown tint often found in cast lead.
Cast lead sheet is produced by rotating a half-submerged water-cooled drum in a bath of molten lead. As the drum rotates, the hot molten lead sticks to the side of the cold drum and solidifies. This is then peeled of the drum and onto a cutting bed.
Unlike milled lead where the thickness is precisely controlled by the spacing between two rollers, the thickness of cast lead is controlled by varying the depth that the roller is immersed and the speed at which the drum rotates.
This difference in production methods means that cast lead can never match the exact tolerance as that of milled or rolled lead. Users will often find that cast lead varies in thickness along the length of the roll. On high specification work this could cause the lead to buckle or split.
Because it is produced with less intense production methods, cost can be slightly cheaper. However as most roofers and architects now insist on using Milled or Rolled Lead, this cost difference has significantly decreased over the last years, as the laws of supply and demand catch up.
It is worth noting that many experienced roofers remark that it Cast Lead harder to work and easier to split. Often it contains a rusty / brownish tint that spoils the look of detailed lead-work.
Sand Cast Lead
Produced by pouring molten lead onto a table of compressed sand, the liquid lead is quickly leveled by hand to the desired thickness.
Sand Cast Lead’s unique dimpled surface (caused by rapid air cooling along the uppermost layer of the metal) is often specified by architects wanting to capture an age old character for their buildings. Because it is produced by hand, thickness along the length and the width can vary significantly. It is disliked by some roofers and contractors who require lead sheet produced to an exact thickness.
Due to the high cost and low consistency of this production method, most sand cast lead is produced to special order, and in thicknesses of Code 6 (2.65mm) and above.
As such it is typically desired by those re-building historic or heritage properties where materials must match those used on the original construction.
Choosing the right types of lead flashing can help you deliver a better finish on your building. Ashbrook Roofing stock a wide range of milled lead flashing, much of which is available for next day delivery.
Do you have experience of using lead? We would love to hear your thoughts and views on the best types of lead flashing: