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

[DIYNotIan]

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

 

[datarebal]

Depending on your stone , it might stick to the lead and detached from the stone in a short time . Then water will get in behind .
Something I've come across

 

[DIYNotIan]

Hmm, that doesn't sound good! The stone is sandstone, if that makes any difference? And it has been pointed with NHL3.5.

So would you choose to use mortar to point the leadwork, rather than sealant?

 

[datarebal]

I would use NHL 3.5 and a 3 to 1 mix with a good washed aggregate. (Wareham washed we use depending on stone)
We generally use st Astier lime as found it consistently reliable .

 

[johnny2007]

Don't worry too much about technicalities.
That lead sealant sticks to everything in any shape or form.
Just make sure you clean very thoroughly before applying.
Vacuuming the joints is best.

 

[datarebal]

Not so