What does the drop (on apron flashing) actually do? 240mm roll. 12x5.5 pitch polycarb roof?

[Safety First]

Hi folks,
Building a polycarbonate covered lean to that will get some harsh weather over the years- getting lashed right now...
I have a roll of lead 240mm deep (6 m long).

The pitch is 12x5.5 inch which I thought was pretty steep so thought I would not need so much on the overlap...then I read that upstand must be 75mm?

Re the overlap I get that more is better as wind will have to drive rain uphill further before it goes over the polycarbonate, but after it has done so, why would it matter that the drop goes on upwards 75mm or more before turning into the chase.

Can I squirt a band of sealant under the overlap and stick the flashing onto the polycarbonate? Or CT1?

The chase will be cut blind into render- no idea where the brick courses will be? Does this make any difference with the option of mortar or mastic? Never done this before but think I read that mortar will more often fail due to lead expansion-contraction.... But would the mortar not need something solid to support it?

 

[Mw Roofline]

75mm isn’t that much really. Especially if you have tall bars. We always use a 12” lead on our poly jobs. Yes you can put a nice bead of fixall crystal clear between the lead and the poly. Ct1 is going to look like squashed bugs from the inside looking up. As long as it’s original render and not that stupid insulation stuff then you can choose your chase height and go for it. As it won’t be a wide chase you can tuck the lead in nice and tight and use leadsealant or fix all high tack white sealant and adhesive. Both can be painted over if you wanted.

 

[Safety First]

75mm isn’t that much really. Especially if you have tall bars. We always use a 12” lead on our poly jobs. Yes you can put a nice bead of fixall crystal clear between the lead and the poly. Ct1 is going to look like squashed bugs from the inside looking up. As long as it’s original render and not that stupid insulation stuff then you can choose your chase height and go for it. As it won’t be a wide chase you can tuck the lead in nice and tight and use leadsealant or fix all high tack white sealant and adhesive. Both can be painted over if you wanted.

Thanks for input
What are bars- 'tall bars'?
Good point squashed bugs-ct1, they do a clear I think, but its damn expensive.
Chase into render- you mean cut maybe twice lead thickness and feed it in deep into a leadsealant filled chase type thing?

And what does drop do? I.e. once the water has run up under the overlap, it's purpose has failed. Is there really a notion that the water will then climb a 50mm drop and climb into the wall fabric?

 

[Mw Roofline]

The glazing bars in between the sheets. Some self support bars are 60mm higher than the poly.

 

[Safety First]

The glazing bars in between the sheets. Some self support bars are 60mm higher than the poly.

Ah, I see. Thanks for clarifying.
Mine are 5"x2" timbers. Will be using the very low profile, screw down glazing bars composed of a 2" wide, rubber gasket strip with a aluminium crown section that you screw down over each side of sheet. Underside of strip is double gasketed underneath/facing sheet.
They look rather basic but appear to be better suited to taking a ladder/crawling board at 90 degrees- the snap fit type need to not be shoved in too far/deep, which a ladder/crawing board would do.

.....Rather off topic, but hoping rationale/detail is of use to a.n.other.

 

[^woody^]

What's the pitch?