Edwin Pawlowski wrote:
....
...Just look at the weight of the older slate roofs.

Slate roofs were certainly not placed on stick-framed houses only sized
for asphalt...that's totally unrelated to the OP's situation.

I agree it's highly unlikely if it's been there any length of time
there's going to be a problem tomorrow.

While weight can be a consdieration, the primary proscription on the
third layer is it tends to shorten life of the new shingles from
additional heat and poor conditions underneath.

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