Drain lead

what’s the reason for the 3x3 sheet lead in the roof drain

Suposidly because with the drain being the area with the most water around it the lead wont rot away like asphalt will. And its flexible and will form into the drain pan where felts wont without cutting them.

Yep, just bullet-proofing. Same as when you run metal under a closed cut asphalt shingle valley. It’s a high water traffic area and merits extra protection IMO.

to me its a strength issue.
it makes the roof stronger at drain.
usually theres is a clamping ring and the drain is in a tapered area,
so lead is needed for it can be formed into the drain
circle and to strengthen the roof when its clamped to the drain base by the clamping ring.

that was a mouth full.