Green Building Forum - Lead Flashing Sealant - how important is the spec's width to depth ratio?

Lead Flashing Sealant - how important is the spec's width to depth ratio?

greenfinger — Thu, 18 Apr 2024 13:53:17 +0100

Hi

I'm finishing a lead flashing job and now need to seal the flashing that's turned into the chase. The lead has been fixed using hall clips. It's an old stone wall, with undressed stone. So nothing is straight or flat!

Although some seem to favour mortar, I've decided to use lead sealant. Some of the sealants specify that a width to depth ratio of 2:1 must be used (I guess at the end of the say this is a silicone, and the same ratio is also specified for other silicones such as sanitary types, etc).

Where this ratio cannot be achieved what might the implications be?

Having read/watch videos about the topic, it has got me to thinking. Lead flashing is commonly used against brick walls. The typical mortar joint is 10mm wide. The recommended chase depth of the lead is 25mm. So in this scenario, there would be a width to depth ratio of 1:2.5 and therefore well off the recommended 2:1 ratio. In fact, here's a couple of manufacturer's videos which seem to show the full depth of the chase being filled with sealant. The brickwork looks standard, so I presume the chase is no more than 10mm wide.

https://www.youtube.com/watch?v=PIVhKIbyxk4
especially at 1:29.

https://www.youtube.com/watch?v=GBtvHif6bTE
at 6:30

Similarly, I can't imagine every plumber using silicone adheres to the ratio.

So I wondered whether you guys think about this ratio business?

Cheers

Lead Flashing Sealant - how important is the spec's width to depth ratio?

Cliff Pope — Thu, 18 Apr 2024 17:41:37 +0100

Commom sense says to me that it's less likely to fall out if it is deeply embedded.

Lead Flashing Sealant - how important is the spec's width to depth ratio?

greenfinger — Thu, 18 Apr 2024 17:58:47 +0100

I can see that too.

But the manufacturer is insisting the opposite, saying that if the width of the chase if 10mm, the depth of sealant should be 5mm.

I'm sure the older I get the less certainty there is on anything, even ubiquitous tasks which are so often done you wouldn't expect polar opposite approaches!

Lead Flashing Sealant - how important is the spec's width to depth ratio?

djh — Thu, 18 Apr 2024 20:10:31 +0100

I think it makes sense for the lead to be deeply embedded. Maybe turned up at the back to fill the width of the slot?

With silicone, there's a thing about keeping it thinner than it is in extent, so it can stretch to fit when required instead of pulling free of the surround. That's why there are backing rods for use when sealing windows etc. So maybe that's the overriding concern for silicone sealants? What do other sealant types say?

Lead Flashing Sealant - how important is the spec's width to depth ratio?

greenfinger — Thu, 18 Apr 2024 23:30:56 +0100

After having done the flashing I came across the technique of turning up the lead at the back end of the chase. On really wide chases they even recommend to fix the turned up piece lead to the wall at the back of the chase, using screws and washers. I thought I'd missed a trick, but on reflection it wouldn't have been possible. Firstly because the chase is mostly quite narrow (10mm ish). And secondly because there is too much unevenness in the wall. and variance as a result. It was a job enough (plenty of swearing) dressing the lead and having ample amount turned into the chase (to "deeply embed it", as you suggest). Allowing for another turn up at the back of the chase would have been impossible.

I can see your point about keeping it thinner for more flexibility. It does make sense. Now I come to think of it, I remember watching vids for using Fuji kits in the past, and how the ideal joint in bathrooms etc will have two point adhesion, not three. So I think if I stick with using sealant I'll use backer rods and aim for 2:1, even though in places
it will be different.

The other option would be to use lime mortar. It would be easier and cheaper. I've read several people say they prefer it, some quite strongly. And I guess if lead has been used for centuries, and lime mortar has been used for centuries, then it is a tried and tested combination. But then again, all these sealants claim to solve the long standing problem of mortar not moving at the same rate as the lead, thereby cracking and causing leaks. Whether it's a modern solution to an age old problem, or a snake oil (more convenience than solution), I've no idea!

Posted By: djhWhat do other sealant types say?

Some say to fill the whole chase. And within that, some say it must be done in one go, while others say it is ok to layer up in stages. Some don't mention maximum depth, width, nor ratio. Some say maximum depth 15mm. Some say minimum depth 25mm. Some say to prime, other say not. I'm sure I can find other discrepancies but I think you get the picture!

Lead Flashing Sealant - how important is the spec's width to depth ratio?

Artiglio — Fri, 19 Apr 2024 08:06:05 +0100

A third option would be to use burnt sand mastic, either to fill smaller voids entirely or as a “cap” to a lime mortar partially filled void. Gives a very nice traditional looking finish.