Lime Mortar + Lead Flashing = "Corrosion"?

[Flyfisher]

I was looking around a friend's new extension building work (1950s brick semi) at the weekend and got chatting to the builders, as you do. Talking about lead flashing, one builder said that lead and lime is a bad combination and leads to 'a reaction', by which I inferred he meant corrosion of the lead. He said that on old buildings he always uses clear silicone sealer to seal lead into brickwork.

I don't think I'm tempted by the silicone sealer but has anyone heard about detrimental 'reactions' between lime and lead? I couldn't find anything using Google that supports his view - could it have been a wind-up? (if it was, he was very, very convincing!).

 

[Me!]

A few years ago I took a lead flashing off a chimney stack. The stack was built from brick and lime mortar in 1906 and as far as I am aware the flashing was original. It was absolutely fine, I even kept the lead to re-use.

 

[Ian Bond]

Lead corrosion is something that I've read about previously when looking into roof conservation work.

I made a note to consider using bitumastic paint to protect the lead - example in following spec (search term bitumastic:
http://www.collywestonstoneslaterstrust.org.uk/collywestonspecification2.htm

Further info here
http://www.rsc.org/delivery/_Articl...leForFree.cfm?doi=AN9356000321&JournalCode=AN

 

[Flyfisher]

Well, there's a can of worms!

I've also removed old lead that had been in contact with lime mortar for some time, certainly many decades, and it looked OK, although it showed some evidence of lead oxide (white powder).

The paper Ian linked to states that lime mortar is actually better than cement although it also states that wood, especially oak, will also corrode lead! It is quite an old paper though (1934) so perhaps there is more up to date research somewhere?

The protection of the lead with bitumastic paint is interesting, and makes sense in the light of the above, but I'm a bit surprised it wasn't mentioned in the installers guide I got from the Lead Sheet Association.

I guess I'm a bit surprised there seems to be no industry standard/best practice for such things, or if there is, it seems hard to find. Given all the things that seem to corrode lead, perhaps the builder I spoke to is right about using silicone.

 

[Ian Bond]

Condensation will also cause corrosion of lead.

As you'd expect for materials that have been used for 1000s of years, the problems are well understood, even by the 1930s. Not by everyone, of course.

Here's something a bit more recent.... (protecting lead from underside corrosion, using a chalk emulsion)

http://www.rowantechnologies.co.uk/underside_lead_paper-2006.pdf

I'm not too worried about my lead soakers and flashings in contact with lime to be honest.

Perhaps if I was redoing a church roof in cast, code 7 lead - but then again, the more serious threat at the moment is someone pinching the stuff in the dead of night.

 

[Lime]

The silicone sealer used so commonly today has good adhesion to lead and clean brick.
It allows for movement so unskilled workmen can use it and it is quick and easy.
Plumbing is a skill and care has to be taken where and how lead is affixed to a building, this silicone allows those skills to be waived.
Most of the corrosion of lead is caused either by acidic materials nearby and/or in contact with the lead and acid rain.
.......or coca cola poured out of the window as one of my french students did to my roof!!!!!kill kill kill kill :evil:
I have always used mortar to fix lead to brickwork and have never seen any corrosion because of it.
However the silicone looks awful and I have seen large strips of it pulled out of the brickwork by children with access to it from windows.

 

[middi]

Lead has been used in direct contact with lime for centuries with no ill effect. Lead will not withstand direct contact with ferrous materials and tannic acids( such as Oak sap) for too many decades.

 

[Craig]

Silicone has a limited life when exposed to our climate - certainly shorter than the lead or lime.

 

[Ian Bond]

Last place I saw it was smeared all over cracked window putty. Guess what? It had peeled off.

So much quicker and easier than redoing the putty though, I'll give 'em that