It’s so common it has become a cliche: The church roof needs replacing. We climbed up to see the re-laying of the lead roof.
Many of the UK’s churches, and buildings, have lead roofs in need of replacement. The lead used here is sheet lead, which is a great material for roofing, although it does have it’s issues.
As you can see here, the existing lead was old, worn and needed to be removed. A common problem with lead is corrosion to the underside, caused by condensation from below and/or carboxylic acids. This doesn’t happen on the face side because as the rain falls, the carbon dioxide in the rain reacts with the lead to make lead carbonate, which thickens over time to create a protective patina which protects it. Read more on buildingconservation.com
To prevent this corrosion, lead sheets are now given a chalk rich coating before laying. This is then heated, which causes the chalk coating to set in minutes, as opposed to hours left unheated.
As a side note, on some heritage projects you are not allowed to carry out hot works within 5m of the building (not this one, however). If you want to know why, read this beautifully written piece about Uppark.
To aid the corrosion problem, a breathable felt is now laid atop the existing roof deck to allow natural ventilation. Then Wood Rolls are laid, around which the lead will be bossed. I have drawn an example, but these are better: See fig. 24
You can attach lead by welding or bossing. Bossing is “The shaping of sheet metal with a former and a mallet.” (Penguin Dictionary of Building, Maclean and Scott.. MUST HAVE). The metal is bent into position using purpose built bossing mallets/sticks, flat dressers, chase wedges etc. I’d love to give it a go one day, but I’m sure it can be quite tiring.
Bossing and Welding courses are provided by the Lead Sheet Association (LSA)
The lead is laid in sheets and attached with two layers of copper nails at the top and a single line going 1/3rd of the way down each side. The nails are copper so as not to react with the lead, and only cover some of the sheet to allow for thermal movement. The lead will naturally expand as it heats up, and if the sheet is attached too rigidly this can cause the nails to fail, and the sheet will fall off. For the same reason the clips (nailed to the underside and folded – bossed – over) should be laid with a gap at the bottom. The LSA recommends 6mm, but I must admit even this seems a little small.
The south facing side is almost complete. They still need to add some ventilation gaps in the top and bottom of the roof.
P.s. If you’re interested in the codes; codes = thickness. Typically, roofs are made with code 7/8 lead (3.15-3.55mm thick) and the flashing will be 5/6 (2.24-2.65mm)
Websites like Building Conservation are brilliant because they have articles written by people from all areas of conservation and you can just search for your topic. I found this article on Repairs to Lead Sheet Roofing particularly relevant.
Visit the Lead Sheet Association for a wealth of resources to guide you from design to maintenance.