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

[Oxbow16]

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.  

 

especially at 1:29.

 

 

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

Edited April 18 by Oxbow16

[Mike]

5 hours ago, Oxbow16 said:

I wondered whether you guys think about this ratio business?

I'd follow the manufactures recommendations in full.

 

Well actually, I'd use mortar...

[Oxbow16]

Fair play re following manufacturer's advice. But what I find interesting is that nearly all of the sealants are a low modulus, neutral cure silicone. And yet the advice on how to use them is not consistent between manufacturers. One example is how some say it is best to fill the chase with sealant, while others say the point adhesion must be avoided (by using a backer rod or similar). 

 

Perhaps while the main elements of the sealants are the same, there are other ingredients which mean they perform differently? And this explains the variance in application. Or perhaps not?!!

 

I think mortar would be easier and cheaper. I've read several others who 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!